I have questions regarding technical details.
A technical introduction can be found in the 'Information Center'. Read 'Slicing the Spectrum' by David Lunt.
What's so interesting about Hydrogen alpha?
The red light of hydrogen alpha with a wavelength of 656.3 nm (1 nm = 1 billionth of a meter) is a very prominent feature in the solar light spectrum since hydrogen makes up 75% by weight of the outer layers of the sun. This light shows us layers up to 1700 km above the sun's visible surface and is especially important for imaging the lower chromosphere. The lower chromosphere is the coolest layer in the sun's atmosphere. The H alpha picture of the sun is very useful in predicting eruptions. The bright regions around sunspots, called plages, and brilliant solar flares are easily seen at this wavelength. Filaments, vivid string-like regions, and sunspots, large blotches on the solar surface, appear dark. Filaments are common sources of eruptions. Filaments on the solar limb appear bright against the blackness of space and
are called prominences.
What are sunspots and what causes them?
Dark splotches on the sun are sunspots. They are almost always seen in pairs and in groups of pairs.
Sunspots are dark because they are the coolest places on the sun.
Magnetic fields about as strong as those of a horseshoe magnet (which is 1000 times stronger than the Earth's surface magnetic field) keep heat from flowing up to the surface here. The sun's differential rotation (faster at low than high latitudes is critical in producing sunspots).
Solar flares (enormous explosive releases of energy from the sun) are most likely to occur in sunspot groups that are growing rapidly and rotating like a hurricane. The number of sunspots on the sun at any given time varies in an ll-year cycle as does the number and severity of disturbances in space weather. Constantly monitoring the development of sunspot groups is one important job of spaceweather forecasters.
What are prominences and filaments?
Prominences and filaments are really the same thing, but they look bright or dark depending on what is in the picture's background.
The part on the face of the Sun is dark, those parts of the filament that show beyond the edge of the sun are bright. Filaments last for a few weeks or months. The gas in a filament will eventually move to different layer in the Sun and will no longer be visible in an image of the chromosphere. But at the same time, other gas may move into the chromosphere and create a new filament someplace else. The birth and death of filaments is a mystery and observing them can be a facinating hobby.
Will I be able to see sun spots, prominences , filaments and surface detail using CIG filters?
Under optimal seeing conditions you should be able to observe all of the above. Remember that Solar observing is a learned art.
One CIG customer writes as follows:
My general ignorance of solar observation led me to believe that it would be as simple and easy as planetary observation. Of course it is not--as I have learned. One must train the eye, learn to tune the filter and optical system, and deal with the effects of heat on the various components of the instruments.
Any suggestions for a good all round Solar observing book?
SOLAR ASTRONOMY HANDBOOK Beck, Hilbrecht, Reinsch, & Volker. Available through Willman Bell , Inc. Richmond, Va. (804) 320-7060 or www.willbell.com In stock. $24.95 plus $1.00 for US shipping.
Excellent resource book.
Any hints on using the Helios for imaging?
We'll share our experience on this subject but actively solicit hints /tips/techniques from other users. Keep in mind that we are absolute beginners at CCD imaging.
Using a Starlight Express MX516 16 Bit CCD we did the following: TO FIND THE FOCUS
With the eyepiece in place we measured the distance from a specific point on the Helios....the point from the rear of the Helios where the helical focusser joins the Helios(in other words, the last fixed point on the Helios)....to the eyepiece lens including the 90 degree angle.We then removed the eyepiece and replaced it with the CCD camera and its 1 1/4" mount.
With the camera replacing the eyepiece, the camera chip will be focused when at the same relative distance that you measured from the eyepiece lens.
We found that in the beginning tht finding the focus was a bit tricky but after using the above method to identify the required distance and marking that distance with a piece of tape we were able to set up for CCD imaging fairly quickly.
It's important to coordinate with the computer operator who should take several shots to calculate the best focus position.
The image posted in the Image Gallery was our first ever attempt at CCD imaging.
The chief difficulty in maintaining focus is caused by the fact that the sun moves! In the time it takes to remove the eyepiece and insert the camera in focus position the sun will have moved. To compensate we set the eyepiece up with the sun just in the edge of field of view figuring that by the time we set up, there was a chance of getting a shot! Somewhat a three stooges approach but it worked! We were not using an equatorial mount which would obviously make life easier.
As always, necessity is the mother of invention and we have on the drawing board an attachment which includes an adjustable blocking filter. This device will enable the user to use simutaneously an eyepiece and CCD camera. It will be available later in the year. Meanwhile we'd welcome any other user comments.
I know I'm an H-alpha newbie, but I'm having a little trouble getting the image I want.
First, with an Astro-Physics 2" standard diagonal, I couldn't come to focus. I had to use a 1.25" diagonal, but that was OK.
Then, I had trouble getting a good H-alpha image. I saw great sunspot detail, but the image did not look like the image on your webpage. I tried tuning the filter with the adjuster,and eventually got prominences slightly off center. However, the other image still looked like a "red" white light image.
The temperature was about 75 deg F--does this affect anything?
Derek Wong
Regarding the blocking filter adjustment;- turn the blocking filter ring at least four turns anti-clockwise, (looking from the eyepiece). This will tilt the filter as far as it goes. Slowly turning the ring back again, should bring H-alpha detail into view. The significance of this is that the optimum position should be very obvious and should not be super critical;- you should be able to see the detail even if the ring is as much as half a turn from the optimum position.
The second point is that we have found that, if you are not used to looking at the Sun, it takes quite a while to realise what you are seeing and to get used to the type of detail visible. We have noticed that users see more and more the longer they observe. The biggest barrier is that one expects to see the surface somewhat like one sees the photos in magazines, etc.. It should be kept in mind that these have usually been greatly enhanced. However, it is not that the detail is not there, it just takes a while to get used to the way it appears in the telescope.
Regarding temperature;- the narrowband element of the filter is extremely temperature stable and will not be affected by any ambient temperature. The blocking filter will but this is the reason for the tilt ring. The blocking filter will adjust with this from about 30degF to approximately 100deg F. Hope you have more luck with the Helios. Please let us know if there is anything we can help with.
CHECK MR. WONG'S RESPONSE (UNDER INTRODUCTION /CUSTOMER COMMENTS) FOLLOWING THIS ADVICE. THE BOOK IN QUESTION IS THE ONE NOTED IN FAQ
I'm interested in sharing with fellow CIG users photography tips. Any suggestions welcome.
We are collecting comments suggestions etc, and will post them here. Also suggest you start posting your questions in Solar Chat.
When I look through the etalon assembly (ASP-60) "naked eye" I see a string of solar images along one line, and when I turn the assembly the line also turns. It has nothing to do with the blocking filter or the scope optics?
A word of warning. You should never look through the unmounted filter! I see what you are getting at now. The images seen under those circumstances are caused by multiple reflections between the high reflector on the narrow element etalon and the ERF. The ERF is mounted in the front cell at an angle of 1.5 degrees so that these reflections produce images well off axis and, normally, outside the field of the eyepiece.
What is the expected life of an ASP60 etalon?
With careful handling there should be no limit on it's life time. There is nothing degradable
What is the best way to keep the filter safe.
Keep in its case when not in use. Avoid sharp blows, drops etc. and extremes of temperature....particularly dampness and high heat.
Are there any problems due to the focal lenght of the telescope?
Which is the best f/ ratio for the ASP60?
Which is the minimum f/ ratio for the ASP60?
With the filter installed the focal ratio ideally should be greater than f10 to maintain performance of the blocking filter....but in fact you will see good results at considerably shorter f ratios than this.
From Giovanni dal Lago
Coelum Astronomia
1) Does HELIOS work with the same principle as ASP-60, that is, is the filter positioned before the lens? If not, with which principle does it work?
The Helios etalon is positioned between a set of ancilliary lenses, half way along the tube. By choosing the focal lengths and position of these lenses, we can arrange for the angles through the etalon to be almost the same as though the etalon was in front of the telescope while keeping its diameter to approximately half the telescope aperture. This is the configuration that we would use in, for instance, a NASA project so that we could maximise the aperture of the instrument and use the largest etalon that the budget would support.
2) With the two instruments that I have, the ASP- 60 is less bright than Helios-1. Is this due to the lightly lower diameter or to the narrower band-pass (0,7 instead of 0,8), or to both?
In the ASP-60, we use a slightly 'darker' secondary filter in front of the blocking filter in order to reduce spurious reflections between this and the front element. We have just started to use a different colour glass for this purpose which has had the effect of making the image a little brighter. Other than this, as you point out, the bandpass is a little wider in the Helios and, probably more importantly,- it is working at F/5.7 instead of F/8 as the ASP-60 does on a Pronto.
3) ASP-60 is composed of two parts: the filter and the tuning module (stardiagonal); where is the etalon positioned?
The etalon is in the front element. The diagonal (PROM-15T) contains the blocking/prominence filter. This does not 'tune' the whole filter. The narrow element (etalon) has a temperature coefficient of passband shift of <1 Angstrom/ 2,000 degrees C. However, the blocking filter is somewhat temperature sensitive and needs compensating,- by tilt,- for large differences in ambient temperature. This filter isolates the H-alpha passband in the etalon and eliminates all the others. If it moves with temperature, without being compensated, it would allow one of the other etalon orders to come through and, thereby, lower the contrast.
4.)Does the Helios work in the same way? As described in previous answer.
Yes. The blocking filter in the Helios is adjusted for temperature changes with the large knurled ring on the body.
5) I have noticed that with Helios it is more convenient to position the Sun always slightly low in the field of the eyepiece, no matter the tuning. Do you agree?
The Helios varies a little in this respect. Due to the tolerance on tuning the etalon to H-alpha, it is sometimes beneficial to have the image slightly off-axis in the view. With others, you have to put it right in the middle. The irony is that the view of prominences is usually a bit better through the unit that needs the image off-axis due to collecting a few more Doppler shifted events. However, in the ones that require the Sun to be right in the centre, the surface detail usually comes out a little better.
6) In the ASP-60 the observation is slightly easier than in the Helios; why?(for example, with the ASP there are no problems if I move the Sun at the edge of the field
Using the ASP-60 is rather like using the telescope through a 'monochromatic window.' Also any rays coming from the Sun traverse the etalon at no more than 0.26 degrees. Thus the etalon works absolutely at its nominal performance. In the Helios, these angles can be up to 0.4 degrees and, although this is perfectly adequate for an etalon of this type, I think that, once you have become accustomed to observing with both instruments, you will notice the difference.
A final question; my trial ASP-60 has a diagonal tuning module, is
it possible to have it different, as it has been published in your web pages?
In this way it would be much more practical for the photography CCD
Yes. The ASP-60 is available with with the BF-30 as an option.
This is a 2" unit and is a 'straight-through' design.
The technical information refers to AST and SMn models but they are not mentioned in the catalogue. Are they available?
See OBSERVATORY SERIES FILTERS at the top of the product catalogue.
HELIOS 1 and Photography.
I have just bought a Helios 1 solar telescope and would like to take some pictures with it. I have seen your pictures on their web site and have been incredibly impressed. In fact I think your latest picture which David Lunt sent me "adair surface new.TIF" convinced me to get the Helios. I have been procrastinating since I first saw the Helios and the ASP60 at Astrofest trying to decide between them . I was put off the Helios because the surface detail did not seem to be as good as the ASP60, and then there was the problem of off axis images on the Helios. Your picture convinced me that good surface detail was possible. It's really prominances I am interested in and I think there is not much between the two on prominances.
Anyway, to get to the point, I would be grateful if you could tell me how you took the pictures, ie film, exposure info, and what type of camera adapter you used please. Did you enhance the contrast to get those results in software, and did the visual image look as good as the picture?
I have a Nikon Coolpix 950 digital camera and hope to take some pictures with this and maybe an SLR as well. I guess that with an SLR one would have to use some sort on eyepiece projection.
I decided early on that I would want to take high resolution and high magnification images so the first requirement was to obtain a suitable tripod. I choose the Celestron G3 equatorial mount as it was relatively inexpensive and was powered in one axis to compensate for the earth's rotation. If you don't want to take high magnification images then a sturdy tripod, e.g. a Manfrotto tripod, is probably all you'll need.
I initially tried to use an SLR camera to take images but I really did not get good results at all. Getting the focus was reasonably OK when used at the prime focus of the Helios. However the constantly changing atmospheric conditions resulted in never getting a sharp image. Also contributing to the poor focused images was undoubtedly the small amount of movement introduced by the shutter on the camera. This movement being magnified because of the distance from the camera to the mounting hole on the Helios.It also meant that high magnification images were out of the question for the money I was prepared to spend on the tripod.So I decided to move to CCD imaging.I read up on all sorts of CCD camera's and imaging techniques.What seemed to be crucial was being able to obtain absolute critical focus.I was also aware of the differing brightness levels between the surface of the sun and prominence and therefore the importance of being able to control the shutter speed.
Whilst I felt that the normal astronomical CCDs were excellent for long exposure images I didn't feel they were best suited for solar images.I finally decided on video CCD equipment and in fact settled for the Astrovid 1000 Video Camera but would also recommend the Astrovid 2000.The AV2000 has a slightly larger chip and has more lines of resolution.The AV1000 has the benefit of giving an apparent increase in magnification and is cheaper than the AV2000.It comes with a standard 1 1/4" adapter which fits all telescopes.As always once you go down this route there are all the video capture and PC requirements to consider.So it depends on how much of this you already own, and how adept your PC skills are, to make this a viable and affordable approach.
With my set up I can now obtain critical focus and adjust the image brightness and contrast, using the camera control box,for the subject in question and see the results live.I am able to move easily from surface to prominence details. Whilst my preference is to capture images direct to PC I also attach the camera to a video recorder.Images captured on the VCR are not as high resolution as the camera direct to PC, but it does have the benefit of being able to do time lapse work and actually see the surface details change.Allows me to extend my observing when the weather doesn't allow direct viewing or I have some spare time in the evenings. A normal problem with the UK weather at the moment.
As a consequence of using video I can now attach additional equipment to vastly increase the magnification.For almost all the recent images on
the Coronado site I've used a Televue 4X powermate.Occasionally I use a 5X Powermate when the atmospheric conditions allow it with no breakdown in image quality.This is where the slightly brighter image of the Helios comes in very useful.As I'm sure you know image brightness reduces rapidly with increasing magnification.The image quality of the Helios I is superb and is also testament to the fine optical quality of the filtering produced by Coronado.I also happen to believe they are an excellent company to deal with and give good honest advice and support.(I also think they have one of the most open web sites for customers to communicate ie. Solar Chat )You also asked about how much image enhancing I do. I use as little enhancing as possible, usually just a little unsharp masking and smoothing and occasionally re-balance the brightness and contrast.I find that if you can capture a good sharp image you really don't need to do very much.
All my images are taken in black and white so I do use the same software to colour enhance. I use a pretty basic graphics pack, Corel Photopaint,in preference to expensive specialised astronomy imaging software. Having practiced I can now regularly produce colour results on par with the Learmonth Solar Observatory in Australia. If you are interested let me know and I'll send you a comparison of one of my images with that taken by Learmonth at simialr times.
As with all things it has taken me some time to get all this integrated and working smoothly. It now only takes me about ten minutes to setup and start capturing images.
All setups have advantages and disadvantages.The advantage of the setup you are proposing to start with are ease of use, portability, speed of setup, colour images, inexpensive and instantaneous results to keep and share with others. All this whilst not missing out on direct observation. I think this is an excellent place to start and become familiar with the Helios and "learn to see" at the eyepiece.
I do hope this has been helpful and you enjoy the Helios and solar observing. If you want any additional information please don't hesitate to contact me. Perhaps we will meet at one of the future Astrofests.
John Adair, UK
Check The Image Gallery for a stunning collection of images by John Adair.
Can you give a brief explanation of the differences between the Helios 1 and the ASP-60?
The basic differences between the Helios and the ASP-60 are a bandwidth of <0.8A and <0.7A and aperture of 70mm versus 60mm respectively.
Personally, I am always hard pressed to determine which one gives the best view. They will both show prominences superbly and a great amount of surface detail. To me, the real difference is simply that the Helios is an integrated Solar telescope and the ASP-60 can be used on an already existing telescope of pretty much any size and type. You don't have to mount the Helios piggy-back'on another telescope,- it can go on a small equatorial or even a sturdy tripod arrangement.
The central obstruction is a feature of our filters, (all but a couple of specialised models for specific applications). It is, in fact what we have recently received a patent on. Without this feature of construction, the filter would cost
approximately 5 times as much! The obstruction is about 30% diameter (<10%area),- similar to a standard Cassegrain. This does not have as much effect on Solar observing as in planetary viewing, due to the higher contrast detail on the Solar disc and the smallest detail on the disc being around 1 arcsec anyway.
The off-axis degradation you refer to in the Helios is a result of the filter being configured within the optical train. In this position one can not keep the various incident ray angles as small as when it is mounted on the front.The maximum angle that such a filter will accept is 0.5 degrees. With low power in the Helios, one could have an apparent field of considerably greater than this and so the Sun would only be in H-alpha within the central 0.5 degrees. This problem, of course, disappears when the power is increased and one only has maybe 0.5 degrees visible anyway.
The maximum magnification is dependent on the quality of the optics,- not the filter. Actually, with monochromatic light, one can usually push the
magnification a bit further than in white light in the daytime because of generally poorer seeing conditions. We regularly use over x100 on the Helios and this is normally limited by seeing than the optics.
David Lunt
Bearing in mind the perceived "problems" with the Helios because of the placement of the Etalon, why did you not make the Helios with the Etalon at the front and a simple 2element achromat? You have used four elements which would seem complicate it. If you did this, you would then have pretty much the same instrument as an ASP60 with a Pronto. I would have thought that because you are only interested in one wavelength, then false colour is simply not a problem. One might even get away with a single element. I assume it is cost related as the cost of the Etalon is probaly proportional to the area. You didn't say in your letter, but the Etalon looks smaller in the Helios than the ASP60, so I presume it costs quite a bit less to make which is offset by the cost of the rest of the
scope.
The cost of a 70mm filter for front end mounting would put the cost of the Helios well out of the range we intended for it. The cost of the etalon element rises somewhat more than the square of the aperture. The objective in the Helios is only a two element design, the additional lenses are to create an afocal position for the etalon. I would not say that this is a problem with the Helios;- any etalon is very limited to the range of off-axis incident rays no matter where it is situated. In actual use,one would,by default, position the image near the centre of the field, so one doesn't normally notice the effect unless the image drifts towards to edge.
Can one use the ASP60 with reflecting telescopes?
Yes, you can use the ASP-60 with just about any telescope, regardless of its configuration.
Do you have a figure that would indicate what the ratio of the maximum possible light from the prominance (ie the light level one would see with a 70mm scope during an eclipse) to the actual light received by the scope? Looking at eclipses (I've seen four), I am always amazed how bright the prominances are.
The brightness of prominences is always relative. During an eclipse, the entire light from the disc is missing, so they seem very bright. One gets the same effect if you use a 'prominence viewer' of the type that has a central cone to cover the disc of the Sun.
You will notice the relative effect if you see a prominence that also covers part of the surface as well as being off the edge of the disc;- being cooler than the chromosphere, where it is on the surface it shows up as a dark filament.
Once I buy an ASP60 from Coronado, what else do I need to start observing?
A telescope! Coronado filters consist of three elements: 1. the energy rejection filter (ERF) which removes the UV, the bulk of the visible spectrum and all the infra red. The ERF is an integral part of the second element the (2.) narrowband element which gives the narrow peak at Ha plus the adjacent peaks in the spectrum . This unit mounts on the front end of your telescope. And thirdly the (3.) blocking filter (BF30 or Prom 15T) which removes all the peaks except Ha. You do not need two blocking filters.The blocking filter mounts at the eyepiece end of the telescope. An eyepiece is inserted into the blocking filter.
Our ASP60 model is threaded to fit a TV Pronto but with an adaptor plate it will fit most other telescopes.
I have experimented with a 0.6 Angstrom Daystar ATM oven-heated filter for 20 years. The filter
produces good resolution in a specific field only appearing brighter and less contrasty in other zones. My system is a Meade 102ED coupled to a 2X astrophysics Barlow, a Telecentric lens, and the Daystar (in that order). The telecentric actually produces a reflection image also - faint in disk photography but very obvious in prominence photos - (because of increased exposure). I have tried to baffle the telecentric just behind the unit with a 1" aperture metal diaphragm offering little or
no improvement. The photographic result is a very "blotchy disk". Examining the telecentric lens : reveals that it is a "mild" telenegative" with a plano front surface and a convex back. Why would anyone polish the front surface flat when the very strong possibility of a reflection one-to-one that will result between the Barlow lens and the telecentric? If I remove the telecentric lens I get good prominence, lose the secondary reflection image, but get poor detail on the surface of the disk. Anyone had similar problems?
Message : Re: H-alpha filter performance
Posted by David Lunt on September 28, 2001 at 11:14:59:
In reply to : H-alpha filter performance posted by John Hicks - Newforest Solar Observatory on September 24, 2001 at 12:54:36:
What John describes is a very typical result of using a narrowband filter near the focus. The problem is that, although this type of filter has to be constructed to tolerances tighter than almost any other optic, these tolerances, in reality, can never be good enough. As a general 'rule-of-thumb' the total error for a F-P etalon, in waves, is the reciprocal of twice the Finesse. As an example, if building an etalon with a Finesse of 25, the total error budget is 1/50wave. This includes flatness and parallelism. Dividing this between the flatness of each plate and the parallelism between them, converts to actual plate quality requirement (at least statistically), of at least 1/150 wave flatness. However, the sting in the tail here is that this rule of thumb allows for an 'acceptable' 50% increase in the filter's bandwidth. If less compromise than this is required, the flatness and parallelism tolerance becomes much tighter than this. The result of this is that it is almost impossible to not have some variation across the aperture of the filter. This can include bandwidth differences as well as variation in the central passband. If the filter is positioned near the focus of the instrument, different parts of the Solar disc are imaged by different parts of the filter causing the above described filter variations to be imparted on the image. It is commonly considered that such variations across the field are solved by the use of a telecentric lens or system which will render all the rays parallel through the filter. This is incorrect. A telecentric component only converts the primary rays in the off-axis bundles parallel. The bundles traversing the filter still have the focal ratio of the system and are still specific to a portion of the image.
Considering a couple of the other points raised,- one of the most difficult problems in designing Solar instrumentation is the elimination of spurious light and ghost images. Many components can be tilted to sufficient angles to put these outside the field of view. Obviously, this is not the case with lenses which have to be carefully designed, configured and positioned so that the residual reflections from curved surfaces do not image anywhere they can be a problem. This can be very difficult and much effort is expended optimising this situation. It is 'de rigeur' that all lenses have extremely efficient anti-reflection coatings and, if it is at all possible, none have plane surfaces.
The reason for the lack of surface detail when removing the telecentric is that the bandwidth of the filter is then compromised not only by the instrument angles inherent in the focal ratio, but also be the field angles. This broadens the bandpass the point were, probably, only prominences are visible and even they will have severely reduced contrast.
It is for the above reasoning that we position our filters as far from the focal plane as possible or position them within an afocal lens system. The downside of moving away from the focal plane is a consequent necessary increase in the aperture of the filter. However, with the construction that we use, the increase in aperture is not as much of a problem as trying to solve the problems associated with proximity to the focal plane. The afocal configuration is far superior to the telecentric one when it comes to maintaining the performance of the filter. This mode does result in parallel rays traversing the filter, only limited by the field of view corresponding to the angular acceptance of the filter;- a better condition than having no part of the field of view imaged at the nominal spectral resolution of the filter due to the cone of the focal ratio.
We are frequently asked what are the differences between Coronado filters and other filters.
There are many ways of constructing a narrowband filter based on the Fabry-Perot etalon. We have chosen, as our standard, a solution that we feel gives the amateur the best opportunity of achieving an optimised view of the Sun. In optical terms, the Coronado, DayStar, Lumicon and Thousand Oaks filters are all similar. The latter two are not sub-Angstrom filters and, therefore, fall into a different category;- they are very similar to the Coronado blocking filters. DayStar's filter is based on a solid etalon design and requires external heating for temperature compensation. Coronado has the SMn-35 model which is virtually the same and also requires thermal control.
All these filters are good. The real differences in the performance of any of them are associated with the telescope/filter configuration. The sophistication of the configuration of an ultra-narrowband filter relative to its performance is not generally well known. It is quite common to have to re-design a proposed layout from even professional organisations in order to optimise the filter performance. This is the main reason that Coronado standardises on the front mounted filter. In this configuration, the optical system (telescope) behind the filter becomes irrelevant to its performance. Once the filter is configured within the optical train, the situation becomes much more difficult and, generally, ancillary lens are required to maintain the characteristics.
The standard front mounted SolarMax 40, 60 and 90 mm Coronado filters utilize a patented design that does not require any external heating to bring them to bandwidth as the filters are temperature stable and always on bandwidth. This design has the added advantage of allowing for set up and observing in a matter of minutes.
The rear mounted filter (DayStar and Coronado’s Smn) can, in theory, be used with a larger aperture than the front mounted type. However, in practice, this is not completely true as the usual way of achieving the required long focal ratio (at least on amateur instruments) is to stop down the aperture in which case it is comparable with the aperture of the front mounted filter. To utilise the full aperture of a larger telescope would require an afocalising set of lenses to control the ray angles traversing the filter. There are very few amateurs who use this configuration, although it is fairly common in professional circles, and so they end up, for practical reasons using a focal ratio which is not adequately long enough and thereby compromise the characteristics of the filter resulting in a less than optimised performance.
What should I look for when choosing a telescope for Solar observing?
The principle consideration in this question is the vastly varying quality of the telescopes on the market,- usually related to their price. Much to a lot of peoples' surprise, I generally recommend a good APO, or at least 'ED,' for Solar observation. The reason for this is not the usual consideration of excellent colour correction (which, obviously, is not required),- but the fact that the extra degrees of freedom resulting from having more surfaces and elements to work with generally result in having broader spectral correction for the other aberrations. In addition, the APO provides a compact, portable system with better aberration correction than an achromat of similar focal ratio. Many of the cheaper achromats are poorly corrected in the deep red end of the spectrum. This is something that in many cases can be improved by changing the air-space (if there is one) between the elements;- but this is not something that the average user can accomplish. Such telescopes are also usually anti-reflection coated with a single layer of MgF2. Such a coating, optimised for the middle of the visible at 550nm, is falling off quite badly at H-alpha, resulting in significant reflectance from each surface and, therefore the possibility of producing stray light and 'ghost' images near the image plane. Higher priced telescopes and APOs, usually have a broadband anti-reflection coating on all surfaces. However, even these can vary widely in their performance and we have measured many that are not as good as they should be at H-alpha.
Generally speaking, therefore, a good APO from one of the recognised manufacturers will perform excellently for H-alpha observing. In addition, many of the achromats on the market will also be adequate although one usually has to try it out to see if it meets the individual's criteria. None of this has anything to do with the telescope's colour correction;- from that criterion a single lens would be adequate. However, with reasonable focal ratios, this solution would be more expensive than a doublet.
The significance of the MaxScopes is that Coronado is offering a telescope that is completely optimised for H-alpha regarding aberration correction, anti-reflection coating and anti-'ghosting,' - but ONLY for H-alpha. This is an easy way of making sure that the instrument on which you use the filter is optimised to realise the best performance from the filter from an IMAGING point-of-view and, as was pointed out by Rick Fienberg in S&T August 2002, results in a very black background giving high contrast and easier viewing of prominence detail. The spectral characteristics come solely from the quality of the filter.
In answer to a number of questions we have received,- the type of filter used for H-alpha observation of the Sun is not usable in its standard form for deep sky imaging of H-alpha nebular regions. The transmission is deliberately reduced to allow visual observation and would be completely inadequate for the deep sky purpose. Although most deep sky filters have a fairly wide bandpass (1 to 10nm) this is generally for cost purposes. The bandwidth of a Solar filter (<0.1nm) would give very high contrast in such an application if the transmission could be suitably increased.
David
Why do the Coronado SolarMax series filters have a central obstruction?
The Central Obstruction
A noticeable characteristic of the SolarMax series of filters is the central obstruction. This is an extremely important part of the design and is, in fact, the principle of the patents issued.
Ultra-narrowband filters are based on the Fabry-Perot etalon. This can be of either a solid spaced or air-spaced design. Because the solid spaced etalon is constructed of an optical material, such as glass or fused silica, the temperature coefficient of the material combined with the temperature coefficient of its refractive index, mean that the etalon is thermally sensitive and has to be kept at a constant temperature in order to maintain the tuned wavelength. Air-spaced etalons have been constructed for many years as an alternative based on their thermal stability. However, in this respect, one has to carefully define the thermal stability. For instance, if such an etalon is tuned at a temperature of 200C and the temperature is changed to 300C, it will also be tuned, and have the same characteristics, at this new temperature WHEN THE WHOLE ETALON HAS COME TO THERMAL EQUILIBRIUM at the new temperature. However, it will NOT be tuned during the time that the temperature is changing and its bandwidth will be considerably BROADENED, due to thermal gradients within the components, causing distortion of the air-gap. Such an etalon, when used as a Solar filter, will have unacceptable, variable characteristics and, therefore, performance. As with the solid spaced filter, it would have to be temperature controlled.
It was this dilemma that prevented the use of air-spaced etalons as Solar filters until the breakthrough brought about by the patented Coronado design. The reason for this, of course, is that, in use as a Solar filter, facing the Sun,- the filter is never in thermal equilibrium due to continuous incident radiation. In the Coronado design, an extra ‘spacer’ in the etalon, controls the shape of the etalon components during temperature changes and maintains all the etalon’s characteristics. Thus the filter is kept tuned to the correct wavelength, and maintains its proper bandwidth, at all temperatures and UNDER CHANGING CONDITIONS. This CAN NOT be the case for an etalon that does not have this controlling central spacer.
Because of this central spacer, a mask has to be put over the center of the filter to prevent direct sunlight from getting to the image plane. This is the same situation as the central obstruction in a Cassegrain type telescope (or a SCT or Newtonian or Maksutov/Cassegrain). However, whereas a typical cassegrain has a central obstruction of approximately 35%,- giving an area obstruction of ~12%,- that in the SolarMax filters is only 2.5% to 5.5% depending on the model. This is well within the obstruction generally considered to be completely unnoticeable to image quality.
How can I post my images in Solar Chat?
Answer courtesy of Paul Hyndman. Several people have mentioned the desire to share their images here in the "Solar Chat" section, but were unsure of how to do it. Ergo, the following brief how-to:
1) The image is not stored here, so must already be present on some other server elsewhere. If you do not have your own web-site or access to one now, you can post a JPEG version of the image to one of the free services such as:
http://geocities.yahoo.com/
http://www.pbase.com/
2) Fill in the Post a New Message info here the way you normally would, with a message describing the image or other particulars you want to pass along.
3) Go to the lower box, labeled "Optional Image Url" and insert the URL of the image you wish to post. If you are unsure of the exact URL, just go to the image (wherever it is posted), position the mouse over the image and "right click" on it.
a box will pop up that includes "Properties" on the bottom of the list.
Left click on "Properties" and a box showing the image URL will come up. That's the address you want to put in the "Optional Image URL" box, and it should end with ".jpg" (ie: http://www.astro-nut.com/sun-surface09sep02.jpg)
4) That's all there is to it... now select the "Post Image" button and your message and picture will come up!
If you'd like to include a link to a page on the web, you can use the "Optional Link URL" box to do so. Just insert the link there, which can be an image (.jpg) or page (.html). You can then use the "link Title" box to caption the link with whatever you'd like.
For this example, I am posting the picture "sun-surface09sep02.jpg" by putting its URL in the Image URL box:
http://www.astro-nut.com/sun-surface09sep02.jpg
and optionally making a link to the full page that it is on (so as to include the other info I want to pass along with the image) by posting the link URL in the appropriate box:
http://www.astro-nut.com/sun-ha-09-sep02.html
(note that the link is also active on the screen if posted in the body of the message as was done here).
Then making a name for the link and posting it in the "Link Title" box. Let's call it:
"Here's how to post an image and link!"
Hope this helps, and hope to see some more of your images here!
Ciao,
Paul
Can I image with the PST?
If you're planning on using an afocal projection (eyepiece projection) setup with a fixed-lens digital camera and can focus the eyepiece there should be no problem.
Prime-focus digital SLR cameras are another matter. The internal focusing mechanism has a limited amount of accommodation and in these configurations it will be far more difficult to reach focus. This may also be true of webcams depending on individual adapters and cameras.