I eventually settled on an bought a stock Coronado SM40 from Telescope House in the UK, I used this setup for a year or so before I purchased a second SM40 etalon in order to double stack for increased contrast. I used a Pentax 6.5-19.5mm zoom eyepiece for visual work, this is a quality peice of glass that works very well. Imaging was principally full disk and quadrant work done with an Imaging Source DMK31. A variety of barlows and extender nosepieces are used to achieve different image scales. A CaK PST was used for ultraviolet visual and imaging work. Here a 15mm Televue plossl worked very well, although at only 40mm aperture it has to be admitted that the view is pretty faint! In addition I used a Meade ETX90 with Baader solar film and Baader Continium filter for white light observations and imaging. The optics were mounted using Geoptik hardware on a Skywatcher EQ6 Pro mount.
As I used this Ha setup more and more I became ever aware of a hatched pattern that was
visible on my images, after much deliberation the verdict was that the objective lens on the SM40 was astigmatic. I could not remove it from its cell at all, not even with a lens spanner or else would have replaced with a 'spare' objective lens from a PST I had come across, as a result the only option was to mount the etalons on a new OTA. I chose a 70mm f6 achromatic refractor from Ian King Imaging in the UK as this had a focal length of 420mm - ideal for full disk imaging, it had a decent 1.25"/2" dual speed focuser compared to the atrocious helical of the Coronado, and also importantly the dew cap was made of metal which would allow me to turn it on a lathe to make the necessary adjustments to mount the double stacked etalons.
I did a write up of the conversion process on Stephen Ramsdens excellent Solarchat forum - there's alot of information on there, including alot of relevant input from other people; rather than reprint it all here i'm going to include the link to the thread...
I would recommend a read if you are interesting in the specifics of this mod.
I really like this 70mm OTA to the point I since bought a whitelight solar wedge from Lunt, and also removed the filter elements from my CaK PST and built a module that allows me to do full disk imaging at 3 different wavelengths, all at the same image scale. The whitelight and CaK modules are also usable with my 127mm Meade refractor, though, it has to be said that poor seeing really limits the occasions that this large aperture setup can be used at these shorter wavelengths.
It wasn't long before aperture fever had got the better of me; there's a limit to what can be realistically achieved in Ha at 40mm aperture, I had got a Tele Vue 2.5x powermate that could only be used once or twice a year due to the urban seeing conditions I have, the time had come to get more aperture to be able to get a larger image scale. While there are alot of attractive commercial units from the likes of Coronado, Lunt, Solarscope, Daystar and others, the cost of these was just way to expensive for the aperture I wanted to be working with. As a result decided to go the route of a stage 2 PST mod. This essentially involves removing the etalon from a conventional PST and fitting it along with an energy rejection filter (ERF) and blocking filter to a larger aperture refractor. Certain parameters have to be followed in order for this to work, but all this is well documented on the internet and is a relatively straight forward procedure.
For my 'donor scope' I chose a Tal100R refractor. This is a Russian made 100mm f10 refractor that features fantastic optics and is a bargain price at £249 - these are widely available in the UK. I used a 105mm Baader D-ERF as the primary filter, and made a laser cut acrylic cell for this to sit inside the Tal dewshield. The D-ERF was purchased from Teleskop Service in Germany. I also used the Moonlite SCT focuser from my C8 in this setup. The following picture shows the PST mod sat side saddle alongside the DS40 on the 70mm scope...
All the scopes in this setup are fully modular and is possible to use for all 3 solar wavelengths. The following image shows the DS40 for Ha viewing, with the 100mm Tal being used with the Lunt solar wedge for white light. The Tal is particulary effective at CaK wavelengths and produces some very good images. My only (slight) criticism is the focuser tends to lose traction when used with the heavy WL wedge, but, given the cost of the scope this is something I can forgive!
This arrangement works very well with both the Geoptik tube rings on the 100mm, and using a Skywatcher guidescope mounting on the 70mm scope for adjusting so that the fov is the same in both instruments. When at home the HEQ6 equatorial head is mounted on a pier, which offers excellent stability and is really easy to level and to point north to ensure excellent tracking. For portable setup I use the stock tripod with this mount, while this is ok when the legs are not extended, things can get a bit wobbly, especially at full extension when multiple scopes are mounted like this. The mount is powered by a 70Ah gel battery from a golf cart, which holds sufficient charge for months of continous use of the mount.
The setup is very portable, and easily fits into the back of a car along with all the accessories - eyepeices, filters, cables etc. The following series of pictures show it being demonstrated at an outreach event at Keele University - sadly, in true British tradition we had cloudy skies, so were only able to demonstrate the scopes inside the observatory building.
I've also been fortunate enough to be able to use this scope setup for numerous outreach events at the local academy I work at, both for students who attend there and also students from local feeder schools who have attended taster days or STEM events that have been organised. I am also able to use the scopes as part of the GCSE astronomy curriculum that I teach. We are fortunate enough to also own a PST which is always setup and ready to go, many students have used this and are always amazed at the features that are visible on our star and also how rapidly these can change. I was also featured in an article in our local newspaper as a result of this, typically, in true British fashion, it was actually foggy when the reporter came round to photograph for the article, you'll notice a lack of counterweights, lack of power source and hand controller and also the scope is actually pointing to the north celestial pole rather than the sun - despite the fog on the morning it is nice to get some good promotion for solar astronomy.
As is normally the case with me and astronomy, it wasn't very long before aperture fever struck again; while 100mm at Ha wavelengths is quite large, I had a 5" (127mm) Meade AR5 refractor that was just sitting around, and my thoughts turned to this to add to my Ha arsenal.
I had, as mentioned above, been using this for some whitelight and also CaK imaging, but was being limited by poor local seeing conditions to how often I could realistically use the scope at these shorter wavelengths. Knowing that the longer the wavelength that is used the less prone and susceptible the image is to poor seeing I started to consider using this scope for a Ha 'big gun'. I had done alot of work on this scope in terms of modifications prior to this - it had a Moonlite focuser fitted, I had made a harvington strut as can be seen in the photograph to stabilise what was a large scope when on the mount, It had been retrofitted with a much larger dew shield, and had its lens cell rebuilt and then fully collimated. All this made it a very capable performer.
My attention also shifted to the donor PST etalon; with the 100mm Tal refractor in Ha the results I was getting were variable, sometimes I got really contrasty images that were nice and even, sometimes these were quite clearly showing a much wider bandpass and also a noticeable gradient in terms of bandpass and also brightness across the fov. While this wasn't too obvious visually, unfortunately it was really apparent when using the camera and showed up in the images very obviouslly.It was very apparent that the cell that housed the PST was very 'sloppy' and that this slop was causing the angle that the face of the etalon made to the optical axis to vary, when it did this it caused the tuning to vary, widening the bandpass and also causing gradients to be visible. I learned alot about etalons in the process of this and found this to be 'normal' behaviour in this situation. The picture to the left shows a gap of 2-3mm, which is where this 'play' was generated - this would be accentuated by simply touching the tuning ring on the etalon or even by sag caused by gravity. This meant that the tuning was not staying constant which is what is required. This can be seen in the video below:
When the etalon assembly is dismantled, it is clear the etalon itself sits snugly in this threaded housing as can be seen in the photograph to the left, it is very nearly an interference fit. However, as can be seen in the photograph below, the sub assembly that this screws into has a foam gasket in its base and 2-3mm of gap around the periphery of the etalon between the housing. Coupled with quite alot of play in the threads themselves in the housing explains why there is so much movement in the assembly and hence the etalon itself.
Fortunately the fix was relatively straightforward; if I could prevent the etalon from moving around at its base on the foam gasket then I could reduce or remove the play and hence tuning issues that I was experiencing. This took alot of trial and error to get the exact measurements, but by cutting out an acrlic ring on a laser cutter, and placing this on the base of the etalon, this effectively filled the gap and stopped the undesirable etalon movement. This acrylic ring can be seen on the etalon in the picture below:
In addition I also used some PTFE plumbers tape on the threads on the etalon cell to make for a much tighter fit of the screwed components. What was also clear from this diassembly of the PST etalon housing was the way the etalon itself was tuned - previouslly it was thought that the etalon spacers were compressed hence tuning the etalon, however it can be seen that this is not the case, rather the etalon itself is ever so slightly wedge shaped and relied on a tilt - tuning arrangement as the threaded insert applies pressure to the tilted etalon face, hence tilting it as it pushes against the foam insert in the base of the housing cell. Observation of how the image varies when tuning takes place backs this up. This subject was discussed at some length on the Solarchat forum and can be found here:-
The following video shows with the etalon sleeve and PTFE tape in place there is virtually no slop of the cell housing, and resultant observations and imaging show this solves the problem of changing bandwidth and illumination across the fov.
The full video showing how to disassemble (and re-assemble) a Coronado PST etalon can be seen below:
With all things fixed now with regards to the etalon I could finally turn my attention to modding the 127mm scope. Completely by chance the cell that I had made for the ERF for the 100mm Tal was a perfect interference fit on the inside of the 5" Meade scope. This had implications in terms of removing the lens cell to insert it, however this was the perfect opportunity to recollimate the lens cell.
I initially had a problem; in so much as with a PST mod the front collimating lens of the etalon assembly has to to sit 200mm inside the focal position of the donor scope used. With the Tal100R this was not a problem as I cut the ota shorter to accomodate for this, however with the 127mm Meade this was not an option as I did not want to cut the tube shorter as has fitted several hundred pounds worth of moonlite focuser onto it, and this was all very well collimated and I did not want to upset this. The solution was relatively simple though; the front collimating lens assembly is in a cell that screws into the etalon assembly, by chance this is pretty close to a standard M48 fitting as used by all 2" telescope accessories. I initially mounted this in the front of the 2" to M50 PST nosepeice adapter, however this still offered insufficient in focus, so decided to mount it in an old 2" barlow nosepeice and mount this in turn on the 2"/M50 adapter. This worked and brought the front collimating lens to the magical -200mm focal length position.
With that issue now resolved I had a 'module' that could be inserted into any suitable f10 telescope (with ERF) to give Ha solar views. This made use of the lovely Moonlite focuser that was sitting idle on my C8, along with the Coronado B10 blocker and trusty Pentax zoom eyepeice. The PST adapters for either end of the etalon were purchased from AOK Swiss - in Switzerland http://www.aokswiss.ch/index_zub.html and can wholeheartedly recommend them for PST mods. One final fact is important to get the best out of the PST mod is the relative rotation of the etalon with respect to the optical train. There will be an optimum rotational position for the etalon that gives the most contrasty views with the tightest bandwidth. It is important when setting up a PST mod to find this an 'clock' its position so that if the PST mod scope is disassembled at any point, for transportation for instance, that the ideal etalon raotion can be returned to quickly.
The following video shows a timelapse setup of the 127mm PST mod taken with the excellent GoPro Hero2 camera:
So, where now in terms of solar scopes and equipment? Well, I already have a 70mm Baader D-ERF hopefully arriving for Christmas which I will be using on the 70mm f6 ota to make a PST mod. This is obviouslly f6, and, if used with the standard assembly above would result in vignetting of the light cone, so, in order to remedy this the plan is to replace the front collimating lens with a negative lens that will have 20mm aperture and -120mm focal length. This should work perfectly and allow some great high res full disk views. Longer term I am toying with a few projects; firstly a 6" short focus PST mod - at f6 this willl allow ultra high resolution while still remaining compact. The other project I want to work on is reducing the bandwidth of my PST mod; I have a couple of options to explore in order to achieve this - firstly is to use the Omega EBay Ha filters or secondly to use a second PST etalon. Given what I now know about the importance of the rotation of PST etalons in order to achieve optimum performance, I think, at this stage, this is the route I will likely take. Watch this space for further details and developments!