Coronado PST MOD written by Dennis Put
I See Astrophotography
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The etalon of a PST is a Fabry-Perot etalon with a bandpass of <1.0 Angstrom at the H-alpha wavelenght. It is quite cost-effective because it is rear-mounted, so the etalon doesn't have to be very big. In case of the PST the etalon is 20mm in diameter. To make the etalon work optimally at its rear-mounted position correctional lenses are placed in front and after the etalon to make the light beam that enters the etalon parallel, which is necessary for it to function. For the mod, I have used a Skywatcher Evostar 102/1000mm f9.8 achromat refractor, at which the ideal position of the etalon is closer to the focuser. The 40mm aperture of the PST doesn't stress the etalon, but when placing a 102mm objective in front of it, the stress has increased. To protect the etalon from thermal stress it is important to place an Energy Rejection Filter in front of the etalon, and in this case in front of the front objective. For this I used a Baader D-ERF 110mm filter in a custom made cell. When mounted in the cell the free aperture of the ERF is still 104mm and, therefore, ideal for the 102mm aperture of the telescope that I have used. The Coronado PST also has a small ERF filter just under the BF05 blocking filter I left in place. The ERF is often placed in front of the front objective, but it is also possible to use a smaller ERF and place it inside the tube. The Baader D-ERF is a high-quality filter and the custom cell from TS has black felt on the inside of the cell, so that no scratches or damage can be made on the telescope lens cell. In some situations, it may be useful to tilt the ERF a little bit in its cell to eliminate ghost images. A picture of the D-ERF:
My Coronado PST was a hand-selected piece, so the contrast of the H-alpha features was very good. When the PST etalon is used on a bigger telescope and especially in prime focus the small 5mm blocking filter can cause serious vignetting. The image is much more equal in contrast when using a barlow. Sometimes, the image was not equal in contrast over the entire view, but software processing was often able to significantly reduce those differences. When imaging prominences, you might need a little different tuning. It is also interesting to watch filaments as you tune the etalon. For both prominences and surface features tuning of the etalon is critical. When modding a Coronado PST you will want to remove the PST etalon from the PST's black box and the golden tube. Sometimes the etalon thread has Locktite on it which can make it quite challenging to remove it. The blockingfilter can also have this. I was lucky that my PST did not have any Locktite on the threads and the etalon was removed easily. For the etalon to work optimal, it should be placed 200mm inside focus of the telescope. For this my 102mm refractor probably needed to be shortened by some centimeters, which I haven't done because I found the results to be very good. The 102mm refractor was also perfectly aligned/collimated by the shop where I bought it. The results of other amateurs also show that excellent results can be achieved when the etalon is a bit off that -200mm. For an optimal etalon-BF distance I used a couple of Borg parts including a Borg Helical focuser with a focus travel of 16mm. The benefit of the Borg parts is that the thread of the etalon could be attached directly to one of the adapters. To connect the other side of the etalon to the telescope a 2" adapter was used. I used the standard BF05 blockingfilter that came with the PST. When using a small camera chip such as the one in a DMK21AU618, fine results can be achieved especially when using a barlow. To mount the BF05 with small ERF in one of the Borg parts a 2" to 1.25" adapter from Robtics was used and machinally modified to fit the barrel of the BF/small ERF holder. A benefit of the Helical focuser is that focus adjustments could be made without altering the place of the etalon, which is preferably at a fixed position because of the optimal position. The Borg parts that I used:  - 7857 Helical Focuser M57  - 7424 SCT to M57/60 adaptor  - 7505 2" Eyepiece holder 40mm  A picture of the parts in action (from left to right: DMK, barlow, BF05 blockingfilter, 2" to 1.25" adaptor, Borg adaptors, etalon, Steeltrack focuser).
In the spring of 2012, Dennis Put and I decided to modificate a Coronado PST H-alpha telescope to take better solar images. The 4cm aperture of the PST allows for fine viewing of H- alpha features, however, resolution is limited. The idea of a PST modification is to use the etalon and blocking filter of the PST on a larger telescope to get a greater image resolution. It is very important to use an Energy Rejection Filter in such a modification. He began with a Skywatcher while I bought a Bresser AR102, both F10. Meanwhile this telescope went to Neo and finally came into my possession. The rest of the story are Dennis’s findings.