PST mod built with foam board and a hot glue gun

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thesmiths
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PST mod built with foam board and a hot glue gun

Post by thesmiths »

I put up a post in the section on Solar Astronomy Shop and Swap with a bunch of PST mod parts I had around the house for a few years viewtopic.php?f=11&t=31314. I didn't get any response and I happened upon a used Sky-Watcher Evostar 90 refractor for a very good price (see the end of the post for a photo of the very dusty telescope). I therefore decided to complete the mod that I started many years ago. Here is a short report on how the mod went and some comparisons to standard commercial equipment.
PST mod telescope
PST mod telescope
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To start at the end of the story, this is how the 90mm PST mod looks when completed. It actually appears much larger in this photo than it really is. The total length from end to end is 90cm and the weight is only 2.7kg, which includes a Lunt B1200 blocking filter, William Optics 90mm tube rings and dovetail, ZWO ASI camera, and a small helical focuser.

The telescope has a 90mm, two-element, air-spaced lens with a focal length of 910mm. The distance from the front of the lens holder to the focal point was measured to be 920mm. The length of tube from the front of the lens to the end of the tube was measured to be 695mm (once the focuser was removed). Therefore, the focal point lay 225mm beyond the end of the tube. This distance minus the "magic distance" of 200mm suggests the PST etalon should be 25mm beyond the end of the tube.

I used the Swiss adapters acquired from Beat Kohler many years ago. He had recommended putting 2-inch IR filter on the front to protect the etalon components. Here is a photo of the etalon unit. The B1200 is from my Lunt 60PT.
PST etalon module
PST etalon module
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To attach the etalon unit to the tube, I used my advanced arts and craft skills with foam board and a hot glue gun. I cut a series of circular rings from 5mm-thick black foam board with a compass cutter. The inside diameter was cut to give a tight fit to the Swiss adapter nose. The outside diameter of the rings were actually of two sizes: the larger to fit snuggly on the inside diameter of the telescope tube and the smaller diameter allowed applying hot glue in the channel formed between the large rings. The following photo shows the stack of six rings (so total of 30mm), which was sufficient to firmly hold the etalon unit with friction alone. A small outside diameter ring projects beyond the end of the tube to provide the correct etalon spacing.
etalon foam board rings
etalon foam board rings
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Some hot glue was injected into the focuser screw holes to secure the foam board to the telescope tube. This arrangement proved quite robust yet allowed the etalon unit to be easily removed. The following photo shows the centre of the PST etalon 25mm beyond the end of the telescope tube. A match stick was used as a tilt tuning assistant.
etalon positioning
etalon positioning
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At the objective end of telescope, a 25A Hoya 82mm red photographic filter was used as an energy rejection filter. The inside diameter of this filter was 78mm; therefore, to not waste any of the 90mm objective, I decided to mount this filter inside the telescope tube at the point of the first baffle. At this location, the diameter of the light cone is 70mm. To mount the filter, more foam board skills were used (although no hot glue). Two circular rings were cut from 5mm-thick black foam board. Both had an outside diameter that fit snuggly to the inside diameter of the telescope tube. The inside diameter of the rear disk was cut to enable screwing in the 82mm filter threads. The front disk was cut to a 70mm inside diameter to match the light cone.
red filter
red filter
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As a final foam board endeavour, I decided to make a home-built solar finder and attached this to the top of telescope tube (also with hot glue).
foam board solar finder
foam board solar finder
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With the Lunt B1200 inserted into the rear Swiss adapter, focus could be achieved with a mini helical focuser and a short 1.25 inch adapter. By chance, all the distances worked out well.

The month of May had very few opportunities for testing but by the end of the month the weather had improved. With the "first light" images, the etalon was a little wobbly so I followed the video of Mark T and disassembled the PST etalon to tighten everything up a bit. This seemed to improve the "sweet spot" and the tuning stability. The "second light" showed improved H-alpha contrast but the seeing was worse so the images were less than ideal. However, I could already tell that using either a 2x Cemax Barlow or a 2.5 PowerMate produced a lot of interference fringes. I determined that this particular PST mod was really only good using its native 910mm focal length.
PST mod third light
PST mod third light
pst-mod.jpg (345.43 KiB) Viewed 2685 times
With the "third light", the weather was quite good and I had learned how to use the telescope a bit better. The above photo was taken at its native focal length using an ASI 290MM, AutoStakkert, ImPPG, PhotoShop Elements. The resolution is quite pleasing and the prominences are quite appealing. The edges of the photo needed to be cropped since the "sweet spot" did not extend to the edges of even the ASI 290 sensor.
Lunt60 SS PM2.5
Lunt60 SS PM2.5
lunt-60-ss-pm25.jpg (359.06 KiB) Viewed 2685 times
For comparison purposes, the above image was taken with a Lunt 60PT and a 2.5x PowerMate. This means the focal length was 1250mm, so 37% greater than the PST mod. The processing of the two images was done identically. The quality of the PST mod image is not that different than the Lunt 60. The resolution of the prominences with the PST mod may be a little higher. The Lunt with a PowerMate does have some small evidence of interference fringes.
Lunt60 DS PM2.5
Lunt60 DS PM2.5
lunt-60-ds-pm25.jpg (431.72 KiB) Viewed 2682 times
Finally, the above image was taken with the Lunt 60 double stacked with a 60mm front etalon. In this case, the image is somewhat better than the PST mod, but of course this telescope is much more expensive.

All in all: the PST mod using the Sky-Watcher Evostar 90 was very easy to implement. However, the quality of the outcome seems to depend very much on the characteristics of the PST etalon itself, which is very hit and miss and requires a bit of tuning up and fiddling around in order to get satisfactory images. This particular PST mod only really worked at its native focal length and had a very specific field of view since the "sweet spot" prevented very large views and the strong fringes with a Barlow/PowerMate prevented increasing the magnification. The Lunt60 could provide a full disk image using a ASI 178MM at its native focal length of 500mm (see below) and its focal length could be increased to 1250mm without interference fringes causing too much distraction.
Lunt60 SS 500mm
Lunt60 SS 500mm
lunt-60-ss.jpg (428.5 KiB) Viewed 2685 times


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Re: PST mod built with foam board and a hot glue gun

Post by marktownley »

Great account of the mod and very promising results Douglas!


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