PST Solarscope Modifications

The Coronado PST Solarscope is an excellent instrument for viewing H-a solar activity and lends itself to some radical modifications to increase focal length and aperture resulting in significant improvement in resolution and visual/imaging quality.

With the expert help of -the solar imagers on Stargazers Lounge (SGL), including Merlin66 and JohninDerby,ย  this post covers the journey of one Coronado PST solarscope as it is converted into an 1100mm f/11 solar imaging system using a Lyra Optic 4″ f/11 refractor telescope. The standard PST has a focal length of 400mm and an aperture of f/10 and is very capable of taking excellent images like the one below but we always want more.

The Stage 1 modification allows the PST black box to be used in the back of a larger telescope by the addition of a nose piece to the PST and the fitting of an Energy Rejection Filter (D-ERF) on the front of the telescope. The nose piece needs to be anodised and then lined with 0.020″ Protostar Flockboard to minimise any internal reflections. This simple Stage 1 modification will allow me to check the amount of telescope tube I need to remove in order to place the etalon 200mm inside the prime focus of the telescope.

The Stage 2 modification replaces the PST black box with a Borg helical focuser and Coronado BF-15 blocking filter.

The heart of the PST system is the Fabry-Perot Etalon. The PST can tune the filter by the rotation of the “tilting” element, an aluminum grid covered in textured rubber. This way it is possible to tune the filter onto the “correct” frequency, and mechanically the effect is that of translating one of the two filter components, and to compress or relax the orange elastic seal in between the two filters.

The next step will be to calculate the optimum position of the PST etalon relative to the objective lens of the Lyra Optic telescope

….. to be continued

The Baader D-ERF has arrived and I collected the Lyra Optic 4″ f11 refractor telescope today. The Lyra Optic scope is almost too pretty to cut up but it is being sacrificed for a good cause and the very smooth high quality dual speed crayford focuser will be refitted the back of the scope during the conversion. A Celestron CG5-GT mount is due to be delivered tomorrow, more toys to play with ๐Ÿ™‚

I decided to return the Baader 75mm D-ERF which was going to be installed internally and I’ve ordered the Baader 110mm D-ERF from Teleskop-Service so I can make up a D-ERF cell that will simply fit on the end of the Lyra Optic telescope. This will give me the full 4″ aperture and remove any issues with thermal currents that an internal ERF may of caused.

The Celestron CG5-GT mount also arrived today, if the weather is ok tomorrow I’ll take a few pictures of the scope and mount.

The BF-15 blocking filter arrived from HandsOnOptics (LP = 80mm)

The Lyra Optic 4″ f/11 refractor telescope

A little bit of the sun playing hide and seek with the clouds allowed me to measure the optimum cutting distance on the scope tube. With the focuser drawtube at 15mm it looks like I need to remove 75mm of tube to put the front of the etalon ‘rubber band’ 200mm inside the focal point of the objective lens.

Superb service from Teleskop-Service saw the replacement Baader 110mm D-ERF arrive today ๐Ÿ™‚

The Lyra Optic scope tube is in workshop being chopped down and re-threaded, and a 4″ male to male adaptor is being made to allow me to restore the telescope back to its original length. Things are moving along very nicely for the Stage 1 modification, my thanks go to BICO Engineering for their support.

I still need to source the Helical focuser and extension tubes for the Stage 2 modification.

The Borg Helical Focuser M – M68.8 – Part #7835 has been located and ordered from Hutech in the States, this was the main piece missing for the Stage 2 modification. (LP = 63-83mm)

Borg Helical Focuser M - M68.8 - Part #7835

Borg Helical Focuser M – M68.8 – Part #7835

The Lyra optic refractor came back from the workshop today with the modifications complete.

The modified Lyra Optic refractor with the 4″ diameter 75mm long extension tube made from the cutoff section of scope tube

The 4″ diameter 75mm long extension tube made from the cutoff section of scope tube

The 4″ diameter x 1mm pitch male to male adaptor piece.

Teleskop-Service excelled again with a very fast delivery of the TS 8×50 right angle finder scope from Germany

Modified Lyra Optic 4″ f/11 refractor telescope with Teleskop-Service 8×50 right angle finder scope fitted with a Baader solar film cap to act as a sunfinder.

The Borg Helical Focuser M – M68.8 – Part #7835 also arrived today from Hutech in the USA. A beautifully engineered helical focuser that has a remarkably smooth and precise focusing adjustment.

Focuser details

  • Index Marks – 80 microns
  • Length – 63 to 82 mm (19 mm travel)
  • Weight – 320 g
  • Front interface – M68.8 P0.75 Male
  • Rear interface – M60 P0.75 Male, M57 P0.75 Female

A few adaptors to acquired and machine in the new year and the Stage 2 PST modification will be complete.

Borg adaptors#7522 (LP=12mm) and #7317 (LP=24mm) will convert the rear of the focuser into a 1.25″ EP holder.

Borg #7522 – M57 to M36.4 /M42p0.75 T-thread Adapter

Borg #7317- 1.25″ eyepiece holder

and modifying the Borg SCT to Helical Focuser M/T Adapter – Part #7428 (LP = 29mm) will give me the adaptor for connecting the helical focuser to the PST Etalon.

Borg #7428 - SCT to Helical Focuser M/T Adapter

Borg #7428 – SCT to Helical Focuser M/T Adapter

These extra Borg components have been ordered from so they should be with me in the new year.

First Light

The image above was taken using a DMK41 CCD without the 2.5x Powermate.

The images below are with the 2.5x Powermate

During First Light yesterday I noticed that my normal tuning position of the etalon, which is typically about 5mm from the travel end stop, had shifted slightly and I was having to put the tuning ring right against the travel end stop for the imaging. As I wasn’t totally sure that I had reached the optimum tuning position I opted to reset the travel end stop of the PST etalon. I moved the end stop by 2 holes and then tried some imaging today and sure enough I wasn’t quite tuned correctly yesterday and todays images are an improvement.

<———————————————– 343,000 kilometers ————————————————–>

I also did some magnification and resolution calculations.

The Lyra Optic 4″ f/11 has an 1100mm focal length and using the DMK41 CCD camera I am achieving the following fields of view (FoV) when imaging the sun which has a subtended angle of 0.52 degrees.

  • Without a 2.5x Powermate the FoV is 0.32 x 0.24 degrees
  • With a 2.5x Powermate the FoV is 0.13 x 0.10 degrees

Given that the sun diameter is 1,392,000 kilometers this means that at the maximum magnification with the Powermate my horizontal field of view is 343,000 kilometers. Putting this in perspective in the high magnification image above I could put about 27 Earths, which has a diameter ofย  12,756 km, across the image!

Dawes Limit: The theoretical Dawes limit for a 4″ telescope is 1.14 arcsecs so:

  • Using the DMK41 without the 2.5X Powermate I get 0.85 arcsec/pixel
  • Using the DMK41 with the 2.5X Powermate I get 0.34 arcsec/pixel

The atmospheric turbulence typically limits resolution to 1-2 arcsecs.

The Baader D-ERF needs to be securely mounted to the front of the Lyra Optic refractor telescope and needs to be manufactured. If it comes off then either the camera or my eyes are at risk.

The Baader D-ERF will have nitrile o-rings around the sides and top and bottom to act as shock absorbers and allow some movement of the filter so it can be tilted to avoid any ghost reflections.

A quick play, in white light, with the Lyra Optic 4″ f/11 refractor fitted with a DMK41 and 2.5x Powermate to image Jupiter. I used a 80mm 2″ extension tube and Williams Optics Dielectric diagonal to achieve focus rather than fitting the specially manufactured 4″ extension tube.


The D-ERF lens cell has been completed, it was made from Tufnol in the end as I couldn’t locate any 150mm diameter aluminium bar stock. The Borg components have arrived but are currently going through the UK customs system. One final piece will need to be manufactured, this piece will allow the Borg #7428 – SCT to Helical Focuser M/T Adapter to be connected to the rear of the PST Etalon.


Trial fit of the BF-15 blocking filter, the Borg focuser and adaptors and the PST Etalon with 2″ nose piece.

The front Borg adaptor has a SCT thread (2″ x 24 tpi) on the scope end which I need to remove and replace with a M50 x1P thread to connect to the PST Etalon. At the moment the working distance from the front of the BF-15 housing to the front of the PST Etalon knurled ring is 145mm when the focuser is at 10mm i.e. mid travel.

I think when I modify the Borg adaptor I’ll shorten it another 5-10mm as it will be easier to make the optical path longer but more difficult to shorten it.

All of the machining is now completed and all the optics have been cleaned off ready for ‘first light’ with the PST Stage 2 modified Lyra Optic 4″ f/11 refractor. Hopefully the weather will be good this weekend ! The SCT adaptor was machined out to remove the 2″ x 24tpi thread and an aluminium bush press fitted into the recess, this bush was then threaded with a female M50 x 1p thread to fit the rear of the PST etalon.


PST Stage 2 – First Light

The following images are taken with the PST Stage 2 modification, the system still needs tweaking but I’m pleased with the first light images considering the high hazy cloud. With the components I’ve use the system comes to focus using a DMK41 ccd camera with the Borg focuser at 8mm, very close to the midpoint of the 19mm of travel so I’m pleased with the adjustment I have.

I’ve just taken delivery of a Skywatcher N-EQ6 Pro syntrek mount and Hitec EQMod adaptor to stiffen up the telescope and mount to improve the image quality. I’ve also started a small project to remote control the focus on the telescope to remove any vibration issues.

On the advice of the more experienced Solar imagers I’ve removed the mini-ERF from the front (scope side) of the Coronado BF-15 blocking filter diagonal. This should reduce some ghosting which is apparent where the light is hitting one glass surface and bouncing back onto another surface in the optical path. A benefit of controlling the ghost problem is the increase in contrast which will improve the image quality.

The 3M HTD pulleys and belts have also arrived to finish off the remote focusing project which will be driven using a Hitec DcFocuser and Skywatcher focus motor controlled with the Ascom drivers.

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8 Responses to PST Solarscope Modifications

  1. Tad says:

    Great page! Two days ago, I purchased a PST sight unseen for $250, expecting badness. The objective ERF looks like a smooth Mars. Strangely, the visual image of the Sun is pretty good – or about what I expected from a healthy PST. But your work and images blow me away … Any updates these last 5 years?

  2. Pingback: ISS Solar Transit 110712 - PhotosbyKev

  3. John Hicks says:

    I’m making a PST Frankenscope, having cut the 48 mm thread and also a recess in the 2″ nosepiece to go on the front of the PST Etalon. I have heard that if you can screw a standard 2″ extension tube into the front of the etalon, (with its 48 mm thread) you can re-mount the collimating lens with its 48 mm thread in the nose of the extension tube, and slide that up into the new telescope tube 2″ focuser. Providing
    you locate the Collimating lens a minus 200 mm up the focal length, (i.e. Your focal length minus 200 mm) you will not have to cut off your telescope tube to accomodate the PST focal position. Is this true?
    If so, it saves cutting tubes off and ruining a perfectly good telescope. The Stage 1 photos are more appealing to me because the chromospheric network is stunningly
    resolved and I don’t care about full disk or even partial disk capture since I have a variety of H-alpha equipment for that purpose. The little PST etalon does a surprisingly good job. You realize you are only replicating a standard Daystar optical
    train in that the ERF is on the objective while the etalon is at focus, only with a small diameter, tilted etalon. It does surprisingly well – equal or better than my 0.5 A University Model Daystar without having to plug it in and always having to achieve an f/30 focal ratio. I wonder what Del Woods the inventor of Daystar would have to say about all this. His technology was a pure secret for so long. Well, no longer.
    Long time 35 year solar observer and Daystar User Group Member

  4. Michael says:

    I’ve gotten drool on my keyboard.

    I’ve been thinking of getting a solar telescope. I live in the PNW, and night viewing during the winter months are often poor to non existent. I’ve been trying to talk my wife into moving, but she doesn’t want to.

    In fairness, it is the only real downside of living in the area.

  5. c.molders says:


    I have found your nice website by info from Ken Harrison.
    I have made the stage 1 with my pst. Ist work and i have a lot of nice images.
    Next goal is the stage 2, but first I am searching for a blocking filter BF10.

    I tell you I have seen some pictures on your site has a wrong date.
    For instance uncropped solar image with date 17/12/12.
    Sorry for the interupt and a lot of succes with solar imaging.

    Regards C.Molders alias Solar.

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