Modification of a Coronado PST

The intention of this modification is to amplify the image of the Sun which you can see through a Coronado personal solar telescope and get a view of the chromosphere which can only be obtained by larger and much more expensive solar telescopes. You will need a PST, a “donner telescope” with an objective of 80 mm or more, preferably with an f10 aperture. It doesn’t need to be expensive and certainly it doesn’t have to be  apochromatic. The other item you will need is an Energy Reduction Filter in front of the objective. As this is expensive, don't buy it before making sure that everything else is ready and works.



Detach the golden tube from the main body of the PST. Coronado tends to glue the tube so this is a job which has to be done with care. You will have to turn clockwise with your hands only - you should not use any tool as the turning part of the etalon has a small screw which acts as stopper and excessive force will damage it.


You must construct an adaptor to fit into the threads of the etalon housing. The outer diameter of the threaded part should be 50mm and the thread step 1mm - I found out that the threaded part should not exceed in length 10mm.  The adaptor should then expand to a diameter of 2” to fit into a 2” visual back or focuser. The overall construction is unique and uncommon. I found out that the only workshop which makes these adaptors is Lazlo Sanzo’s

Teleskop Austria at relatively low cost (Tommy in charge of Wien shop is a very nice and helpful chap). Nevertheless I decided to make it my myself using my lathe. “Sevah” a fellow amateur astronomer provided some guidelines. I used a 50mm aluminum tube, set the racks of my lathe to 1mm and there it was. To expand the diameter of the tube from 50mm to 2” I first “thinned” the unthreaded part by 1mm and then inserted a plastic “sleeve” made out of an ordinary plastic 50mm drain pipe which I heated to expand and pressed it to the aluminum tube. I then thinned the sleeve to fit into a 2” visual back. I painted the inner and the outer part of the tube black to prevent reflections et voila ...

In the picture below the adaptor has been fitted on the body of my PST.


The next step was the construction of a secure mount to hold the Baader D-ERF filter in front of the objective. This filter is the most expensive item in this modification. I decided to cut some corners and choose a filter with 90mm diameter. I thought - and I was right - that there wouldn’t be much difference between 90 and 100mm. On top of this I needed the extra space to construct the filter holder.  As you see in the picture the filter is thick - 8mm thick and heavy so the

holder had to be strong. I made two things. I bought an 125mm plastic drain pipe’s tap. The tap has two parts. The part that fits into the pipe and has external threads and the actual tap with female threads.  I cut an 85mm hole with my lathe at the tap to make an opening to catch the light. Next I made two rings each with an angled tooth of 3mm depth difference in the cross section. The reason for this was to avoid reflections between the objective and the filter. I used allen screws to hold everything in place. In the picture below you can see the D-ERF filter securely mounted in front of the  objective.

STEP 4 - Cutting the “Donner Telescope's tube

The telescope which I used to make the modification was a Bresser Messier 102/1000 achromatic refractor. It is a cheap telescope but this didn’t really matter as at the end you aim to get a very narrow bandwidth of the visible light. So expensive apo’s don’t make sense. Another reason is the tube has to be cut to reach focus due to the added length of the body of the PST. The cut needs precision and if something went wrong the loss would have been minimal.

To perform the cut I devised a “cutting bench” on which I secured an electric wheel cutter (see at the workshop section for details).  I roughly calculated that I had to cut the tube at approximately the length of the body of the PST - slightly less. 12cm looked right so I crossed my fingers and took the rear part of the tube off.

Mounting and collimating the focuser proved to be a piece of cake with the Bresser. Other telescopes may need more precise collimation. Photographs of the assembled units are in the main page.

STEP 5- Constructing a “Sol Searcher”

The lathe is the most valuable tool when it comes to optics. The “sol searcher” serves to roughly locate the Sun and point your telescope without looking  to the sun directly. I used an 18mm copper pipe which I tapped at both ends with a piece of bronze rod. At the front end I drilled an 1.5 mm hole. At the rear end I drilled a 6mm hole. I used a small piece of white plastic (cut it off a white plastic plate) at the inner part of the rear end to serve as a projection screen. I glued everything in place and then mounted the searcher on a small aluminum plate. This “device” helps to adjust the direction of the searcher. In the photograph below the “sol searcher” mounted on my now modified PST. You can also see the sun projected on the makeshift screen.

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Modification of a Coronado PST - STAGE II

This second modification has two targets. The first is to replace the standard 5mm Blocking Filter of the PST, which does not have sufficient diameter o view the full disk image with a 10 mm BF. The replacement part was a Coronado BF10 on a 1.25” diagonal. The second target is to replace the prism focuser of the PST which to put it lightly sucks.

Soon I found out that there is no adaptor in the market for the rear part of the Etalon.

The adaptor which is needed must have a 50mm X 1mm female thread and by googling it, I saw that most of the people were using 2” adaptor and were patching the male thread of the etalon with teflon tape or similar. This was not good enough to me, so I designed the adaptor and asked Teleskop Austria if they could manufacture it for me. The answer was YES and so they did. It took them a couple of weeks to make it to my specs and anodize it. When it arrived it was exactly as I had ask them to make it. Tommy Navratil many thanks. The BF10 was purchased from OPT Corp, California, who offered the best price (in Europe it costed twice as much as I paid).

Note: At some stage I changed the donor Bresser Messier R102 telescope to an achromatic SW 102/1000. The new donor telescope has the same characteristics with the old one. The reason I made this change was due to a damage at the objective of the Bresser.

Images of the parts for this second stage are here bellow.

The parts used in the STAGE II  modification

The STAGE II  tube assembled

The STAGE II  tube on the visual back of the donor telescope