Modification of a Coronado PST 40 (Mod Stage 1)

The Coronado PST40 is a very good H-alpha solar telescope. It offers excellent image quality, is lightweight, portable, and simply fun to use.

At some point, the desire for more resolution and magnification often arises.

There are two possible conversion options:

The original black box of the PST40 is retained and attached to a large telescope.

The PST40 is completely rebuilt, so that the black box is also eliminated.

This guide refers to the first option.

Aim of the conversion:

A Skywatcher Evostar 120 with a 1000 mm focal length is to be converted into an H-alpha telescope.

One note beforehand: Evostars and most refractors need to be shortened (more on this later), but reflecting telescopes such as Maksutovs and Schmidt-Cassegrains usually do not, as they have sufficient backfocus.

Materials required:

Skywatcher Evostar 120 (link)
Coronado PST 40 (link)
Baader Planetarium DERF energy protection filter in the appropriate size (link follows)
Metal saw / or a very good miter saw
Drill
PST 50 adapter (link follows)

Modifying the PST40

The PST 40 is first modified:

The gold tube is being removed because it will be replaced by the Evostar 120. On most PST40s, the gold tube is now screwed in without threadlock, making removal very easy.

An important request: When unscrewing the gold tube, please NEVER use the etalon, the area with the rubber ring, as a counter-hold. This rotating ring with the rubber ring on it is only attached by a small M2 screw. You would definitely destroy it and thus create additional work with a lot of swear words. Therefore, please only touch the black box.

Now, attach the PST50 adapter.

Modifying the Evostar120

Unfortunately, the Evostar 120 doesn't have enough backfocus for the PST40 in its original state.

We need sufficient backfocus because the front lens of the etalon must be 200 mm in front of the telescope's primary focus. This front lens has a negative focal length of 200 mm, so when correctly positioned, the beam path is collimated so that the rays impinge on the etalon almost parallel.

The position should be accurate to within +/- 5 mm. Outside this range, you'll still see details in the H-alpha wavelength, but the half-width of the etalon's transmission line will broaden. This isn't what we want.

How do you determine the position of the prime focus?

First, the DERF energy protection filter is placed in front of the front lens of the Evostar 120. When installing the filter, make sure the arrow on the side edge points toward the sun.

The DERF is mounted in an adapter, which we also offer.

The Evostar is then pointed at the sun, and the focus position, also known as the focal spot, is determined using white cardboard on the eyepiece side. This is where you immediately notice how the DERF works: the spot doesn't even get warm to the touch.

Once the primary focus position has been determined, it's time to do the calculations:

Primary focus position - 200 mm toward the telescope lens = is it sufficient?

With the Evostar I used, I had to saw off 80 mm from the telescope tube to attach the PST40.

Tips for shortening the telescope tube:

In the absence of a good miter saw, I used a small metal saw. I applied masking tape all around the area where the cut edge would be and used several markers to mark the determined 80 mm on the tube circumference. I also marked the positions of the three mounting holes for the focuser, also on the masking tape.

The manual work now consists of gradually rotating the tube slightly, cutting into it slightly with a metal saw, and gradually getting deeper and deeper. This allowed me to achieve a precise section using simple tools.

Then drill the holes for the focuser.

The focuser is now deeper in; it's possible that one of the lens hoods in the tube is hitting the inner tube of the focuser. Interestingly, these hoods can simply be moved further toward the front lens of the telescope. They fit tightly, so don't be surprised.

The final result

The H-alpha telescope is now ready.

Have fun and success!

Pictures: See german version