My 70mm stage 2 PST mod

[Roel]

I've used my stage 1 70mm PST mod (Bresser Skylux donor OTA, UV/IR cut filter in front of etalon, original PST black box) for several months now, and although I'm extremely happy with the enormous improvement over the standard 40mm setup, I had some minor issues.

-the pentaprism inside the black box suffered from some astigmatism. Even after improved alignment the focused image just wasn't 100% sharp.

-although the etalon was sufficiently kept cool by the UV/IR blocker in front of the etalon, the image degraded after some minutes of observing. I believe it is caused by heating around the area of the filter, causing tube currents.

This could only be solved by going for a stage 2 mod (for solving the astigmatism problem) and by using a C-ERF in front of the objective (to keep everything cool inside the tube). But those C-ERF's are so darn expensive! I was therefore very pleased to see an advertisement for a second hand 90mm Baader C-ERF. It was extremely cheap because of a large chip on the edge filter. But there still was a clear, undamaged aperture of at least 80mm, which was more than enough for my 70mm scope! Of course I immediately bought the filter!

So yesterday I began making the stage 2 mod by dismantling the compete PST. I put the black box firmly in a padded vice and used a BOA wrench to break the Loctite... I had to use A LOT of force to overcome the friction and loosen the part, but it worked!

I screwed the blocking filter into an old Celestron 1.25" diagonal (it just fits, lucky me!) and for now that was all I could do at the eyepiece end, because the SCT focuser hasn't arrived yet...

So I began making the objective filter cell for the ERF. I used 2 aluminium lids from a coffee container and placed the ERF slighty tilted inside (to prevent internal reflections). It fit firmly around the shortened dewshield.

-to be continued-

(Kudo's for Merlin66 for the information about modding PST's!!)

[Ransu]

Your mod looks very good so far, interesting to wait more..

btw, sketches at your blog are awesome, and i like those dobsons too

[Merlin66]

Roel,

I did say coming over to the bright side was addictive!!!

Looking good so far - just get that -200mm and majic happens

[beamish]

looks superb !

The filter cell does not look like coffee tin lids ! Very nice indeed looks like a good pro job !

[Merlin]

Instead of taking a PST apart, would it be possible just to remove the 40mm objective and insert the gold tube into the carrier 'scope's focuser? It might be necessary to shorten the gold tube to prevent it pushing against glare stops in the carrier 'scope and, of course, there would still need to be an heat rejection filter over the carrier 'scope's objective. Waddya think?

[Merlin66]

Yes, It can be done that way. You could even cut the gold tube down to about 50mm long as a nosepiece. It's not exactly 2" so you'd need some packing.

[Merlin]

Thanks for the assurance, it sounds too easy to be true. Apart from getting the correct focal position, is that all there is to this type of mod?

[Merlin66]

Basically, yes..that's it.

Fit the gold tube to the donor so the etalon is 200mm inside prime focus and fit an ERF up front.

[step_hen]

I have a spare PST gold tube nose piece that is already cut down to use as you describe Merlin

[Merlin]

Regarding the 200mmm focal position. Doesn't the position of the etalon depend on the focal length of the donor 'scope's objective?

[Merlin66]

Merlin,

You really need to read my write-up on Modding the PST. Available on request. Just need an email address.

The answer is no. The etalon will act as an aperture stop and bring it down to f10.

The 200mm distance is critical due to the lens around the etalon.

So if you used say a 150mm f5 (750mm focal length) as a donor, it would end up only a 75mm f10 system.

This is why it's important to get the right Donor. f10 or above is ideal.

[Roel]

Still waiting for the focuser to arrive, this afternoon I tested the unit for tube currents with the ERF on the front and with the black box still attached in a "stage 1" configuration on the back. It works, the front ERF is a big improvement! Even after 30 minutes of non stop observing the image remained stable (except for atmospheric seeing of course).

By the way: the donor scope I use is a cheap 70mm f/10 Bresser Skylux which tube I shortened a few centimeters to get the etalon at a fixed position of 200mm inside focus (VERY important). I'll post more detailed pictures of the back end when the focuser arrives, but here are some pictures of how the black box is now attached to the rear end:

[Photosbykev]

no access to the images without registration Roel. Could you add them to your SGL gallery and repost them?

[Roel]

no access to the images without registration Roel. Could you add them to your SGL gallery and repost them?

Done!

[Merlin]

I'm using a 50mm Lunt over-the-aperture filter at the present time, but, as far as I know, modding isn't possible with these front-ended filters.

I have a 70mm/f.10 refractor that would lend itself well to a PST mod, but it isn't something I'm considering at the moment.

Thanks everyone on the thread for the information you've given.

[Roel]

Joepie! Mission completed!

I used a TS SCT Crayford focuser, and if fits the etalon quite well. Just screwed it in, no teflon tape required. (for the stage1 mod I already had the tube cut and the etalon fixed at 200mm inside focus) I placed the diagonal (with the blocking filter) in the focuser, put in an eyepiece and... magic happened!

No visible astigmatism at all, tack sharp prominences and filaments at 100x, and all my eyepieces come into focus. A homebrew 70mm H-Alpha solar telescope!

I am a happy man.

[Merlin]

Very nice! A further improvement would be to use metal tube rings in place of the plastic tube strap.

[Roel]

@Merlin, you're so right! Unfortunately tube rings with an inner diameter of 70mm are surprisingly difficult to find...

[Roel]

This afternoon I was wondering what to do with the remaining PST parts...

I decided to keep the original 40mm objective and use it with the Crayford focuser as a lightweight travel setup! Because the PST objective has no ERF filtering like the 70mm OTA, the little extension tube with the ERF built in must be used in this configuration.

It's such a simple and relatively cheap solution for solving the astigmatism and alignment problems (from which many PST's suffer), it makes me wonder why Coronado/Meade doesn't build all PST like this...

[Merlin66]

Roel,

I don't honestly know - as you say - a simple and effective solution....

[Merlin]

A final question on safety. The Cloudynights forum, which frowns on PST mods, warns that if the blocking filter distance from the etalon is changed in a mod, this could allow more light through than intended and damage vision. Have you paid due attention to this detail?

[Merlin66]

Merlin,

I've seen that comment as well as others re bandwidth...I have yet to find any evidence of any problem.

The bandwidth and transmission efficiency doesn't change with distance - based on my spectroscope analysis and results.

If you can find any 'definitive" measured data I'd love to see it.

I honestly don't know what it is with some of those guys on CN - an "assumption" becomes a "fact" if more than two people are seen to nod their head......

[Merlin]

In CN's solar section there's a safety statement and a blanket ban on articles concerning PST modding.

It seems that they don't want someone to sue the pants off them if PST modding advice on their forum turns out to be bad advice.

[Merlin66]

Welcome to America

[Roel]

if the blocking filter distance from the etalon is changed in a mod, this could allow more light through than intended and damage vision.

I don't understand. In an 100% original PST you change the distance between the etalon and the blocking filter every time... you touch the focuser!

If that distance really is so important the focusing would be after the blocking filter.