PST mod - donor scope advice

[Sqyff]

Feel your pain Luke. Just picked up an Evostar this afternoon and have been looking at what I want to do next.

[keithatrochdale]

A little bird tells me that Peter might just have a PST/TAL for sale.......

[allcart]

Well first thing to do is measure the focal length and subtract 200mm. Mark that position on the focuser or tube with a bit of tape, and then devise a plan of how you would attach the etalon at that position if the focuser was removed. Mull it over, and draw plans on paper. Share your thoughts and see what transpires . Depending on where the -200mm mark is will obviously influence your design plans.

Then when you've finished tearing your hair out, get out the hacksaws you bunch of sissies.

[JamesF]

I've just measured the lengths of the focusers on my ST102 and ST120. They come out at a smidge over 150mm. Given that the diagonal adds probably about 60mm to the optical path, it's perhaps not infeasible that subtracting 200mm from the focal length of the Tal might still leave you outside the end of the tube. In that case machining up an adapter to fit the end of the tube that would accept the fitting on the PST might be possible.

Of course, you still have to work out how to get the ERF inside and cutting may be the only way to do that, so it may be entirely academic.

James

[Sqyff]

Well first thing to do is measure the focal length and subtract 200mm. Mark that position on the focuser or tube with a bit of tape, and then devise a plan of how you would attach the etalon at that position if the focuser was removed. Mull it over, and draw plans on paper. Share your thoughts and see what transpires . Depending on where the -200mm mark is will obviously influence your design plans.

Then when you've finished tearing your hair out, get out the hacksaws you bunch of sissies.

I've gotta buy a hacksaw first.

[allcart]

I found with my Tal that the baffles were not fixed permanently and could be removed quite easily. I even removed all three after measuring their respective positions. It would be possible to make an ERF cell that could slide up or down the tube and be placed at any position. A simple method of removal would have to be considered, but should be easy enough. This would mean you don't have to cut the slot in the tube either.

I'm lucky in the fact that my C-ERF in its cell, fits perfectly on the end of my Tal. When I mod my 120mm XLT I will have to mount the ERF sub aperture. Not looked into how to do that one yet.

[Stu]

That's exactly the kind of thing I had been considering Allcart, pleased I'm not totally mad thinking it up!

Stu

[Steep]

I'm keen to mod my own pst this way, nearly fainted when I saw the price of an erf though! I like the idea of fitting the ota of the pst into the 2" focuser, that seems to simplify the whole process and if it means the donor scope can be used normally as well it's a bonus. Makes good sense to buy a better scope like the Tal then.

[Merlin66]

Steep,

There are a couple of issues using the "gold" tube to mount to a donor...

1. You have to remove the PST objective from the gold tube

2. The diameter of the gold tube is not exactly 2" - you'll need to add some packing strips

3. The length of the gold tube inside the donor may interfer with the baffles and cause vignetting

4. You may still have to cut the tube to position the etalon at the majic -200mm spacing from the prime focus.

[Steep]

Cheers Merlin, I would I think cut the gold tube back to a couple of inches? leaving just enough to act as a nose piece, my pst is one of the earlier models and the tube may be a bit difficult to remove. I think first things first though, get a donor scope and work out what is possible/best with that model.

[keithatrochdale]

Do you guys know about this very useful site;

http://solarchat.natca.net

[Steep]

Yup, got it bookmarked. Not got around to joining to post there yet though, he/they don't make it easy.

[Peter Drew]

Re gold tube use, I have done some mods with the full tube less objective and some with the tubes cut short, in each case I machined out some 2" tube to make thin packing sleeves. The full gold tube doesn't seem to vignet the incoming light cone but will foul the first baffle on a Tal 100 although this is easily removed. The objective can be screwed back on to the gold tube to return the PST back to it's original state for grab and go, a cut short gold tube can be "repaired" by making a suitable connector sleeve. You still have to cut the donor OTA.

[Steep]

This sounds more feasible by the minute, I'll order a Tal ota first and see if it will sit happily on the 127slt goto mount (trying to reduce cost if possible), if not then another mount will be needed.

[David Galvin]

I am gradually gathering what I need to do a Stage 1 mod with a Vixen 80M. Will probably try to use a 75 D-ERF internally mounted to save cost. Stage 2 is a future ambition but the blocking filter is too expensive at the moment.

Stu

I am doing the same mod with the Vixen 80M and will purchase my next piece as reccomended by Pete Drew.

http://www.thorlabs.co.uk/thorProduct.cfm?partNumber=LMR75/M

I strongly recommend that all potentential pst 'modders' follow the words and advice of Merlin66 (Ken) and Peter Drew and you will find the mod not only easier but also safe.

Thanks for all your advice Ken and Pete.

Merlin66

[keithatrochdale]

Yup, got it bookmarked. Not got around to joining to post there yet though, he/they don't make it easy.

It is not to difficult to join - honest.

It was done this way to stop spam etc and seems to work well and the wealth of knowledge is astounding.

The full gold tube is used in my PST/TAL, removing the baffle is easy, in fact I removed them all and flocked the TAL, not bothering to replace them which has caused no problems.

If you are in the area anytime, you are welcome to have a look - just bring some sun with you!

[Steep]

Plenty sun up here this week, clouds only seem to come over at night one reason I bought the pst in the first place!

[Sqyff]

A quick question about the Thorlab mounts, I don't want to hijack the thread. As I am using a 90mm donor with an internal 86mm dim, do you think that the lens holder(s) below would work for a 75mm internal D-ERF?

I was looking at the SM3L05 or the SM3L10. The outside diameter on these is 81.3mm, so really close to the internal tube dim so there is a possibility of removing the objective and sliding the filter down the tube.

http://www.thorlabs.co.uk/NewGroupPage9.cfm?ObjectGroup_ID=3384

Cheers

Cliff

[Steep]

Would this Lunt filter do the same job as the Baader D ERF does anyone know? http://www.apm-telescopes.de/en/Observe-the-Sun/Optical-Accessories/ERF-Filter/Energy-Rejection-Filter-75mm-2725.html

[Merlin66]

Steep,

Yes, but.....

The Lunt offering is just a red glass filter.

The Baader has a restricted bandwidth, UV-IR blocking and is much thicker ( better stability) and excellent optical quality (1/10 wave)

You pay your money...you take your chance.

[Steep]

Cheers and point taken, it would have made a decent saving if it had done the same job. I'll stick with the Baader.

[Peter Drew]

Cliff. It looks like the Thorlabs holder you specify would be suitable, be aware though that you need to be able to tilt the ERF maybe 3mm in order to eliminate reflections.

[lukebl]

Just a thought. Everyone seems to shorten the OTA at the focuser end when doing the mod, but is there anything wrong with shortening it from the objective end? For one thing, you wouldn't need to tap new holes for the finderscope holder, or do anything at all to the focuser. Presumably, though, the baffles in the tube would be in wrong position? Just thinking out loud, and I may well be missing something obvious!

[Merlin66]

Luke,

I think, when you check it out, that cutting the front of the OTA, at the objective end, is a PITA!!!

The objective cell, in smaller refractors is threaded onto the tube.....

To cut and then replace this thread would be more trouble than it's worth.

[lukebl]

Thanks for that. I thought there must be a very good reason why folk don't cut it at that end!