My PST - TAL100RS mod

[lukebl]

Hi all,

Well, here's my new TAL100RS PST mod. Tube shortened and ERF filter attached. Now wait for some sunny skies to see if it works. Unfortunately, the eyepiece holder is really firmly glued with Loctite, so I'll just have to accept that I can't shorten the holder to make it possible to image without a Barlow.

Another thing, though. I opened up the PST box to check on the alignment of the prism, and it seems definitely out of alignment, by about 3 degrees. Is that a problem and do I really have to wrench it off and then re-glue it so that it's more accurately aligned, or is there another way? It's just that there's so much glue in the thing, I'm afraid of damaging something.

Cheers!

[WhiteShiva]

I saw an example of this at the astronomy show today. Looks like a great solution to getting a bigger aperture!

[Merlin66]

Luke,

Nice set-up!

Well done.

For visual work you may or may not notice the astigmatism introduced by the prism....if it does become noticable ( when you finally get the camera to focus- small barlow lens?) then twist it off the support and re-glue (RTV). I'm sure you will then notice the difference.

[lukebl]

Thanks, Ken. All seems to be working. I know I shouldn't complain, but I was a little disappointed by the vignetted view you get through the eyepiece, but the crisp views of the prominences are excellent. One thing I found was that I could fine focus better by tweaking the original focuser, not the one on the PST unit. I'd always under the impression that the original Crayford focuser was meant to be permanently fixed so that the etalon was always at the magic 200mm inside the focal length. No-one else seems to have picked up on this point.

[Steep]

Some time ago in an attempt to get a camera to focus I cut down the (earlier version non removable) eyepiece holder on my PST by about 10mm and also cut down the nose piece on the camera to bring the two closer together, It worked well but I've not had a chance to use it since I TAL modded.

[allcart]

You seem to see this prism misalignment so often I'm beginning to think its part of the design. I had the same problem with mine as well. I looked at the sun with the side cover off and saw that the light cone did not go centrally up the eyepiece, so I adjusted mine as well. I didn't unglue anything, i just twisted the prism gently until it was better. If you want to go further, you can unscrew the focuser all the way and remove the prism. On mine there are 2 locating nodules that slide in a guide track. Make sure they are both in the track when you put the prism back in place.

With regard to the scope focuser........yes you can still focus with it, and its maybe useful for fine tuning, but using the prism focuser is the way its supposed to be done. I'm not sure of the effect of lessening the magic -200, say to -190, but if you increase it you will vignette the sun disc even more.

All attempts by me to deviate away from the -200 have not improved anything so far. Certainly not with an f10 scope.

[Merlin66]

Various "trials" over the past few years have definately shown that moving >5mm or so away from the "majic" 200mm does have an impact on the performance of the etalon. It may be that the "feel" of the better focuser on the OTA gives an impression of improvement, but the optical design demands that the front lens of the etalon assembly be in a prescribed position. It is was a long term solution, others would have fitted a helical focuser into the original gold tube to replace the black box focuser......stick to the majic number....

When you eventually go for a Stage 2 mod - you will immediately see the benefit of having a good focuser between the etalon and the blocking filter!

[JamesF]

Is that really thread lock on the sides of the "black box" in the picture, Luke?

When Meade's pre-takeover accounts are finally analysed I feel sure it will turn out that the company was brought to its knees because of the amount they spent on thread lock.

James

[lukebl]

Yes. Hard glue appears to be oozing everywhere! The eyepiece holder is worst, and impossible to unscrew. So I can't shorten it to enable imaging without a Barlow. I can see why people go straight for the Stage 2 mod.

[Uplooker]

Hi Luke,

I had real problems getting the EP holder from my PST. I used my wifes hair drier to apply some heat and then used a boa strap wrench - shifted easily after the application of heat. Hair drier is best, definitely not a blow torch, as it will induce far too much heat.

Ian

[JamesF]

To be sure I've understood properly...

Is it correct that the eyepiece holder (in more recent models) is in two parts, one of which has an additional filter required for visual use, but that can be left out for imaging? And if it were left out, would that mean there was nothing between the imaging sensor and the prism?

James

[Merlin66]

James,

There are two optical elements in the current eyepiece holder assembly:

1. At the bottom, closest to the black box there is an "mini-erf" (ITF-interference transmission filter) to reduce the energy throughput.

2. At the top, closest to the eyepiece/ camera, is the actual blocking (sort) filter. This is absolutely mandatory.

In a PST mod, where an ERF is placed over the donor objective, the "mini-erf" can be removed (being effectively replaced by the new ERF.)

Some users find that the upfront ERF can pass some IR, which can be blocked with a KG3 filter (blocks 900nm up to 2500nm).

Oliver Smie (Beloptik) has produced a very effective UV-IR/KG3 filter which does the job well.

http://en.beloptik.de/uvxir_cut_on_kg3_blockingfilter.html

[JamesF]

Thanks.

In that case I shall shelve the idea of replacing the eyepiece holder with one from a Celestron diagonal. Perhaps not the smartest idea anyhow.

James

[Merlin]

Regarding what you said Ken about some upfront ERFs letting through IR, does this apply to the BAADER ERFs too, which I thought were impregnable?

I was going to opt for a cheap and cheerful square piece of uncoated Schott KG3 filter for £15, used in conjunction with the 75mm BAADER ERF on my Stage 1.

[Merlin66]

Merlin,

It definately applies to the "Red" ERF's and there is some indication that >1500nm the Baader does let some IR through - caused I think by the resonance bands of the filter coatings. This doesn't affect the CCD as there's no silicon responce above 1200nm.

I haven't seen/ heard of any major "energy" transfer issues with the Baader.

All this reseach comes about in trying to find/ test suitable replacements for the "mini-erf" / ITF filters in the Ha telescopes (Coronado/ Lunt). Certainly the KG3 glass blocks (by absorption) the IR region.

[Merlin]

Thanks for that. Now I feel confident enough to get the KG3 filters: one for the Stage 1 mod and another for the B600 that has a "bloom" on the inside face of the ERF.

[JamesF]

Does anyone happen to have a figure for the optical length of the entire PST "black box"? Obviously moving the pentaprism changes it slightly, but I'm only after a figure to the nearest 10mm or so. Save me taking mine apart and guessing wildly measuring approximately.

James

[Merlin66]

James,

The answer is 200mm.

(The focal length of the objective is 200mm, the front lens of the etalon is at -200mm and the focal length of the rear etalon lens is 200mm....this gives the effective focal length as (400-200+200) = 400mm)

[JamesF]

Thank you. I feel embarrassed now. I should have realised that myself

James

[lukebl]

Hi all.

So far, I have to say that I'm wondering whether this mod was worth all the cost! The results so far haven't been that great, and I'm wondering if it's just that the prism isn't square enough. Perhaps I should have gone straight for the stage 2 mod, or maybe not bothered at all! Or perhaps my expectations were too high.

Anyway, visually the results are OK, but imaging is a problem. I just can't get a sharp image, although seeing is pretty poor. Here's a result from today, May 25th 2013, in false colour. 1000 frames stacked with Registax. QHY5, TAL 2x Barlow, TAL 100RS PST Stage 1 mod.

[Bizibilder]

That seems pretty impressive considering the seeing was so poor today.

[JamesF]

I agree. The seeing here today was dreadful. I'd give it some time and practice first Luke.

James

[Merlin66]

Luke,

Could certainly be the seeing conditions...

Working at f22 ( f11 with x2 barlow) demands good conditions.

How did the detail look visually?

QHY5 mono or clour version?

What wavelets settings did you use?

Check the focus and etalon position, just be be sure.

[allcart]

Don't be disheartened luke, that is pretty good for a stage 1. I went straight to the stage 2 and finally got my helical focusing system working the way it should, but it has cost more than I expected. I chose the wrong focuser at the start and the optical train was too long, but its sorted now. The good thing is that it will work with any scope I put it on. I can even adapt it to suit other focal ratios, although i am finding that f10 is better.

Going to stage 2 will make life easier if you can bear the cost of the bigger blocking filter and focuser. Look at Mark Townleys pics with his small frac. so impressive.

[lukebl]

Well, I've now managed to separate the eyepiece holder and extension tube from the etalon/prism unit (after managing to break a Boa Constrictor wrench in the process), so that I should now be able to achieve focus with a camera. I see that both the extension tube and the eyepiece holder have red (presumably Ha) filters.

My question is, will I now need to add a filter to the imaging train to replace the one in the extension tube?