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Does anyone near me have a Stage 1 PST mod....?


lukebl
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It's just that I'm really disappointed with mine, particularly after the phenomenal expense of getting the ERF filter and TAL 100RS. The results are more pleasing when viewing through the basic unmodded PST. I'd like to have a look through someone else's, and see if I'm just expecting too much, or if I've done something obviously wrong.

Or else, I guess I'll wait till the next Kelling where I'm sure someone will have one to have a look at.

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I wish I lived nearer Luke. I'd come over and take a look for you. Its obviously hard to evaluate your pst without seeing it and testing it. Comparing it to another is a very good idea if you can locate one.

I had similar frustrations with mine when I first tried it. I saw so many great images that I could not get mine to create, then I began to understand about the sweetspot and that the images were taken with ccd cams that frame the sweetspot and cut out all the off band parts.

I have the same sweetspot effect with my Lunt mods, but the DMK41 just about fits it in.

Hope you can find another to compare with.

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It's frustrating to hear you're not making much progress with this, Luke :( Modded PST users do seem a bit thin on the ground. Could be worth asking on the Solar Chat forum too?

James

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Well I don't suppose Belgium is close enough?

The question is moot anyway as we haven't seen the Sun for 4 years now.

OK I maybe exaggerated a touch.

The difference should be noticeable, I would find it hard going back to the regular PST now.

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Thanks, gents. In the unlikely event of my being near Basingstoke, Belgium or Wiveliscombe in the near future, expect to hear from me!

James, with your focuser mod, where did you get your aluminium threaded adapter made up? And was it expensive? I'd like to find someone to fabricate something like that for me, as I have a spare Crayford focuser, but have no idea where to find someone to make it.

Edited by lukebl
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James, with your focuser mod, where did you get your aluminium threaded adapter made up? And was it expensive? I'd like to find someone to fabricate something like that for me, as I have a spare Crayford focuser, but have no idea where to find someone to make it.

I made it and the camera extension myself :) Had to shorten the camera extension today, too, but that's another story :D

James

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Luke,

I've been involved in many (MANY!) PST mods and you're the first to say that there was no improvement.....

There's obviously something wrong somewhere.

What's the main difference you see? Better for visual or for imaging?

I can only suggest you go back to basics and check the position and alignment of the PST etalon in the donor scope, the tilt of the ERF and the position of the final focus.

Based on previous mods, it should be able to be brought up to par....

If I can assist further let me know.

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Cheers, folks. I guess my issues with the mod are:

Imaging (I have tried using an IMG132e, QHY5 and a DFK21AU618.AS, probably not the best choice of cams):

1. The need to take out the spacer unit from the PST eyepiece unit, otherwise it won't focus with the camera. The spacer has a built-in filter which is presumably important. In any case, it must reduce the light intensity, as it's so bright that even exposures of 1ms can be overexposed. Using a Barlow just makes the image too big, at a focal length of 2m, and at the mercy of bad seeing.

2. Good focus is very difficult to determine, and the focusing prism in the focuser box often sticks in position. So I have to open it up and release it.

3. Vignetting and gradients in the resultant images mean that mosaics aren't possible.

Visually:

4. The image is soft, lacking in contrast

5. The severe vignetting is annoying, like viewing through a tube.

6. Again, finding focus is very difficult.

I've tried various eyepieces, ranging from 25mm to 6mm, although I don't have any of particular quality.

I know, this is a bottom-end solar telescope intended for grab-and-go visual use and it's not intended for this type of mod. Which is why it's such a joy to use in its original form and with its original eyepiece. I guess the Stage 2 mod might improve things, but I'm loathe to throw any more cash at it on current form!

Edited by lukebl
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Your focusing mechanism sounds even worse than mine, which "grabbed" as it moved. I've since acquired a second black box and the focuser on that is very smooth by comparison. If I ever reassemble the original scope I shall use that one. I wonder if it can be improved by lubricating it?

I wonder if the tube has not been shortened enough? Taking 26mm more off the tube would presumably allow the mini-ERF to be added back in without moving the focal plane?

Gradients in the image and the lack of contrast sound like some sort of etalon tuning issue? I was reading up on this last night and it sounds like Mark Townley(?) found a way to stabilise the etalon inside the tuning ring that improved things. Have you tried altering the etalon tuning though? I was fiddling with mine last night and found that there appear to be several places where the tuning is right, none of which were easily accessible using the tuning ring as it was initially set up. Is the pentaprism correctly positioned? I guess that might cause trouble too.

I can't see why everyone wouldn't have issues with vignetting though. By my calculations the image size of the Sun from a 1000mm focal length scope should be around 9.3mm across at the focal plane. Yet the light cone has to pass through a 5mm filter (the BF5) first which must surely act as a sort of field stop. Why would it not vignette in every case?

My understanding of the focusing issue with the DMK41 is that the nosepiece is too long to allow the sensor reaching the focal plane because the nosepiece reaches the bottom of the upper section of the eyepiece holder first. The fix is to use a shorter nosepiece without a shoulder. If the DFK21 is the same design (I believe it is), perhaps the same replacement nosepiece would fit? Do the QHY5 and IMG32e have the same problem?

James

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Luke,

Using the Stage 1 mod doesn't change the focus position relative to the "black box" - if you had problems focusing the camera on the PST, you will have the same issues...

removing the "mini-erf" is not a good move...

IMHO you should at least be seeing the same visual performance (if not better) with the mod.

Using the original PST blocking filter (5mm) with any donor with a longer fl then 400mm will give some vignetting.....

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Using the original PST blocking filter (5mm) with any donor with a longer fl then 400mm will give some vignetting.....

I accept this is the case, but I don't recall seeing it mentioned that much in discussions of the stage 1 or stage 2 mods. is it just something people live with, or is it less of a problem because mostly people are imaging and it's handled in processing?

James

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I agree that imaging without the extra 'mini-ERF' can't a good idea at all! However, I have the usual problem people have with PSTs, in that you can't reach focus for imaging even with a low-profile nosepiece.

What I'd like to do is a focuser mod like yours, James. I have this spare Low-profile Dual-speed Crayford for a Newtonian which I'd like to modify somehow. Unfortunately, I don't have the engineering kit or knowhow to fashion a suitable adapter. Any ideas where I could get this done affordably?

post-3895-0-76566800-1372327977_thumb.jp

Edited by lukebl
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I agree that imaging without the extra 'mini-ERF' can't a good idea at all! However, I have the usual problem people have with PSTs, in that you can't reach focus for imaging even with a low-profile nosepiece.

What I'd like to do is a focuser mod like yours, James. I have this spare Low-profile Dual-speed Crayford for a Newtonian which I'd like to modify somehow. Unfortunately, I don't have the engineering kit or knowhow to fashion a suitable adapter. Any ideas where I could get this done affordably?

Ah, I didn't realise you'd already tried the low profile nosepiece too. That's not helpful :(

It looks like what you need is a plate to fit the scope end of the focuser with an M50x1 thread inside to take the etalon unit together with a fitting for the eyepiece similar to the one I made that takes the M33x0.75 (off the top of my head) thread of the mini-ERF mount and fits in the 1.25" or 2" focuser adaptor.

You don't know anyone locally who has a lathe who might be able to do that for you? For an experienced machinist I bet they could probably do both in little more than an hour (doing both parts for mine probably took me quite a few, but I was learning :) Could be worth a PM to Roger (Bizibilder) asking if he knows anyone local to you (he's in the same county at least :)

Failing that, a google for "norfolk model engineers" might turn up people you could ask for help.

I'd offer to do it myself, but I'm going on holiday next week and to be honest I'm not that confident in my ability yet that I'd want to offer to do bits for other people :D I'd also need the focuser of course, which makes it more of a pain. It looks very similar to the one I have on my dob, but it's not identical.

James

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Guys,

A Stage 1 mod is always a compromise.....

Most modders acknowledge the limitations of the BF5 and quickly move to the larger BF10 or BF15.

Ouch! They're not cheap, are they? Those sort of prices make me wonder about fitting a 2" ERF in front of the objective.

Is the mini-ERF only required with the bare PST? Presumably if you have the D-ERF in the OTA you don't need it?

James

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Ouch indeed.

However, I see that in the US, the BF10s seem to be about half the European price, and I'll be there in a month's time. Hmm. More expense, I think.

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Ouch indeed.

However, I see that in the US, the BF10s seem to be about half the European price, and I'll be there in a month's time. Hmm. More expense, I think.

I'd hardly describe them as cheap in the US even, but I've only seen them available as part of a diagonal when you really only need the filter part itself. I wonder if it's possible to buy them on their own?

James

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I wonder what the specifications of the ERF and BF5 are?

Could one, for example, put either one of the Baader 2" H-alpha filters in front of the objective using a 52mm to 48mm step-down ring and dispose of one or other of the mini-ERF or BF5? I'd have thought having the filter up front might be a good plan as the entire OTA then has to handle far less light, potentially increasing the life-expectancy of the more delicate components. I've not been entirely convinced that it's a great idea to reject the light so far down the optical train. It's surely just going to bounce around and decrease contrast?

James

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Well, I've been looking at some graphs that are annotated in German, so I'm not entirely sure I'm interpreting them correctly. If I'm reading correctly then jointly the ERF and blocking filter seem to create a pass filter about 1nm wide centred on the H-a line. I think that's achieved by the overlap between the two, but I'm really not sure. I'm guessing that the mini-ERF chops out most of the visible spectrum and lower-energy light and the BF5 takes care of the higher energy end.

James

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Seems the mini-ERF can't really be removed safely for visual. It's a long thread but an interesting read if you're into this sort of thing:

http://solarchat.natca.net/index.php/en/this-is-solar-chat/12-solar-scope-modifications/11016-blocking-filter-mini-erfs-and-alternatives

Might stil be interesting just to add an Ha filter up front anyhow.

James

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Oh, and if you don't want to read the thread it seems to be that the final red filter is a very narrow filter for the Ha wavelength, so alternatives don't seem like they'd be easy to come by.

James

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Oh, and if you don't want to read the thread it seems to be that the final red filter is a very narrow filter for the Ha wavelength, so alternatives don't seem like they'd be easy to come by.

James

Yes. As I understand it, it's at a much narrower waveband to your regular Ha imaging filter. I know. I've tried it, and it doesn't work.

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