PST and Focus Issues - Nightmare

[johnb]

Hi

So I have a Coronado PST Solar Telescope and a DMK 41AU02.AS Mono

I also bought a short nose piece for the DMK

I tried various combinations in an attempt to get focus and the only way it worked was to

Remove the ring on the camera and attach the short nose piece and then remove the lens from my 2 x Barlow and pop that into the eyepiece in the PST and then mount the camera

It did work but that cant be the answer, there has to be another way.

Advice really appreciated

Regards

John Berman

[Photosbykev]

If the short nosepiece has a shoulder that will stop getting focus. The front of the camera case must touch the top of the eyepiece tube. If it does and you still can't focus then barlows may help. Otherwise you can remove the lower section of the eyepiece tube I believe but I didn't need to on my pst

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[SteveT]

John

I had exactly the same issue, mine was within 1mm of getting focus with the body of the DMK touching the EP holder (with a short nosepiece) of the PST so no way of getting it any closer. In the end I had to use the barlow trick and that's what I use now. If you find another solution I would like to know. I guess the only other way would be to reduce the length of the light path slightly somehow by playing around with the prism positioning but I don't know enough about that to even consider playing with it.

Some people seem to just get focus soot must be a marginal tolerance between PSTs.

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[johnb]

I wonder if I can chop the short nose piece in half ?

John B

[Photosbykev]

I wonder if I can chop the short nose piece in half ?

John B

if the nosepiece doesn't bottom out in the eyepiece tube there is no point in cutting it

The eyepiece tube is made from two pieces of tube screwed together so you could remove the lower section and screw the upper one straight into the black box. There is a filter in there which needs to be retained

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[johnb]

Hmm or remove a few mm from the top section ? John b

[RikM]

Is there a difference in the back focus distance between DMK-41 and 21 cameras? The 41's seem to focus okay.

[Photosbykev]

Hmm or remove a few mm from the top section ? John b

that would work IF the nosepiece doesn't bottom out

Re back focus distance of 41 and 21 I think they are pretty close. I use both on my rig and I only have tweak the focus slightly changing between them

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[jerryTheC]

One other option I've seen is to get a replacement main tube made for the PST - the bits at either end just screw onto/into the main tube, so using a shorter tube gives you a bit more back focus (presumably at the cost of losing a little bit of effective aperture, since the light cone would be a bit wider when it reaches the bits at the back end)

[SteveT]

I vote for taking 4 or 5 mm off of the top tube, this is the easiest way to fix it, you don't need to worry about refitting the filter if you don't remove the lower section. I have 2 short nosepiece adapters, one from FLO and one from Bern at Modern Astronomy, one of them is quite a bit shorter than the other but I don't know which one came from where, so if you take some length of the tube and your adapter bottoms out then you can use the shorter adapter, (assuming you don't already have the very short one).

I would rather not resort to using the Barlow lens if I could help it either.

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[johnb]

So here is my solution, (not actually done it yet) I have removed the ring off the camera and fitted my short nose piece. On the scope I will shorten the top section by x mm and see how it goes, removing x mm until I can achieve focus, i can then drill and tap a new hold to allow the screw to hold the camera - does this make sense ?

regards

John Berman

[Photosbykev]

If you can see the sun as a ring but just can't achieve the fine focus then you will probably only need to remove half a mm or so. You should be able to retain the existing locking screw

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[pmlogg]

I don't have a DMK but had a problem with my imager butting onto the head of the thumb screw in some orientations which prevented reaching focus. I solved the problem by replacing it with another that had a smaller head.

I've been asking questions about the Seibert PST Focus Adaptors (www.siebertoptics.com/PST%20focus%20adaptors.html)to see which one, if any, would be suitable for my setup (currently just a Canon DSLR). If focus is nearly there perhaps the Siebert 1.5x might be sutiable. It looks like quite a neat solution.

Peter

[johnb]

Hi so im going to remove about 4 mm which will leave the thumb screw hold in place, if its to much I can adjust by not putting the nose piece in to far and then marking the spot.

John B

[StevieDvd]

Hi so im going to remove about 4 mm which will leave the thumb screw hold in place, if its to much I can adjust by not putting the nose piece in to far and then marking the spot.

John B

Have you tried the setup with the short nosepiece and the spacer on the camera removed? Before you start removing from the holder as others seem to be able to get focus like that?

[johnb]

Have you tried the setup with the short nosepiece and the spacer on the camera removed? Before you start removing from the holder as others seem to be able to get focus like that?

Already done that and still cant get focus, its a real pain but such is life. John B

[johnb]

Already done that and still cant get focus, its a real pain but such is life. John B

gave it a go but no joy - will be for sale later as I have tried every combination with no luck and its supposed to be pleasure not a laborious hobby - after ready up on the subject it seems to be a common issues with PST sigh.....

[Merlin66]

JerrytheC,

Cutting back on the PST "gold" tube is a very bad idea....this moves the etalon away from it's optomised design position relative to the prime focus...bad, bad news.

To obtain endless backfocus (well, almost!) the "black box" can be removed from the etalon and replaced with a SCT focuser, a spacer and the blocking filter re-used from the original eyepiece and fitted to a diagonal. This "mini-Stage 2 Mod" has been done before and seems to give good results....

[SteveT]

John, did you cut off 4mm from the EP tube in the end, if so, are you saying this did not work?

[johnb]

John, did you cut off 4mm from the EP tube in the end, if so, are you saying this did not work?

Yes that's what was removed, actually to within 1mm of the threaded hold

John B

[Merlin66]

Hmmmmm

Sounds very strange, the backfocus of the DMK and "No shoulder" adaptors should have got you VERY close....

Sounds like we're missing something. What about posting an image of the DMK + noespiece and the eyepiece holder (what's left of it!) of the PST.

[SteveT]

Mine is just the same, I can only achieve focus using the Barlow lens, without it I am sure it needs a good 1 or 2mm or more which was why I was watching this closely to see if this was going to work. Maybe a later design has different focal length as mine is only couple of months old.

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