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GlassWalker

Focusing DMK41 on PST

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Before anyone says it, I have got and fitted the short shoulder-less nose piece and removed the 5mm extension ring it comes with.

With that out of the way, the DMK41 arrived today. I ordered the short nosepiece with it, and putting them on the PST the camera body is flush with the end of the eyepiece holder on the PST. It isn't getting any closer. In a quick and frustrating play, I just can't find focus. The sun spent most of the time hiding behind clouds so I didn't get much time, but I went near enough end to end on the focus scale. Before I gave up the sun was behind a TV aerial and trees, and using that as reference I know I can focus closer by rotating the knob clockwise. Infinity should obviously be the other direction, so I wound it as far as I dare anti-clockwise. It hits some resistance and by this point the knob sticks out quite far. So I'm hesitant to go further. Is this normal? Should I keep turning or will something fall apart before I hit an end stop?

Any other tips? I did manually adjust the exposure so the sun wasn't blown out, but I haven't really had time to play with the settings yet.

Given the sun has gone down now, I'll obviously can't try again until tomorrow (weather allowing).

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There was another thread about focusing issues with a PST and DMK on here a few days ago. I don't own a PST (or DMK), but I have to say I'm confused. I thought you should be able to focus with the short nosepiece.

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I'm beginning to think the current batch of PST scopes hitting the stores have a slightly altered focussing setup inside the black box which is preset at the factory. Does anyone have a Coronado contact who they could ask??

When I got my PST last year the DMK41 and short nosepiece just worked without any issues and it also worked for a lot of other PST users i.e. no one was reporting focusing problems

I have a PST black box left over from my PST Stage 2 modification which I know worked very well as a standard PST and focussed without problems, I could take apart if it would help anyone?

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I'm beginning to think the current batch of PST scopes hitting the stores have a slightly altered focussing setup inside the black box set at the factory. When I got my PST last year the DMK41 and short nosepiece just worked without any issues and it also worked for a lot of other PST users

That's what I thought. Something must have changed ....

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I did have a search before posting, but didn't see anything relevant. I did get the PST quite recently, a month or so ago. If it is scope focus range tolerance I wonder what can I do to work around it? If I'm not mistaken, adding some glass (e.g. some filter) should extend the focus point a little, maybe that'll be enough. I don't want to go to a barlow yet as I want to try a full sun single shot. Or is it crazy to think of using a barlow and focal reducer? :D Before that, I'll see if I can turn the knob some more until either my nerve or the PST breaks. Last resort would be modifying the PST...

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I did have a search before posting, but didn't see anything relevant. I did get the PST quite recently, a month or so ago. If it is scope focus range tolerance I wonder what can I do to work around it? If I'm not mistaken, adding some glass (e.g. some filter) should extend the focus point a little, maybe that'll be enough. I don't want to go to a barlow yet as I want to try a full sun single shot. Or is it crazy to think of using a barlow and focal reducer? :D Before that, I'll see if I can turn the knob some more until either my nerve or the PST breaks. Last resort would be modifying the PST...

Taking the side plate off the black box isn't a big deal (other than voiding the warrenty) there is next to nothing in there other than the pentaprism and focussing screw. It would be simple to then see what was moving when you focus

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Longer term I was considering to adapt the PST to a bigger scope, but I thought I'd at least leave it until after I mastered the PST.

Even if I can't get that to work, I've still got a CaK filter to play with also :)

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Just need a 2" nosepiece for the front of the etalon, a donor scope and D-ERF. What that won't overcome is the focus problem for a ccd camera

Sent from my iPhone using Tapatalk. Blame Apple for the typos

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*if* the etalon can be placed relatively far into the donor scope, wouldn't the focus point then be further out from eyepiece holder? Assuming other things don't get in the way of the light path of course, since I don't know exactly what's inside...

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Merlin66 is the expert with these things but based on my playing there is a 200mm lens built into the rear of the PST etalon and it is this lens that defines the nominal focus point behind the etalon. When you go to the Stage 2 mod you replace the black box with other blocking filters and adaptors to make up this 200mm length http://www.photosbykev.com/wordpress/2011/11/29/pst-modifications/

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I've seen your page before, but there's one major detail I didn't tweak until now. The etalon is the round bit in front of the black box? Doh! I assumed for some reason it was in the black box...

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Can we make a measurement to compare the lengths or is it all internal?

It has never really seemed like Meade are keen for the PST to be an imagers scope so maybe they have changed the lengths as they find out the tricks we come up with.

I don't know but there is enough of us here that with older versions to maybe compare.

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It's all internal Jarrod, unless they have changed the spec of the 200mm lens on the rear of the etalon?

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I've seen your page before, but there's one major detail I didn't tweak until now. The etalon is the round bit in front of the black box? Doh! I assumed for some reason it was in the black box...

The etalon (with the rubber band in front of the black box) has to be tuned in order to see any detail on the surface or prominences. It doesn't change the focus but it does alter the bandwidth to allow the relevant portion of H-a light to be transmitted through the scope.

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I get that the ring sets the etalon. I just didn't realise was where the etalon was physically located. I had assumed the ring was mechanically coupled to an etalon placed inside the black box. Maybe I should take it apart for fun, although my track record of putting things back together again isn't the best :D

On Jarrod's idea that Meade could deliberately cripple the PST for imaging, what would they gain? The PST is priced as an ideal starter solar scope. If I had to pay much more, I wouldn't have bothered at all. They wouldn't have upsold me to their higher models. Or if I was spending that much more, maybe I would end up with a Lunt anyway.

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It's all internal Jarrod, unless they have changed the spec of the 200mm lens on the rear of the etalon?

On Jarrod's idea that Meade could deliberately cripple the PST for imaging, what would they gain? The PST is priced as an ideal starter solar scope. If I had to pay much more, I wouldn't have bothered at all. They wouldn't have upsold me to their higher models. Or if I was spending that much more, maybe I would end up with a Lunt anyway.

It was just that earlier comment that got me thinking, I wouldn't put it past a manufacturer anyway, but I don't know and it may be a rash call.

I have a dmk41 on the way and as there has been a couple of these threads lately so it's caught my eye, my PST is newish but probably over a year old so I'll give it a go and hope for the best.

What I'd think they'd gain is people would get a taste for Ha viewing, it is brilliant after all but if it's a right faff getting the pst to do what you want it to do then maybe you'd spend double the price for a model up.

However like you said there is no way the scope would be as successful as it is if you had to pay much more for it, it could just be a poorly formed marketing strategy. I don't know, speculation and heresay will get us nowhere.

I do hope that a solution can be found though because it's such an enjoyable scope to use.

Hopefully it's just the etalon.

Edited by Pibbles

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To the best of my knowledge there have been no changes to the optical system in the PST since it was first introduced in 2003.

It was conceived and designed primarily as a visual instrument. Imagers could use the Coronado Solarmax or the SM series external etalon filters.

The Lunt's which came along later were designed to accommodate imaging.

(Glasswalker - I have a detailed write-up on the PST Mod - just PM me an email address for a copy - all is reveiled!)

HTH

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Before anyone says it, I have got and fitted the short shoulder-less nose piece and removed the 5mm extension ring it comes with.

... It hits some resistance and by this point the knob sticks out quite far. So I'm hesitant to go further. Is this normal? Should I keep turning or will something fall apart before I hit an end stop?...

yeah, no reason to unscrew it any further. The pentaprism has already reached its maximum travel and is pushed up against the top of the black box. If you really want to know the travel range of the pentaprism via the focusing knob's position, unscrew the side plate of the black box, so you can see the travel of the penatprism by screwing in or out the focusing knob.

Edited by thenews24

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So I am at the end of the focus range then? Not good... I'll have to see if I got my theory the right way around, that adding some glass will extend the focal point a little bit which I hope will be enough to achieve focus. Otherwise I guess I'm looking at a barlow, which will be annoying as I originally wanted to do fill disc imaging and not move onto mosaics just yet...

Speaking of annoying, the sun is out, but there is a lot of thin high cloud around at the moment. Will let my coffee kick in before I consider having a go.

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Set what correctly? Positionally it is as close as it is ever going to get without getting a saw out. Software settings, I manually vary the exposure so it isn't blown out, and I can't get the image in focus. I'm kinda getting there with the focus knob turned as far CCW as it goes, but not close enough. I can indirectly focus on things that are closer - if there's a foreground obstruction, I can focus on its silhouette. This is a classic symptom of the sensor being too far out.

Anyway, I did have a further play earlier today. Still no luck. I turned the focus knob more than I dared yesterday and have definitely hit a hard stop now. The image shifts when I hit that, so something is shifting in ways unintended and I wont do that again. I tried adding a filter as discussed, which I believe would extend the focal point slightly. The only suitable filter I have is a LP one, which I confirmed visually passes Ha. Didn't help either. My barlow I never got focus with anything on the PST, even with eyepieces. Looking inside it, the first element in the barlow is quite far up, so I guess that's my problem and I can't even use that.

I'm running out of options here... the only thing I have left to try is use eyepiece projection with the DMK... this could get... interesting.

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Thanks for the link, and also the earlier e-mail :) gonna have some fun reading that in detail while I wait for the sun to stop semi-hiding. It's still teasing me, popping out long enough for me to start something and disappear before I get anywhere.

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I just opened up my PST black box. The prism was a bit tilted, presumably from my earlier efforts at moving focus more than it would do. Have twisted that back a bit, although it wants to spring back to the tilted position.

Elsewhere, I have opened up my barlow and have extracted the optical assembly. This has a thread on one end which matches that of 1.25" eyepieces so I can fit it on the short nosepiece. But the direction of fit is opposite compared to normal. Past experience shows they can be directional, so just in case I flipped the optics around inside the holder too. I might need to play with the spacing later...

Now I just need some sun to test it. There was a heavy shower that just past and the weather forecast suggests I might not have a chance for a long time... :(

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This sounds strange, I have no problem focusing my DMK41 on my PST. What I have found though, if you put the standard nosepiece on the DMK41 (the long one with the shoulder) add a 2x Barlow, focus on the sun and get the gain and gamma correct. Then remove the long nose piece and put the short one on and remove the barlow, focus is very nearly in the same place, but you have to just reduce the exposure right down. It should at least get you to a similar focus using this method.

Alexandra

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