Another mod question

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JohnM
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Another mod question

Post by JohnM »

I thought I'd ask this here in case anyone has ever had the parts to play with.
I sprung for one of the Denk 60mm PST conversions back in early 2010.
The thing was nice visually, but never would allow any imaging, at least with the DMK21 I have.
I gave up on it and stuffed the pieces in the box and put it away.

Eventually I did away with the Black Box on my old standard PST and replaced it with a motorized Crayford SCT focuser, some adapters and etc to get things in the right position while using the stock 40mm OG and tube up front.
Thing has worked like a charm and doing away with the penta prism was the best thing I ever did.
Double stacked it it does amazing stuff for just 40mm and even in SS mode is superb for capturing prominences.
It's been a blast to use.

I spotted the box with my 600 buck paperweight in it the other day and decided I hadn't been frustrated for a long time, so dug it out and went after it again.
I can get the thing positioned to come CLOSE to focus, but never there.
All the measurements jive with what's generally used, but still no go.

So, my question is, have any of you guys here ever messed around with those?
Mine came with a 10mm BF, but during all my experiments I've always gotten the best results with the original 5mm.
How do you make an objective that won't focus?


colinsk

Re: Another mod question

Post by colinsk »

You must be outside of focus. You will have to find a way to shorten the path from the etalon to the camera.


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Re: Another mod question

Post by JohnM »

That would seem to be the logical answer, but with the focuser I have I can go past best focus both in and out.


colinsk

Re: Another mod question

Post by colinsk »

Collimation error?


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Re: Another mod question

Post by Merlin66 »

Correct me if I'm wrong...the Denk is a just a 60mm objective that screws onto the end of the PST gold tube AFTER removing the original lens? Is this correct?
Measure the focal length of the Denk lens for me (image the sun on paper-measure the distance to the rear of the tube then add the length of the tube...)
To work properly:
1. the PST etalon must be in the -200mm position...
2. the fl of the Denk lens must be enough to provide this distance.
3. The distance from the PST etalon to the CCD chip must be 200mm

(Stange one...I've just had someone else report problems with a Denk60....Hmmm)


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Re: Another mod question

Post by JohnM »

Right the Denk/Spectrum upgrade replaced the 40mm OG, plus tube, with a new 60mm unit and a 10mm BF.
The original PST was supposed to be sent to Denk for the work.
I talked to them quite a bit and was able to get them to just send me the parts.

The objective I have measures dead on at 600mm.

There never has been much about these 60s that I've found.
They're evidently still available, but the last time I looked the price had dropped.
Of the few reports I've found, they all mentioned the soft focus.
I've tried setting it up for 2 years now, but still can't bring myself to toss it.
I did put 600 bucks into it, plus I don't know how many hours trying to figure out where the problem was.
BTW, the OG is a cemented triplet.


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Re: Another mod question

Post by Merlin66 »

OK.
If the fl is 600mm then the objective should be 400mm infront of the etalon/ black box.
Is this correct?
(One step at a time....)

From their website:
"The 60mm Objective lens assembly offers diffraction limited performance, with blocking from ultraviolet to far Infrared to 10^-5 to ensure the safety of the observer"
This infers it has a ERF coating - how does look? No rust?


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Re: Another mod question

Post by JohnM »

Yes, I came up with 452mm and begin to get best focus at that distance.
The weather looks forbidding for doing anything today, but since the set up that works so well using the 40mm OTA is slightly over 200mm at the back end, I'm thinking I might start shortening up that distance some.

The OG is a deep red color, I assume a coating of some type on both exposed surfaces.
That part of the manufacture was done by Spectrum coatings.
And there is no "rust" as was sometimes seen on the older PST glass.


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Re: Another mod question

Post by Merlin66 »

John,
Unless Denk did something to the PST etalon during the changeover (Nothing they gave you to add/ mod?) then I'm sorry to say the distance is wrong.
The barlow at the front of the PST etalon needs exactly -200mm to work properly....
This would mean the distance from the centre of the objective to the front of the rubber knurled ring would be 400mm.
The ERF sounds OK.
Is there any "easy" way of reducing the length of the Denk OTA???


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Re: Another mod question

Post by JohnM »

Unfortunately the 50mm portion of my Denk sample was so poorly made, undersized, I had to junk it long ago and have been making my own sections.
In fact I think I'm almost out of materials again.
I'm sure you're right about distances being wrong some how.
Most of the info I have to work with I got from you ages ago at the UKAI forum.
Without making another tube, and since it seems the front distance, OG to etalon, seems to be in line;
if I get a little clear sky today I'll disassemble the rear end and get that distance from etalon to BF closer to what it should be and see what happens.
A 60mm would be nice, and this thing gets so tantalizingly close. :)


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Re: Another mod question

Post by JohnM »

Back at it again.
I can't get to 400mm distance from the OG to the etalon without doing a bit of machining.
Here are a couple images from this morning.
OG/etalon 415mm
Etalon/BF 155mm
The "mini ERF" is removed, with it in place it's impossible.
With the 5mm BF, it's always done better than the 10mm.




The first is full 60mm.
The second stopped down to 50mm.
I have some milky skies now and so it wasn't a fair test, I had to use gain which added noise and didn't get a good flat.


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Re: Another mod question

Post by Merlin66 »

Hmmm
Close but not close enough....some machining called for I think...when you do achieve the "majic" number - you'll definately know!
Success guaranteed or your money back ;-)


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Re: Another mod question

Post by JohnM »

:lol: Thanks Ken.

There's one thing I keep forgetting to ask.
I thought that the etalon had to end up positioned so that it would still fit inside the light cone.
No?


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Re: Another mod question

Post by JohnM »

It's setting on the mount in the obs waiting for, hopefully, some clear sky tomorrow.
The center of the objective is exactly 400mm forward of the front surface of the element in front of the etalon. I can move it in or out a distance of about 30mm.
The rear end is now at 200mm to the BF. I can change that spacing and the distance to the chip to about anything I want.
So, I guess I'll see if I have finally made this thing into something that works. :unsure:


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Re: Another mod question

Post by Merlin66 »

John,
Sounds promising.....
One last issue - the distance from the rear of the etalon to the CCD chip is the critical one.
It doesn't really matter where you place the BF (closer to the CCD the better).
It should be 200mm between the etalon and the CCD.
Onwards and Upwards.


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Re: Another mod question

Post by JohnM »

Thanks much, I'm as close as I can get things now and at least can easily move components at bit if I need to now that I got done milling on that last part.
Clear sky, that's all I want :)


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