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Author Topic: Etalon and light cone  (Read 2647 times)

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Offline JohnM

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Etalon and light cone
« on: 23:25:50, 13 May, 2011 »
Could someone tell me what the effect is if the front of the etalon, when 200mm forward of the end of the FL, is positioned into the light cone where it's larger than the etalon diameter.

Offline Merlin66

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Re: Etalon and light cone
« Reply #1 on: 04:47:49, 14 May, 2011 »
If you use the PST etalon in a fast scope (ie focal ratio < 10) at the design distance of 200mm, then it will act as an aperture stop in the optical train. For instance: when used in a 80/ 640 scope (f8) the etalon will effectively stop the aperture down to 64mm so you end up with a 64/640 system. The f10 beam is close to optimum.
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
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"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline JohnM

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Re: Etalon and light cone
« Reply #2 on: 14:44:02, 14 May, 2011 »
So it effectively  stops down the scope. No effect on focus or tuning of the etalon?
My 60mm f10 actually measures to 622mm FL, so it's really about f10.4.
I've been looking at the results shown by a design calculator that Simon sent me.
According to it, it gives a Solar disk at the etalon of 24.7mm, but measuring a full sized drawing shows about 19mm.
That should be near ideal size.
A couple days ago I managed a short period of testing in these infernally terrible skies we've been having, now we're under another storm system coming off the Pacific.
Conditions were bad, but I couldn't get a good sharp focus like I can under the same sort of conditions as I can with my 40mm objective.
Next chance I'll put the black box on and try it.
I'm beginning to think the 60mm OG I have just isn't up to the task.
A friend and I have swapped our regular PST 40mm OGs, the one I ended up with is just amazingly sharp compared to his.
We've done endless comparisons and his, while good, can't compare to what mine will do. I guess I just got a really good one and the 60mm I got isn't.
What do you think of masking my aperture to say 50mm? I almost always stop down my achros a little, but haven't tried that yet on this rig.

Offline Merlin66

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Re: Etalon and light cone
« Reply #3 on: 16:25:48, 14 May, 2011 »
Are you allowing for the diameter of the sun; 1/100 the focal length ie 6.2mm for your example.
You should have to stop the lens down....
If the etalon is close to the design 200mm inside prime focus the results should be OK. There are good and bad etalons also...I've handled and tested almost a dozen and would say that two or three actually were worth keeping...the rest were average...OK for proms and "basic" surface detail.
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline JohnM

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Re: Etalon and light cone
« Reply #4 on: 16:51:09, 14 May, 2011 »
This etalon has always worked perfect with the 40mm OG.
I can dial it in so that surface detail will snap in with perfect clarity.
Back together with the 60mm, it's the same old thing again.
I can get fairly good prominences, but surface detail is "soft" at best.
The last tests I was doing showed something like astigmatism, then I found bad flexure caused by some bad threads back by the focuser.
Now that's corrected, but the focus and detail still isn't here.
Just have to wait for some clear sky again, this has sure turned into major frustration.

Offline JohnM

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Re: Etalon and light cone
« Reply #5 on: 19:17:46, 14 May, 2011 »
This is an image from the only AVI I got the other day.
Maybe it isn't so bad and a good day will let me refine things.

Offline Nomis Elfactem

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Re: Etalon and light cone
« Reply #6 on: 16:22:55, 15 May, 2011 »
John - what's the image details (no of frames, number stacked etc)
Simon

Scopes: Astro-Tech AT-111EDT Triplet, TS65ED Quad, Orion ST80, Modded PST-90 Solar Scope, PST Cak (on loan)
Cameras: SXVF H694, Atik 16ic, Canon EOS 600d, DMK41, DMK21, QHY 5L-II (mono & colour)
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Mount: EQ6 (EQMOD), SW Star Adventurer, plus a lot (and I mean a lot) of other bits and pieces

Offline JohnM

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Re: Etalon and light cone
« Reply #7 on: 16:45:24, 15 May, 2011 »
1800 frames, 30 seconds at 60FPS.
Stacked in AS2, PP in CS2.
I don't remember just how many frames were finally stacked, I got deleted everything from that day.
I usually go for the best 25%.
It looks like a possibility of some Sun on Monday, maybe there will be enough to spend some time taking more than 1 AVI.
If I can ever see things begin to snap into focus, I'll figure there is hope, rather than the wide spread of focuser travel I always get with this 60mm with no real visible "best" position.

Offline JohnM

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Re: Etalon and light cone
« Reply #8 on: 17:45:22, 16 May, 2011 »
OK, here's 2 test shots from this morning.
Now, I'm actually ~4mm further apart on the front 200mm setting than I should be, but I've read that should be OK.
I can't get to the exact 200mm without wrecking my last good section of tube with good PST thread.
The seeing was poor and now clouds are coming in, it might get better later in the day.
I tried with the 10mm BF and just couldn't get decent looking images on the screen.
So, I put in the 5mm BF.
The first is stopped to 50mm and the second is the full 60mm.
I had a good half inch I had to crop all the way around both images, worse out of focus around the edges than the last time.
Sooo, this is where I am now. I can't get the tuning to really do its thing with the 60mm objective mounted.
I'm open to any ideas, even including forgetting this objective.

Offline Merlin66

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Re: Etalon and light cone
« Reply #9 on: 19:22:06, 16 May, 2011 »
John,
It could be as simple as seeing conditions...
I'm surprised with the OOF comment...at f10 the depth of focus is almost 0.22mm - so very difficult to be out of focus.
Which camera were you using for the tests?
C11, C9.25 SCT, 4" Genesis, NEQ6pro, modded 1000D,  ATik314L+, DMK41AF04, SM60DS/BF15, and Spectroscopes (many!).
http://tech.groups.yahoo.com/group/astronomical_spectroscopy/
"Astronomical Spectroscopy for Amateurs", "Grating Spectroscopes-How to Use them" -Springer

Offline JohnM

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Re: Etalon and light cone
« Reply #10 on: 20:18:27, 16 May, 2011 »
Using my old DMK21 I've had for years now.
These 2 were at f25.
Yes the seeing was bad, no doubt about that.
I just have to go on what I know the 40mm glass will do under similar conditions.
I'm going to leave everything together for a while now, we're looking for stormy weather for at least a week.
And I had to epoxy the 50mm tube section in place, so to get back to the 40mm, I'll have to disassemble everything so I can go at it with a heat gun to get the parts loose.

 

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