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Old 26-08-2013, 04:27 PM
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Question about OAG focusing

My TSOAG9 arrived today... problem is my lodestar is sequestered away until father's day...
So I'm preparing, hoping for a hassle-free-first-light, if such a thing exists...

I'm trying to come to grips with focusing.

The chip in my CCD is approximately 45mm out from the plane of the OAG prism. Does this mean, that, field curvature aside, the CCD in the lodestar needs to be approx 45mm from the prism in the OAG?

And on the same line, once the guide cam is focused, moving the whole guide-cam-prism-stalk in and out shouldn't appreciably affect the focus at the guide cam CCD? Or will it??
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Old 26-08-2013, 05:05 PM
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multiweb (Marc)
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Quote:
Originally Posted by Lee View Post
My TSOAG9 arrived today... problem is my lodestar is sequestered away until father's day...
So I'm preparing, hoping for a hassle-free-first-light, if such a thing exists...

I'm trying to come to grips with focusing.

The chip in my CCD is approximately 45mm out from the plane of the OAG prism. Does this mean, that, field curvature aside, the CCD in the lodestar needs to be approx 45mm from the prism in the OAG?
Yes.
Quote:
Originally Posted by Lee View Post
And on the same line, once the guide cam is focused, moving the whole guide-cam-prism-stalk in and out shouldn't appreciably affect the focus at the guide cam CCD? Or will it??
Correct again. The further out, the chonkier and fainter the stars will look but it won't affect focus that much. Focus is the distance between the lodestar and the prism.

Also try to rotate the lodestar so the top of the chip is in the brightest part of the field. This way readout is faster when you use subframes and you get a faster guide rate. If you use an AO. The top right corner is darker than the three others when you look at the chip front on. It reads from top left to bottom right.
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Old 26-08-2013, 06:16 PM
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Thanks Marc.... no AO, just OAG here... unless someone makes an AO with a thickness of 1mm, won't be happening anytime soon here Damn newts....

I just realised today also, I'll be limited a bit in rotating the whole she-bang on the focuser, as the OAG stalk will foul on the focuser in places, and the motor hanging off the filter wheel will foul on the focuser stepper motor in other places.... Damn newts....

At present I'm using a fairly small chip, so I have fair bit of lee-way with moving the prism, I think I'll work out where it vignettes, then back off a tad. I'm not expecting to have too much hassle finding guide stars through an f/4 at 800mm FL... hopefully!
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Old 26-08-2013, 06:23 PM
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The easiest way to set up the OAG is during the day on a distant object....
Get the imaging camera into best focus then adjust the guide camera to come to focus..
If these are not moved, then they will be in focus for the evening...
Sometimes, there's not enough inward travel on the guide camera tube. In this case you need to add a spacer after the OAG body, between the OAG and the imaging camera, then re-set the guide camera position.
I use Al's reticule with my Lodestar to position the target star on the spectroscope slit....majic!
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Old 26-08-2013, 07:48 PM
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Originally Posted by Merlin66 View Post
Sometimes, there's not enough inward travel on the guide camera tube. In this case you need to add a spacer after the OAG body, between the OAG and the imaging camera, then re-set the guide camera position.
I hope not - I need to preserve the MPCC's 55mm spacing....
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Old 26-08-2013, 08:12 PM
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Why is your OAG 45mm from camera sensor?
OAG should be as close as possible to camera sensor, shortest distance that can achieve focus..
If you need to preserve 55mm backfocus then add spacer before OAG, not between OAG and camera.
And there is no need to rotate OAG, Lodestar is very sensitive, you'll probably never have to chase guiding star.
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Old 26-08-2013, 08:46 PM
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Originally Posted by orion69 View Post
Why is your OAG 45mm from camera sensor?
OAG should be as close as possible to camera sensor, shortest distance that can achieve focus..
If you need to preserve 55mm backfocus then add spacer before OAG, not between OAG and camera.
And there is no need to rotate OAG, Lodestar is very sensitive, you'll probably never have to chase guiding star.
Thanks - that's great advice... but the OAG is directly screwed to the filter wheel, which is directly screwed to the camera. I am as close as I can get, I don't want to guide through narrow band filters....
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Old 26-08-2013, 09:05 PM
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I see, in that case you have no choice.
You'll probably need spacer between OAG guiding camera holder and Lodestar...
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Old 26-08-2013, 09:31 PM
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Quote:
Originally Posted by orion69 View Post
You'll probably need spacer between OAG guiding camera holder and Lodestar...
I picked up several spacers/adaptors for several contingencies just in case, so there I should be OK...
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Old 26-08-2013, 09:37 PM
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Bought myself the new Orion thin OAG direcly through a friend in the USA. Hopefully, it is thin enough for the Mak Newt I will be using it on - it's 11mm compared to TS's 9mm. SHOULD be OK, especially once I have the Moonlite on it.

The Orion Thin OAG came in at $160 all up, including post to here, so pretty happy with that. Comes with a gullet of adapters too.
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Old 26-08-2013, 09:59 PM
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It's the coma corrector that makes spacing tough with mine.... Even after adding the OAG and filter wheel, there is about 15mm of focuser travel left, thanks to the Moonlite....
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Old 27-08-2013, 10:34 PM
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Gee this lodestar is a nice piece of kit.
I was ecstatic to see a defocused star or two when I first fired her up, a push and pull here and there on the OAG stalk, and focused nicely....
Needless to say no difficulty finding stars.... I was just testing about in Sagittarius tonight.
and... Flexure.???.... GONE!
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Old 28-08-2013, 06:34 AM
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great news Lee
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Old 28-08-2013, 07:39 AM
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Quote:
Originally Posted by Lee View Post
Gee this lodestar is a nice piece of kit.
I was ecstatic to see a defocused star or two when I first fired her up, a push and pull here and there on the OAG stalk, and focused nicely....
Needless to say no difficulty finding stars.... I was just testing about in Sagittarius tonight.
and... Flexure.???.... GONE!
Yeah I love mine too. So sensitive. I even find stars where I thought there would be none.
If you bin it 4x4 and reduce the tracking box size to 16x16 even better.
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Old 28-08-2013, 07:48 AM
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It's so sensitive, it can see through clouds.... Last night as I was testing clouds rolled through, quite thick (astronomically) - the imaging cam was seeing nothing, but the lodestar could still see its star.... it wasn't a hot pixel either (I got PHD to calibrate just to make sure!)....

Last edited by Lee; 28-08-2013 at 09:24 AM.
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