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Old 11-03-2013, 10:18 PM
Nico (John)
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St-8300 oag

G'day all,

I recently purchased the ST-8300 OAG to help improve guiding, but I'm having some difficulty trying to set the thing up. Guide camera is a Starshoot autoguider and while it works fine with the 80mm guide scope I think it may not be sensitive enough for the OAG. There is absolutely no image at all so I can't adjust the prism for optimum. Prism is setup for a F10 system while my 9.25 is F6.3 so I know there is some fine adjustment required but I would have thought that even at F10 I would be able to see an image.

I'd like some advice on how best to set the system up. I'm guessing this is best done during daylight but not sure how to go about it.

Cheers

John
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Old 12-03-2013, 05:06 AM
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naskies (Dave)
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I have the OAG for the STF-8300M (I believe it's identical to the ST-8300 OAG) and ST-i guide camera, as well as an Orion mini guide scope + QHY5 (virtually identical hardware to the Orion SSAG).

Your problem may be that your SSAG is too far back from the focus plane in the OAG?

When I try to use the ST-i on my Orion guide scope, it needs a 1.25" extension tube to focus... but the QHY5 focuses fine with its nose piece.

I think this means that you can't get inwards focus travel with the OAG + SSAG combination? With the ST-i, the imaging chip actually sits *inside* the OAG...
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Old 12-03-2013, 08:55 AM
Nico (John)
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Focus seems fine during daylight at distant target. I must have spent several hours last night trying different things but nothing seems to work. About a week ago when the moon was up I was able to use an eyepiece to focus but the weird thing was that I had to move the OTA considerably to image the same target.
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Old 12-03-2013, 02:54 PM
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Meru (Michael)
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Hey John,

First off congrats on going down this OAG path I too used to use the SSAG with the 80mm scope, and eventually I ended up on the OAG route too. It can be a little tricky getting the distances and camera correctly setup but it is well worth it! Remember that the guidecam has to be at *exactly* same distance from the prism as the imaging camera to the prism. What is really means is that when the main camera is in focus, the guidecam should be too. Don’t use eyepieces to focus, it's always easier (and better) to use both cameras at once.

By far the easiest thing to do is to use a very, very bright star or preferably something larger (like the moon or even terrestrial objects). Reason being that because you are 'picking' off light from the edge, what your guidecam sees might be different to what the main imaging cam sees. Sometimes when I point my RC8 at a bright star, I cant see it at all in my guidecam! So first get your main camera into focus then lock the focuser into place. You can now forget about them both and not worry about having to touch them again. Then, move the guidecam back and forth and watch the image. If it comes into focus at any stage, then that’s it, your done! If it doesn’t, then you need to work out if the guidecam is too far or too close to the prism.

If it starts to become clearer as you move *away* from the OAG, then you are too close. This is very easy to correct, you just need some sort of extension tube to move the camera further away. The exact extension tube depends on how you are connecting the guidecam to the OAG. If however you find that the image starts to become sharper as you move the guidecamera *into* the OAG, then you're too far from the prism. This is the most troublesome spot to be in, because it means you have to find a way to get the guidecam closer to the prism which can be a pain. There are two ways to solve this:

1. You can try moving the prism closer to the edge of the OAG. This brings the prism closer to the guidecam, but does not change the distance to the imaging camera. Problem with this is that as you get to the edge of the OAG, the sharpness and brightness of light coming deteriorates, and unless you have a super sensitive camera, it'll be difficult to see anything at all.

2. Move the main imaging camera back. This means the distance from the prism to the imaging camera increases, meaning you have to push your guidecam further away from the prism. How much you will need to, is something you need to work out. Problem with this is that the further you move away from the focuser, the more chance of 'slop' in the focuser (the drawtube moving up and down).

It seems like a lot of work but if you can get your head around why we are doing all this, then it's really very easy to reach focus However I'd just like to say that if you are using a SSAG at f/10, I can almost guarantee you will have a very hard time finding any guide stars. Don’t be surprised if you can reach focus whilst looking a bright object, and then when you slew to anything else it's too dim to select a star. Hope this helps, and be sure to let us know how you go!

Meru
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Old 12-03-2013, 09:43 PM
Nico (John)
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Thanks Meru for detailed explanation. Seems like SSAG is not a match for OAG, simliar to Z match I guess. I originally thought that the oag prism or optics could have been out so I used a laser 1.25 in to project back through. Pretty rough test but at leas gave some confidence that optics is not the issue. I think that distance to focal plane is the issue something that I cannot really change a great deal with SSAG.
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Old 12-03-2013, 10:40 PM
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naskies (Dave)
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The OAG-8300 has a pretty short optical path for the guide camera since it's designed to fit the ST(F)-8300M/C cameras directly behind it, and also has a 0.7x reducer built-in.

If you have enough back focus with your scope/camera set up, you could potentially add a spacer between the OAG and your *main* camera so that there's enough room to bring your SSAG into focus.

However, since the SSAG is not very sensitive as a guide camera, it may be a futile exercise...
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Old 13-03-2013, 05:51 PM
Nico (John)
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Agree, SSAG NBG. Decided to just order the ST-i since this camera is matched for the oag and is on special. Has a six week wait but no problems, I'll just remove the oag and use 80mm guide scope. I'm also working on a dual focus controller using pic18f4550 through usb and couple of spepper motor translatters. God has scheduled rain this w/e so nothing else to do..

John
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