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Old 01-09-2014, 08:26 PM
Nico (John)
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Auto guiding question

Hi all,

Im having some problem trying to get a decent image through my oag. Currently running st8300, 8pos filter wheel, oag and st-i through C 9.25 with 6.3 focal reducer. I can focus and image through the main but I cant seem to get an image through the st-i. I tried 1 sec exposures to move the st-i back and forth but no image. Is 1 sec long enough or should I be looking at exposure times to get an image?

John
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Old 01-09-2014, 08:48 PM
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Amaranthus (Barry)
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I strongly suspect it is not in focus. Try various distances for the pick-off prism to ST-I chip, and point to an open star cluster to get it working (lots of stars guaranteed to choose from). First focus of an OAG, especially it doesn't have a helical focuser, is the hardest part.
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Old 01-09-2014, 09:09 PM
Nico (John)
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I agree, but I cant seem to get nice looking guide stars, they actually look like a quarter moon and not round. The camera is more than likely not on axis, but yep Ill try on star cluster. BTW what exposure time would you for the sti for use for say mag12 object like a gallaxy. By the way Im going 20sec is not long enough. Maybe I need to check the prism again, but by what Ive read this is not overly critical.
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Old 01-09-2014, 09:25 PM
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Amaranthus (Barry)
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You won't be guiding on the galaxy, you'll be guiding of a nearby star in the FOV. With my ASI120MM I take 100-500ms exposures, which in PHD2 are then stacked as an average of 5-10 frames.

Guide stars will be somewhat misshapen if you are using a focal reducer, because they will be capturing light away from the ideal focal plane. It should not be a major problem for guiding, however.
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Old 01-09-2014, 09:37 PM
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Octane (Humayun)
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Yeah, don't worry about the star shape; the algorithms work on centroids. In fact, it is recommended to guide on slightly out of focus stars. This likely improves sub-pixel guiding accuracy.

H
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Old 02-09-2014, 06:47 AM
Nico (John)
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Problem is I can't get anything usefull without increasing exposure length. I'm using Maxm but find that the image is very grainy compared to PHD. A little confused there I must admit. I have taken care with the prism offset, used a digital vernier, but Ill take advice and redo. At this stage there is no way I could image at .5sec because I just cannot see any guide stars.

john
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Old 02-09-2014, 08:42 AM
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Octane (Humayun)
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At 0.5 seconds, you'll be chasing the seeing. The minimum I'd guide at would be at least around 2.5 seconds.

H
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Old 02-09-2014, 11:14 AM
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Amaranthus (Barry)
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Remember to distinguish exposure time from guiding interventions. When I say 100 to 500 ms for an exposure, this is in the context of a 1 to 3 second guiding interval, and determines how many frames I stack with the ASI120MM (and how bright and well characterized the guide star is). Note, if you can't see any guide stars at 500 ms with your ST-i, then something is wrong - you should be seeing lots. I suspect, as noted above, that you are WAY out of focus.
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Old 03-09-2014, 04:22 PM
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Quote:
Originally Posted by Amaranthus View Post
Note, if you can't see any guide stars at 500 ms with your ST-i, then something is wrong - you should be seeing lots.
That does depend on your subject though... for instance I've been trying to capture NGC 55 lately and with 1s exposures and a QHY5L-II I can only see one star in all orientations of my OAG, with a 600mm OTA. I imagine if I was using my RC8 it's quite possible that I'd see none at all.

As for the OP, I agree, it sounds like you're out of focus. Try reaching focus on the moon or failing that, during the day; it'll be a lot easier. You may need more/less spacers between your guide cam and your OAG.
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Old 03-09-2014, 05:35 PM
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Use 3 second guide exposures or more. Star shape does not matter at all. Check the focus of your guide camera. Maxim will appear more grainy, don't worry about that either.
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Old 03-09-2014, 06:06 PM
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Merlin66 (Ken)
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John,
I usually use a Lodestar on my slit guiding (similar to an on axis guider ;-) ) with the C11 at f10.
For mag 9 stars I usually use around 1.5sec guiding exposures with PHD2 (or AstroArtV5), no issues...
I'm trialling the QHY5II L(mono) and find the same....
Hope this helps.
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Old 03-09-2014, 07:49 PM
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John,

It may also help you to take some dark frames - cannot remember if the Sti does this automatically?? etc - I use a Loadstar with an OAG and PHD2 and use 0.5 or 1 second exposures - from a dark sky with a 12.5" never have any issues getting a guide star - getting dark frames reduces the noise even further.

As the other guys have said, get the focus right - move the Sti up and down in the OAG barrel - does not matter if the guidestars look like seagulls either. If this is annoying you, then with most OAG's you should be able to rotate slightly the prism to reduce this affect.

Are you taken images from Melbourne or from a dark sky? Light pollution would also be an issue with fainter guide stars.

John K.
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Old 03-09-2014, 09:19 PM
Nico (John)
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I was hoping to roll back the roof tonight but the weather's been crap the last few days, way to much cloud and for what I need to accomplish would just be plain painfull.
But I'll take all the advice you all have provided and hopefully over the next day or so have some good results despite the light polluted skies!

As for adjusting the prism angle relative to the guider I don't think it is possible with the 8300 oag, the prism is adjustable in one direction only (back &forth), kind of glad really as I would have already played around it and stuffed it right up. The guider adjustment allows the camera to be moved in & out and also off centre as there is a bit of free play between camera and housing, this could part of my problem as well as focusing. Anyway let you know how it all goes, otherwise there should be a for sale add going up soon enough...

John
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Old 07-09-2014, 07:15 PM
Nico (John)
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Well I've had some success. I measured the distance of the prism from the attached assembly and found that it was 2-3 mm toward the centre than it should have been. It just took a few moments to re adjust. I would imagine that the only issue here would have been shadow in the main imager.

Initially there was no difference but then realised that the powered USB was off. Once turned on I had much better results despite the bright moon. I'm yet to do an imaging run but hopefully this will go well.

John
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