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Old 18-02-2011, 10:58 AM
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pmrid (Peter)
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Guiding at Long Focal Lengths

Celestron have yet to come out with a reducer for the HD range of scopes so imaging options are very limited - either HyperStar at F1.9 or native at F11. Guiding at F11 (which equates to a FL just a whisker under 4 metres) is a real challenge. I've tried SBS and Piggy-Back guiding but neither really gives me the stability I want.
So I am about to go back to using an OAG. Added to that, I have sourced a non-Celestron reducer (Optec "Lepus" .63) that is said to be designed for highly corrected scopes including the HD line. But I'm a bit up in the air about how to combine the reducer and an OAG.
Reducers have very specific requirements governing the distance from reducer to CCD imaging plane and the Optec Lepus is certainly no exception - so much so that you have to buy a camera-specific adapter with the reducer. Similarly, the manufacturer says, you have to position the reducer as close as possible to the back of the OTA and the corrective optics that sit inside the baffle tube of the HD scopes. That's where my dilemma comes in.
If I position the OAG in front of the reducer, the guide camera has to work at F11. I can't position it behind the reducer without screwing up the distance to CCD measurement.
Here is a rough graphic of the setup as I see it at the moment.
Can you guide at F11. I'm imagining the image scale on a QHY5 at F11 and, to say the least, it would be minute. So finding a guidestar would be awfully hit-and-miss. I'm assuming I can overcome differential focus issues by using adapters on the QHY5 and that I can get the QHY8 into focus without screwing up my guiding focus too much.
But can you guide at a FL of 4 metres? I have my doubts so I'm hoping either for reassurance or the sound of distant laughter.
Peter
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Old 18-02-2011, 11:22 AM
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multiweb (Marc)
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Yes you can guide with an OAG and a QHY5 on a C11. Bit hard to find a guide star at times but it can be done. A lodestar is best in this case as it's more sensitive. Focus the primary camera first then move your guider up and down to focus the guide star. Problem on the C11 is the field curvature which makes stars on the edges a bit chonky and dimmer.
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Old 18-02-2011, 01:12 PM
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DavidTrap (David)
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I'm going to give the Astro-Physics CCDT67 a go. It's meant to be just a reducer, so appropriate for corrected SCTs. You can vary the reduction and illuminated circle size by varying the distance. Good info on AP website. I'm ordering a custom adapter from Precise Parts. The illuminated circle should be ~33mm.

Will let you know how I go with OAG.

DT
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Old 18-02-2011, 03:03 PM
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Octane (Humayun)
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Peter,

You might need to talk to Frank (spearo). He guides his C14 on his G-11 and gets pinpoint round stars every time. He is using the STL-11000M, though, with self-guiding capability.

Still, I think it can be done.

H
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Old 18-02-2011, 08:18 PM
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pmrid (Peter)
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Thanks guys. Advice and suggestions all welcome and appreciated.

As a matter of interest, I set up the optical train as per the diagram I posted earlier and ran a couple of daylight test shots. Awful.

I attach 3 images - one is very out-of-focus and the next is more in-focus showing the serious image distortions going on with the Lepus reducer. The third image shows the optical train I was using - and you'll note that the OAG and the Celestron SCT adapter have both pushed the reducer well back from the back of the scope. It may be that this is the cause. IUf it is, then I have to look at a way to put the OAG behind the reducer but to do this I'll have to have a special-purpose adapter made in order to maintain the 100mm distance between reducer and CCD.
Peter
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Old 18-02-2011, 08:59 PM
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DavidTrap (David)
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I agree that there certainly some serious distortion. Is it just me, or does the imaging train look like it is "hanging" in that picture - doesn't look square to the back of the scope.

My understanding of the Edge HD scope is that there is a lens in the barrel of the scope where the attachments screw onto the back. If this is the case, wouldn't the Lepus reducer need to be the "correct" distance from that lens too? Try screwing the reducer directly to the back of the scope, without the OAG, and see if the distortion goes? I that works, you're up for some adaptors from Precision Parts...

DT
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Old 18-02-2011, 09:05 PM
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Reducers have a specified distance from the metal most edge fo the reducer to the chip of the camera - its called the metal back distance.

Depending on the reducer this will have a specified tolerance - for example a Tak reducer may have a metal back distance of 106.7mm +/-6mm. So you have to be within 6mm of 106.7 for it to work.

They also have a corrected image circle which has to be larger than the size of the chip you are imaging with.

I use a MMOAG from Astrodon with a Planewave CDK17 at 2950mm focal length and I use an ST402ME as a guide camera. I have also used a Lodestar which is OK but mine puts a white horizontal line in the image which can mess up the auto select for a guidestar function of CCDSoft and also can mess up calibrating the autoguider (the software picks the bright line instead of the star). The ST402 of course is cooled and is not that big and works quite well and is quite sensitive.

I don't think you could put the reducer after the OAG as the chip and guide camera need to be in focus together at the same time. Perhaps you could with the right spacer for the guide camera otherwise you may not be able to achieve focus with your guide camera whilst the imaging camera is in focus.

My Planewave has a corrector before the MMOAG and camera. The AP reducer is the go. I wouldn't try these other more dodgy brands. You'll more than likely find out they don't do the job properly.

The AP67 reducer is a well known and respected reducer that isn't too expensive (by AP standards anyway).

Not 100% sure if a reducer would work with an Edge HD (best to get advice from those who use them) as it already has a flattener and flatteners and reducers don't normally work together.

Greg.
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