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Back focus on SCTs?


astolpho
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Hey guys quick question,

I own a Celestron EdgeHD 8" OTA and I have a question on the back focus for AP.

Originally the back focus is set at 133mm (5.25 in). Adding the focal reducer brings it to 105mm (4.133 in).

I'm setting up a OAG with the system and my distance is basically 1.5mm more than the 133 or 105 distances.

Does that affect it at all or is there a "margin of error" on that?

I'm using the EdgeHD T Adapter and a thin off axis guider ring (from Orion) for my system. The OAG is identical to a T-Ring for a camera except that is normally at M48 threads on the scope side and adding the step down ring to a T-Thread adds the 1.5mm.

Is the EdgeHD T Adapter longer than the regular one for a C8? If yes, could maybe getting the regular one and adding t thread spacers to get to 105 or 133 work?

Thanks in advance everyone!

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Hey guys,

So after checking some diagrams, the distance is 0.5mm not 1.5mm.

Does that affect the back focus distance on a SCT? Does it have to be EXACTLY at the 133 or 105mm with the reducer or that 0.5mm is ok to work with?

Thank you!

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I don't really understand "back focus" except it's supposed to be the plane where the image is in focus, but I have used CCD's on my CPC1100 Edge without problem.  The thing is that I find with f10, or even f6.3  the focus range is very broad, and also the focus adjustment on the Celestron OTA is enormous, so the odd few mm either way are no problem.  Don't know if this helps but it's what I found.  I have also used an optical back with a rack and pinion focuser to avoid all the weight hanging off the diagonal - again there was no problem achieving focus.

Chris

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According to the Celestron EdgeHD white paper, the backfocus is critical and needs to be +/-0.5mm of the stated backfocus in order to preserve field flatness. So you are within tolerance, though just barely. You should be fine.

The Celestron OAG can certainly be used with the EdgeHD scopes as long as the correct backfocus spacing can be achieved. The 8" scope is a bit of an oddball in that the use of the reducer actually changes the backfocus (reduces it as you pointed out). For the other EdgeHD scopes, the reducer does not change the backfocus. I use the Celestron OAG on my EdgeHD 1400 and it works great.

Tim

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Tim,

I think the issue is the use of a DSLR rather than a CCD camera.....

The only EdgeHD arrangement for which the Celestron OAG cannot be used is the EdgeHD 800 with a reducer and a DSLR. As I mentioned, the 800 with the reducer has a shorter backfocus than the other EdgeHD scopes. With a standard t-adapter, a DSLR's focal plane is 55mm back from the face of the adapter. The Celestron OAG SCT adapter consumes 25.3mm of backfocus,  the guider body consumes 29mm, the M42 adapter consumes 12.5mm. That's a total of 121.8mm of the available 133mm, so there's plenty of room. All of the other EdgeHD scopes have 146mm of backfocus with or without the reducer, leaving lots of spare backfocus.

It's true that the original poster cannot use the Celestron OAG with his reducer, but he's using the Orion TOAG which will work fine.

Tim

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  • 1 year later...

Yes it is true that we can't use Celestron off-axis guider together with 0.7x focal reducer. But I think that is only if we depend totally on adapters provided with OAG. I think we can save a lot of length by using a shorter SCT to OAG (reducer to OAG in this case) adapter. You can get it done from preciseparts.com (quite expensive though). The adapter provided with OAG has 25mm effective lenght. But if you get an adapter of 8mm from preciseparts then the total length of optical path after the reducer is as follows. 8mm (for reducer to OAG adapter) + 29mm (for OAG body) + 12.5mm (for M42 adapter) + 55mm (for T ring and DSLR) equals 104.5mm. So it is within the acceptable range. I have already ordered the adapter. It is still in transit. I will update as soon as I get a chance to try it. 

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  • 11 months later...
On 6.6.2016 at 11:34, Cmahesh said:

Yes it is true that we can't use Celestron off-axis guider together with 0.7x focal reducer. But I think that is only if we depend totally on adapters provided with OAG. I think we can save a lot of length by using a shorter SCT to OAG (reducer to OAG in this case) adapter. You can get it done from preciseparts.com (quite expensive though). The adapter provided with OAG has 25mm effective lenght. But if you get an adapter of 8mm from preciseparts then the total length of optical path after the reducer is as follows. 8mm (for reducer to OAG adapter) + 29mm (for OAG body) + 12.5mm (for M42 adapter) + 55mm (for T ring and DSLR) equals 104.5mm. So it is within the acceptable range. I have already ordered the adapter. It is still in transit. I will update as soon as I get a chance to try it. 

How did this work out for you? I am considering the same solution and I would really appreciate som feedback before ordering a $150 custom machined adapter :-)

Edited by grukx
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