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Seeking visual-back suggestions please to help me get correct back focus for planetary on Celestron 8 Edge HD


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Does anyone have any recommendations for a short SCT/1.25mm visual back please?

While Jupiter’s favourable I fancied trying my hand at planetary on my 8 Edge HD.

Problem is my image train is about 30mm greater than Celestron’s recommended back focus of 133.35mm.

The only element I think I can change is the visual back which accounts for nearly 60mm.

My train: ASI 224MC > ZWO ADC > Celestron X-Cel LX 3x Barlow > Celestron Edge 1.25mm visual back

TIA 

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I have never bothered my head about "back focus".  I use a CPC800 + Celestron prism diagonal + ZWO manual 5-filter wheel + ADC + ASI462MC for planetary imaging. 

The diagonal is for convenience, and for preventing the image train colliding with the bottom of the forks.  It also gives the right spacing when I use a f6.3 reducer for deep space shots.

A 3x Barlow on a scope with a 2000mm focal length is probably overdoing it.

Edited by Cosmic Geoff
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1 hour ago, Cosmic Geoff said:

I have never bothered my head about "back focus".  I use a CPC800 + Celestron prism diagonal + ZWO manual 5-filter wheel + ADC + ASI462MC for planetary imaging. 

Thanks Geoff

I’d always assumed/read that the camera’s sensor should always be as close to the recommended back focus as practical. Does it not make any difference then?

You’re right about the 3x Barlow, it was a mistake I made a couple of years back but unfortunately I haven’t the funds to buy a replacement any time soon.

My usual set up is for DSOs with the 0.7 reducer, chasing planets is pretty new to me and I’m at the bottom of the learning cliff 😉

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1 hour ago, Ade Turner said:

I’d always assumed/read that the camera’s sensor should always be as close to the recommended back focus as practical. Does it not make any difference then?

Perhaps it makes more difference with the Edge version.  But whether you will see any difference if you deviate by a few cm is the question.

Celestron's quoting the back focus to 0.01mm is clearly idiotic, to my mind.

You can unscrew the top and bottom of the ADC which may allow you to reduce the spacing.

I have yet to demonstrate that using a Barlow improves the planetary imaging results at all.

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Agreed, correct back focus is important to minimise aberrations right out to the corners of a large sensor, but when I use an ASI 178 camera (sensor dimensions 7.4 x 5 mm) on an Esprit 150 I don't worry about precise back focus.

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