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Been looking at qhy 183c and using star diagonal why not?

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#1 robodan


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Posted 24 February 2021 - 09:55 PM

I know most use a camera without star diagonal which is fine with a GEM or Wedge, but limits the angles an alt alz SCT can go before hitting mount.

A star diagonal gets around this issue, and use the diagonal when planetary imaging, 1 can swap out eyepiece centre planet in centre of eyepiece field then slip in the camera, so another advantage if using diagonal.

Been just looking at the QHY 183C not for astrophotography but EAA and using 1.25 eyepiece adapter and using in diagonal.

So how do I go about backfocus and what would it be with a 0.5x reducer screwed into eyepiece adapter and back focus on a 9.25 evo SCT, non HD version, or does back focus not so much a concern with EAA, cheaper cameras its never mentioned like cheap planetary cameras, etc, get into more expensive CCD cooling cameras its suddenly an issue, yet does not the SCT have the most focus available of just about anything?

This may seem stupid, but the diagonal on these 1 arm bandits gives full travel of mount

Edited by robodan, 24 February 2021 - 09:55 PM.

#2 bips3453


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Posted 24 February 2021 - 10:41 PM

Things would be complicated when using a reducer. A reducer would need an exact amount of back-focus. Swapping eye-piece with camera would need you to do precise calculation for each and make sure the distance is very close to perfect. 

Also, diagonal would mean introduction of additional surface in the optical train. 

It's a trade-off between convenience vs quality. 

#3 robodan


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Posted 24 February 2021 - 11:06 PM

Thanks to much a hassle and inconvenience, think will stick by getting a QHY5III462 for EAA seems the cheaper cameras not a big concern.

Edited by robodan, 24 February 2021 - 11:08 PM.

#4 dcweaver


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Posted 25 February 2021 - 12:13 AM

RBD - The thing I would pay attention to with a diagonal reducer combo is whether you have enough "infocus".  A reducer moves the focal point inward toward the secondary mirror.  


When I use my 0.5x GSO reducer, I also use a spacer to set a specific distance between the reducer and the camera sensor.  A 1/4 inch reducer gives me less than 0.5x reduction.  A 1/2 inch spacer gives me the design 0.5x reduction.  A 3/4 inch spacer gives me more than 0.5x reduction.  The bigger the spacer, the more reduction I get.  However, as the spacers get bigger, I have to dial the focus further inward.  With a 1 inch spacer I nearly run out of focus travel.


A diagonal would be similar to having a large spacer, and you might not be able to reach focus.  You would have to try it to know for sure.  An un-cooled camera is much smaller, and might give you good EAA results while avoiding collisions with the mount base.  A Starizona Night Owl reducer might also help with the collision problem.  It inserts directly into a 2 inch visual back.


Here is some more detail of how I use the GSO reducer


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#5 roelb


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Posted 25 February 2021 - 05:17 PM

Using a diagonal on the SCT will not pose a problem.

I use it when attaching the ASI533MC-Pro on the Evolution 8" to avoid "OTA-mount" collision.

The 0.5 reducer at 50 mm from the sensor will give a little bit lower focal reduction (0.44) compared to the "prime focus" assembly.

Which can be corrected by less spacing between FR and sensor.

Only difference is the longer focusing distance which will pose also no problem.

Using a "mirror diagonal" will not degrade the "EAA image quality".

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#6 Yanlisman


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Posted 26 February 2021 - 04:53 AM

I'm on a 8SE with a Celstron, 0.63 reducer and rail extension and use the diagonal to mount my Cannon 550D for viewing via a standard 1.25 adapter. Not perfect but acceptable for a quick fix setup. No issues with getting focus.

I find that if you rotate both the diagonal through 90 degrees clockwise and then the camera through 90 degrees anti-clock the angled "slope" on the diagonal then passes the mount and the view remains in the same relative orientation and I can get to the Zenith without collision.

#7 cpl43uk


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Posted 27 February 2021 - 11:55 AM

Am using an ASI533 in the Baader 2" Diagonal attached to my Nexstar 8SE Alt Az. It certainly has more vignetting verses the straight through option (I have also used) and the image seems "softer" but very acceptable. I do notice stray starlight from bright stars just out of shot more with the diagonal vs the straight through. Not sure why. However it gives mo more room in the sky for a view and since I still set up my scope visually then it is slightly easier just to swap in and out the camera.

Edited by cpl43uk, 27 February 2021 - 11:56 AM.

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