actual backfocus Nexstar 6se asi244mc

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Watcher
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actual backfocus Nexstar 6se asi244mc

Post by Watcher » Sun Jul 19, 2020 5:27 am

I am new to both SharpCap and ZWO cameras
I am not new to astronomy, natural photography and DSLR astrophotography.
I have a single question for my first post on this forum.
1. What is the actual back focus for my system? I have a value I think is too long- about 90mm. Also what is a good workable tolerance for the actual
value?

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Re: actual backfocus Nexstar 6se asi244mc

Post by admin » Sun Jul 19, 2020 9:43 am

Hi,

I'm not sure that there is a single answer to that question. However I think the important value is that the sensor in that camera is 12.5 mm behind the front face of the camera. If you are used to DSLR imaging then you will need to push the camera back further than usual with extra extensions as the distance from the front face of the DSLR to the sensor is typically 40 to 50 mm (exact value depends on brand of camera).

Hope this helps, Robin

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Re: actual backfocus Nexstar 6se asi244mc

Post by turfpit » Sun Jul 19, 2020 11:39 am

Watcher

You might be overthinking this. In my experience, SCT's seem to be very forgiving as regards back focus. This is what I would try first, YMMV:
  • put the supplied visual back on the scope
  • use the supplied eyepiece to focus on a distant (over half a mile away) land based object
  • swap the eyepiece for the camera with its supplied nosepiece
  • adjust focus, focus should be very near to start with
If this works, job done.

If the 6SE is mounted on a single arm then the last thing you want is to be adding extension tubes as this will restrict the scope elevation.

I used to own a 6SE (never had any trouble achieving focus) and currently have a C8 SCT. I use the C8 with an Altair 183C - I use the scheme above and focus is never an issue.

Some links found from a cursory internet search.

https://www.celestron.com/blogs/knowled ... stron-scts

https://bbs.astronomy-imaging-camera.co ... 21&t=10234

When using refractors, the back focus is less forgiving and usually dependent on the specs of the reducer/flattener used. For example, I have an Altair 1.0x flattener which has a back focus (specified) of 55mm. If I don't get a distance of ~55mm from back of the flattener to the face of the camera sensor then I cannot achieve focus on either a Vixen or Altair refractor. This is easy with a Canon DSLR as the T to EOS ring (11mm) + sensor being set back in the camera body by 44mm conveniently makes 55mm. If I am using the Altair 183C camera then I have to make up a length of 42.5mm using tubes + the 12.5mm the sensor is behind the face of the camera (42.5 + 12.5 = 55mm).

Celestron make an SCT to T adapter which cost around £20 and is 50mm long.

Hope this helps.

Dave

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Re: actual backfocus Nexstar 6se asi244mc

Post by BlackWikkett » Sun Jul 19, 2020 2:34 pm

Hi Watcher

From Celestron's site it's 5 inches or 127 mm. This means to achieve focus the imaging plain of the camera (sensor) must be at this distance to achieve focus. The ZWO ASI224MC has 12.5 mm of back focus listed (ss Robin described the distance from the sensor to the camera mounting thread.) Therefore 127 - 12.5 = 114.5 mm from back of scope (visual back) to the camera and you should be close to focus.

https://www.celestron.com/blogs/knowled ... stron-scts

Hope this helps
-Chris

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Re: actual backfocus Nexstar 6se asi244mc

Post by Watcher » Mon Jul 20, 2020 9:43 pm

Thank you for your replies,
Initially your answers were very confusing until I realized you all were calling from a different viewpoint. Some of your answers give measurements that are wildly different, but they are being measured from different places, so the back focus appears to be different. The actual back focus of any given telescope doesn't change but where we measure it does. And to you who though I might be over thinking the situation, you might be right. I am a retired mechanical engineer and I require precise solutions to problems. (Until proven otherwise.)

Anyways, I have devised a solution to this problem that I will post under a new heading in a day or so. I hope to take much of the confusion out of this problem.
I thank you all again, you have pointed me in the right direction.

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