Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

ASI 120 MC-S Back focus

  • Please log in to reply
3 replies to this topic

#1 BobW55

BobW55

    Ranger 4

  • *****
  • topic starter
  • Posts: 338
  • Joined: 06 Jun 2018
  • Loc: Melvin, Michigan (Sanilac County)

Posted 23 February 2020 - 02:33 PM

Starting to play with planetary imaging.

Most ZWO/ASI cameras require a back focus of 55mm.  (I understand what this is)

I plan on using this on my C8 with a 6.3 FR.  and a 2X Barlow.

I am not sure how the Barlow works into the total back focus distance.

 

I am assuming that I need to remove the Diagonal.  

 

Would my set up be:   Scope -->Focal Reducer-->Visual Back-->Barlow-->Camera?

I would think I need some spacing in there somewhere.

 

ZWO has a page about Back focus https://astronomy-im...tions-55mm.html



#2 sg6

sg6

    Fly Me to the Moon

  • *****
  • Posts: 7,237
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 23 February 2020 - 03:11 PM

Starting to play with planetary imaging.

Most ZWO/ASI cameras require a back focus of 55mm.  (I understand what this is)

I plan on using this on my C8 with a 6.3 FR.  and a 2X Barlow.

I am not sure how the Barlow works into the total back focus distance.

 

I am assuming that I need to remove the Diagonal.  

 

Would my set up be:   Scope -->Focal Reducer-->Visual Back-->Barlow-->Camera?

I would think I need some spacing in there somewhere.

 

ZWO has a page about Back focus https://astronomy-im...tions-55mm.html

No, the camera does not have a back focus, the camera requires that the image falls on the sensor and that is all.

That image is usually defined as a distance from some machined face, and the distance from that face to the focal plane is the back focus.

 

Your FR probably has a back focus specified and being for an SCT I do not expect it to be 55mm - what does the specification of the FR state???

 

If you add a barlow in then all bets are off.

Firstly the focal plane will be somewhere undefined and secondly it will likely no longer be flat.

 

The really big question is why use a FR to reduce the image size, then add a barlow in to increase the image size?

Just seems utterly pointless, and expensive.

 

So I would expect an FR and a barlow to result in a non-flat non-reduced image on the sensor at some undefined distance. You are not the only one Slightly Confused.



#3 BobW55

BobW55

    Ranger 4

  • *****
  • topic starter
  • Posts: 338
  • Joined: 06 Jun 2018
  • Loc: Melvin, Michigan (Sanilac County)

Posted 23 February 2020 - 03:35 PM

It is a Celestron FR.  Will have to check on BF for it.  Will have to try it without the FR.  (Guess I still have a DSO imaging mind set)



#4 klaussius

klaussius

    Ranger 4

  • *****
  • Posts: 357
  • Joined: 02 Apr 2019
  • Loc: Buenos Aires

Posted 23 February 2020 - 05:35 PM

I'm not sure you need the barlow. The C8 with the ASI120 alone (no FR, no barlow) is already diffraction-limited.[1]

 

I'm just getting into planetary so I may be wrong, but I think you won't get any extra detail with the barlow, and may in fact lose some (more optical elements = more aberrations).

 

[1] Dawes limit for an 8" apperture: 0.57", image scale w/o barlow 0.38"


Edited by klaussius, 23 February 2020 - 05:39 PM.



CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics