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

Backfocus on Explore Scientific ES-ED102

Refractor CMOS Equipment
  • Please log in to reply
5 replies to this topic

#1 GeoAnn

GeoAnn

    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 19 May 2020

Posted 25 July 2021 - 09:20 AM

I'm hoping someone can help me understand why my (new) Explore Scientific Essential Series ED102 is requiring so much Backfocus.  To achieve focus I must add an additional 3-plus inches.  That's almost double the expected amount.  To be clear I am not suggesting that the telescope has a problem.  I would simply like to understand why it is so far off the manufacturer default.  For reference I only use the telescope for imaging.  My camera is the ZWO ASI533mc Pro.  I do realize the stock focuser is rather short, which I assume is the reason the manufacturer includes two additional 2-inch spacers.  I've experimented with many configurations using the included spacers.



#2 barbarosa

barbarosa

    Gemini

  • *****
  • Posts: 3,179
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 25 July 2021 - 10:00 AM

Refractors have a point of focus, in this case the focal plane is 714mm from the objective. Scopes designed for general use normally provide for the optical path length of a star diagonal. If you remove the diagonal you may have to insert an extension tube to add back the lost optical path length.

 

The other factor is that the designer may intend to keep the focus tube a short as possible, to reduce the load on the focuser or reduce sag or maybe just to save a penny on materials, 


Edited by barbarosa, 25 July 2021 - 10:00 AM.


#3 GeoAnn

GeoAnn

    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 19 May 2020

Posted 25 July 2021 - 03:18 PM

Thank you Gemini.  That does make sense in that the primary design consideration was for visual use. And you are correct this telescope has the shortest focus tube I have ever used.   I just wasn't expecting to add so much more Backfocus spacing than what the manufacturer had anticipated. 



#4 PatrickVt

PatrickVt

    Viking 1

  • ****-
  • Posts: 912
  • Joined: 20 Apr 2018
  • Loc: Vermont, US

Posted 26 July 2021 - 09:00 PM

For simpler, trouble-free use with binoviewers and/or for adding other optical accessories that eat up space in the optical path.  It is far easier to add a bit of length to a tube than it is to shorten the tube.  

 

On the positive side, ES provides threaded extension tubes for this particular telescope that fit the focuser perfectly.

Patrick



#5 compoundw

compoundw

    Lift Off

  • -----
  • Posts: 10
  • Joined: 09 Nov 2020

Posted 01 September 2021 - 02:45 PM

GeoAnn, how long an extension to you end up with to get back focus with your ZWO camera?



#6 GeoAnn

GeoAnn

    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 19 May 2020

Posted 04 September 2021 - 09:18 AM

compoundw, first I apologize for such a delayed reply, sometimes life gets in the way doesn't it!  
  
To achieve focus with my ASI533mc-Pro I had to purchase an additional Explore Scientific 2" extension.  The focuser was at approximately it's index mark of 45.  I suppose I could have added another 1" extension to allow the focuser to move in some since that is almost the full length.




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





Also tagged with one or more of these keywords: Refractor, CMOS, Equipment



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics