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

Focal Reducer for Sky-Watcher Evostar ED80 Pro

  • Please log in to reply
2 replies to this topic

#1 Oscar Szentirmai

Oscar Szentirmai

    Ranger 4

  • -----
  • topic starter
  • Posts: 324
  • Joined: 28 Dec 2006
  • Loc: London - UK

Posted 12 January 2020 - 07:39 PM

Evening everyone

 

The ED80 has a F/7.5. I gather that the matched reducer made by SW is a 0.85x, taking down the focal ratio to f/6.375. The SW option is pretty expensive.

 

There has been a popular cheaper choice in the ED80 community, specifically the Altair Astro Lightwave 0.8x reducer. It gives the scope a ratio of f/6. Unfortunately it seems to be sold out everywhere.

 

Why is it not possible to use a stronger reducer, like the Altair Astro Lightwave 0.6x which would give a ratio of f/4.5? That one is readily available, and on checking specs, it looks to be physically identical to the 0.8x reducer?

 

I must be missing something here? Will it just distort the image? Physics involved?

 

Is there a rule that I am not aware of, or some kind of equation that limits how much an f ratio can be lowered using a reducer?

 

Many thanks



#2 aa5te

aa5te

    Genial Procrastinator

  • *****
  • Posts: 1,901
  • Joined: 30 Aug 2008
  • Loc: Clinton, TN

Posted 12 January 2020 - 08:37 PM

The 0.6x reducer will work, but it will produce a smaller flat field than the 0.8x will. Here's an image (not mine) taken with the ED80 and the same (different label) reducer; make sure you read his description:

https://www.astrobin...300011/?nc=user

You'll get somewhere around an 11-14mm flat field on your sensor; there's a somewhat useful table here that you can extrapolate from:

https://primalucelab...refractors.html

I tried one of these (the same one you mention) on my AT66ED (400mm, f/6), and it was an absolute mess.

Here's a short exposure with the Orion 0.8x thru my AT66ED:

Attached Thumbnails

  • DSC07251.jpg

Edited by aa5te, 12 January 2020 - 08:40 PM.


#3 aa5te

aa5te

    Genial Procrastinator

  • *****
  • Posts: 1,901
  • Joined: 30 Aug 2008
  • Loc: Clinton, TN

Posted 12 January 2020 - 08:38 PM

And a shot with the 0.6x:

Attached Thumbnails

  • DSC07260.jpg



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