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 recuder and Field Flattener combination, is it possible?

  • Please log in to reply
8 replies to this topic

#1 JavierV

JavierV

    Sputnik

  • *****
  • topic starter
  • Posts: 42
  • Joined: 31 Oct 2012
  • Loc: Texas

Posted 15 September 2021 - 12:15 PM

Good day AstroFriends,

 

I have a 0.5 focal reducer and a Field Flattener and i would like to use them both on my AT66 refractor. 

 

Have anybody used such combinations?.  i'd like to know how to place each of them in respect to back focus.  

 

I figure, i should use the reducer first and then "correct it" with the flattener. 

 

My reducer is the TPO 0.5 2" thread on reducer and the FF is the GSO flattener. 

 

thanks for any tips,

clear skies. 

Javier. 



#2 Pictor

Pictor

    Explorer 1

  • -----
  • Posts: 50
  • Joined: 31 May 2020
  • Loc: Seattle, WA

Posted 15 September 2021 - 12:25 PM

Aren't most reduces also field flatteners?  You shouldn't need to double them up. 


  • Ken Sturrock and scadvice like this

#3 JavierV

JavierV

    Sputnik

  • *****
  • topic starter
  • Posts: 42
  • Joined: 31 Oct 2012
  • Loc: Texas

Posted 15 September 2021 - 12:49 PM

Aren't most reduces also field flatteners?  You shouldn't need to double them up. 

hello Pictor,

 

These are not,  FF is zero reduction. 



#4 Pictor

Pictor

    Explorer 1

  • -----
  • Posts: 50
  • Joined: 31 May 2020
  • Loc: Seattle, WA

Posted 15 September 2021 - 12:57 PM

Right, Field Flatteners allow you to image at the native Focal Length of your scope but gives you a flat field. 

 

Focal Reduces will reduce your Focal Length AND give you a flatter field.  Two different use cases.  They're not intended to be used together.  

 

On my Refractor, if I image and want the full 714mm FL then I just use a flattener.  If I want to image with shorter exposure times and a wider field then I use my Reducer (which is also a flattener) and I get an effective FL of 535mm on the same OTA. 


  • JavierV likes this

#5 JavierV

JavierV

    Sputnik

  • *****
  • topic starter
  • Posts: 42
  • Joined: 31 Oct 2012
  • Loc: Texas

Posted 15 September 2021 - 01:15 PM

Right, Field Flatteners allow you to image at the native Focal Length of your scope but gives you a flat field. 

 

Focal Reduces will reduce your Focal Length AND give you a flatter field.  Two different use cases.  They're not intended to be used together.  

 

On my Refractor, if I image and want the full 714mm FL then I just use a flattener.  If I want to image with shorter exposure times and a wider field then I use my Reducer (which is also a flattener) and I get an effective FL of 535mm on the same OTA. 

Hi,

 

this is the Focal Reducer i have: 

https://optcorp.com/...cal-reducer-fr2

 

I don't think it also corrects the field,  but well, let me try it out and report back.

 

kind regads



#6 Tapio

Tapio

    Aurora

  • -----
  • Posts: 4,668
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 15 September 2021 - 01:53 PM

Nope the cheap 0.5x reducers are just reducers.

Haven't heard or read someone trying to combine then.

I'm suspecting it's not a winning combination.

But hey, not saying it's worth trying.

Report back what the results are.


  • Pictor likes this

#7 luxo II

luxo II

    Skylab

  • ****-
  • Posts: 4,328
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 15 September 2021 - 09:18 PM

This is a general problem  - until manufacturers state clearly what scope they're designed to match you're all taking a pure guess on these things as to what their contribution is re field curvature and coma.

 

And they could be designed to match small refractors or newtonians - not just SCT's.

 

Starizona - at least - does, but the rest don't.


Edited by luxo II, 15 September 2021 - 09:19 PM.


#8 rhaskins

rhaskins

    Mariner 2

  • *****
  • Posts: 225
  • Joined: 07 Sep 2015
  • Loc: Minnesota

Posted 16 September 2021 - 08:40 AM

Stellarvue tells you what scope the FF or reducer is for. Stellarvue customer serverice will even discuss this with you. I believe Celestron does the same especially for the Edge scopes.

 

Rick



#9 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 94,562
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 16 September 2021 - 09:34 AM

I suspect the 0.5x TPO reducer will have its own set of off-axis aberrations and any attempt to use it photographically will be frustrating as well as disappointing.  

 

It's not that the TPO provides a perfectly reduced field at all focal ratios and focal lengths, it's just a simple, generic reducer that will probably be pretty good in the center of field but not so good off-axis.

 

Jon




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