8" SCT With Focal Reducer And 2" Diagonal & EPs

[cloudsweeper]

Well, the received wisdom is that this doesn't work, and you should use a 1.25" diag and EPs.

9.25pm Wednesday, the Moon (just past 1st quarter) was high in the south, so I decided to use it and try out the views with the FR and 2" accessories.

42mm, 36mm, and 30mm 2" EPs - all easily gave the 2deg or more of FOV expected, with only a slight loss of focus at the very edge for the longer FLs.  

So maybe I can have the benefit of a faster 'scope as well as that of greater FOV with 2" EPs.  It looks that way.  Unless I'm missing something........,,

Doug.

[newbie alert]

The fr will give you a wider fov.. I'd assume that you would have to slightly refocus on every change of ep regardless of the fr

Is that what you mean?

[Starwatcher2001]

I've done the same with my 9.25 Doug. 6.3 focal reducer, XW 40 2 inch ep, and I get a measured 1.9 degree TFOV.  I don't get much visible vignetting at the edges, BUT the central obstruction becomes visible in the eyepiece. This is very pronounced when looked through in daylight, but also visible on bright objects such as the moon. It dims the centre of the field in bright star clusters. I found it intolerable and don't use the f/r and 2 inch combination any more.

Edit: Actually, after hunting down a previous posting, I did get vignetting at the edges. I just recall the central obstruction being a show-stopper for me. Of course, your 8" may not behave the same way.
 

 

Edited June 9, 2022 by Starwatcher2001

[cloudsweeper]

9 hours ago, newbie alert said:

The fr will give you a wider fov.. I'd assume that you would have to slightly refocus on every change of ep regardless of the fr

Is that what you mean?

FR + 1.25" EPs - best FOV is about the same as w/o the FR, but with only 1.25" EPs the FOV increases when using the FR.  (Typically by 58%.)

Re-focusing, yes (as usual, unless using parfocal EPs).

What I did did seem to show that the FR coupled with the 2" diagonal and EPs does work, and does give greatly increased FOV.  1.34deg with the 42mm Rev increased to easily more than 2deg when using the FR.  (Judged with the size of the extrapolated lunar disc.)

@Starwatcher2001 - your point about the central obstruction being visible is a good one.  I did notice it, but didn't find it too troubling against the advantage of increased FOV.

Doug.

[Fraunhoffer]

I use a 2inch diagonal and fr on my c8 all the time. It works fine. The only issues ive had are with eyepieces with fl greater than 30mm with vignetting and the centre obstruction becomes visible. I think the tube on my c8 has a 38 mm inside diameter so do anything much larger than eg 30 mm 70 deg ep wil start to be degraded.  The fr stays on most of the time to keep the tube sealed. I find the bigger diagonal gives a bit more space between the back and eyepiece making observing more comfortable. 

[cloudsweeper]

9 minutes ago, Fraunhoffer said:

I use a 2inch diagonal and fr on my c8 all the time. It works fine. The only issues ive had are with eyepieces with fl greater than 30mm with vignetting and the centre obstruction becomes visible. I think the tube on my c8 has a 38 mm inside diameter so do anything much larger than eg 30 mm 70 deg ep wil start to be degraded.  The fr stays on most of the time to keep the tube sealed. I find the bigger diagonal gives a bit more space between the back and eyepiece making observing more comfortable. 

Yeah, I'm really coming round to this config now.  I was put off by all the comments about it just not working, but now I have found that it does, the 8SE+FR+2" diag is a winning combination giving:

good mag

good aperture

sharp star images

greatly increased FOV over the 8SE alone (best was 1.34deg, now upped by 59% to a respectable 2.13deg)

...... and all this on GoTo.  Marvellous!

Doug.