•

# Secondary offset question

4 replies to this topic

### #1 tloebl

tloebl

Mariner 2

• topic starter
• Posts: 246
• Joined: 13 Oct 2010

Posted 24 February 2021 - 08:05 PM

I guess this is a geometry question and I hope I can describe it in a way that makes sense using my f3.9 newt.  My secondary vane is exactly centered but the secondary mirror is offset on its stem. Assuming the telescope is perfectly collimated and the secondary is exactly at a 45 degree tilt, If I move the ota to a horizontal position and walk back while looking down the ota, at some distance the secondary will just cover its own reflection in the primary. At that point, should I see it perfectly centered on the primary mirror with 4 equal primary quadrants ?  When I try this with  my f6 newt that has a centered secondary with no offset, the secondary and it's reflection are perfectly centered on the primary.

### #2 TxStars

TxStars

Mercury-Atlas

• Posts: 2,552
• Joined: 01 Oct 2005
• Loc: Lost In Space

Posted 24 February 2021 - 08:36 PM

https://www.cloudyni...onian-how-much/

The secondary should be slightly away from center. (looking in the front of the tube with focuser on right the secondary should be slightly left of center)

Edited by TxStars, 24 February 2021 - 09:00 PM.

### #3 bbasiaga

bbasiaga

Apollo

• Posts: 1,014
• Joined: 10 May 2006

Posted 25 February 2021 - 11:59 AM

No I don't think you should, because the mirror is not mechanically centered on the optical axis.  It should appear slightly closer to the OTA tube on the side opposite the focuser.

-Brian

### #4 MitchAlsup

MitchAlsup

Fly Me to the Moon

• Posts: 5,289
• Joined: 31 Aug 2009

Posted 25 February 2021 - 01:11 PM

In my 13" F/3, the secondary is visibly off center, you can see it looking down the front of the UTA, and you can see it in the EP 20+ waves out of focus, and this is the proper position for it to be in.

### #5 tloebl

tloebl

Mariner 2

• topic starter
• Posts: 246
• Joined: 13 Oct 2010

Posted 25 February 2021 - 01:30 PM

Thanks for both of your inputs. Sounds like the it's normal for the secondary to be offset slightly away from the focuser side when looking from the front of the OTA.

• Asbytec likes this

## Recent Topics

 Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics