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.


Scondary size question??

  • Please log in to reply
7 replies to this topic

#1 Taeyoung



  • -----
  • topic starter
  • Posts: 33
  • Joined: 26 Apr 2010

Posted 29 January 2013 - 05:27 AM

I'm building 13.1" dobsonain now.
I've already have 3.1" secondary.
It is not bad but I want to more samll secondary mirror
for more contrast when observing planets.
I'm considering 2.14" secondary.
I found Lockwood's table for secondary size.
In there 2.14" for my dob meand 0" fully illuminated field size..
My mirror is 13.1" F4.5
and the distance of secondary to focal plane is about 9" long.
What exact meaning of 0 fully illuminated field size?
Is it impossible to use 2.14" secondary?
If it possible for planetary then my plan is
use 2.14" for planet and 3.1" for general observings.

#2 Mirzam



  • *****
  • Posts: 4576
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 29 January 2013 - 08:26 AM

The zero field size means that the secondary is too small. Even for a planetary scope you want a field size of about 0.1" (or 2-3 mm). This ensures that the center of the field is not being vignetted by the secondary. Typical designs for a general purpose scope use a 0.5" fully illuminated field size.

Another problem with using a minimalist secondary is that everything must be perfectly centered or you will not see the whole mirror. In other words the spider must be perfectly adjusted, the secondary position perfectly adjusted and the mirror centered in the tube. This is possible to achieve with careful construction, but it is nice to have a slightly larger secondary--it makes everything more forgiving.

To better answer your question we need to know the f-ratio of the telescope.


#3 bremms



  • *****
  • Posts: 3769
  • Joined: 31 Aug 2012
  • Loc: SC

Posted 29 January 2013 - 08:30 AM

Here we go!! :gotpopcorn:

#4 siriusandthepup


    Mariner 2

  • *****
  • Posts: 287
  • Joined: 14 Feb 2006
  • Loc: Central Texas, USA

Posted 29 January 2013 - 10:17 AM

Mirzam is correct - 2.14" secondary is too small. Try a 2.6" instead. This will allow your scope to continue to work well at lower magnifications and be a great deep sky scope.

I once had a 13.1 f/4.5. It had some noticeable astigmatism at full aperture, but showed beautiful colors in the Orion nebula. I put a 5" off axis stop on it one time and compared the views with my Quantum 6 Maksutov.
The planetary performance was virtually identical and there was no trace of astigmatism visible while using the stop. Something to consider.

#5 Mike I. Jones

Mike I. Jones


  • *****
  • Posts: 3639
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 29 January 2013 - 11:28 AM

Love it! Now we need a tailgate party gremlin!

#6 planet earth

planet earth


  • ****-
  • Posts: 2472
  • Joined: 07 Sep 2010
  • Loc: Ontario

Posted 30 January 2013 - 03:29 AM

If you reread Mike Lockwoods page a couple of times from the beginning all your questions are answered.
Remember the distance is from sec to focal plane not a racked in focuser. Focus the moon on a piece of frosted tape or glass will give you the fp. Try all your eyepieces as some require more in focus and some require more out focus. Take measurements.
It's comman to add a inch to focuser height for differant eyepieces, barlows and paracorr.
So if you measured right your distance 9" will be 10".
I recommend not going below a 2.60" secondary.
That's only 20 percent.
Actually if your sec 3.1 inch (24 percent) is of good quality I would use it.

#7 Achernar



  • *****
  • Posts: 9790
  • Joined: 25 Feb 2006
  • Loc: Mobile, Alabama, USA

Posted 30 January 2013 - 01:44 PM

To add to Mirzam's post, there is another issue to consider. Errors on a secondary mirror, if they are present will usually lie near the edges, and if your secondary mirror is just big enough, they will intrude into the light path. You will also have significant light loss at the edges of the field of view through wide and ultra-wide angle eyepieces. As long as it's 20 percent or less of the diamater of the primary mirror, it will have minimal effect on the views while observing the planets.


#8 ed_turco


    Surveyor 1

  • *****
  • Posts: 1810
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 30 January 2013 - 02:05 PM

I agree 100%. But tell that to an APO owner. Those folks are a bit looney :)

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.

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics