Here's a couple of sites for sizing your secondary. I managed to find the formula which I'll photocopy and pass over the fence. But you might find Mel site a little easier on the brain. http://www.bbastrodesigns.com/diagonal.htm and this once which explains the formula if you're interested. http://www.fisica.edu.uy/~sroland/newton/obstruction/ Cheers,CS Rob From: nswnra-bounce@xxxxxxxxxxxxx [mailto:nswnra-bounce@xxxxxxxxxxxxx] On Behalf Of Corona Australis Sent: Tuesday, 31 August 2010 22:55 To: nswnra@xxxxxxxxxxxxx Subject: [nswnra] Re: Dew Heaters >Now that the elections are over and the footy has quitened down I have an >astronomical question. > >Iam having a few problems with a dew heater for my secondary mirror in my 10" >dob. Astrosystems advise > >they can't supply the units due to the mirror size being 2.27". They have only >2.13" and 2.6". Is it feasible > >to change the mirror to a smaller or larger one, and if so would their be any >draw backs. > >Vince In a word yes, I'll need to dig out the formula and get back to you. Going to a smaller secondary will cause vignetting of your primary mirror. Going larger secondary will create a bigger central obstruction. Both will cause less light hitting the focal plane of your eyepiece. From memory anything below 19% and above 22% central obstruction you'll get diminishing returns of light. At around 19% you get full illumination which is optimum. You might be lucky as most massed produced systems like yours as was with mine in the 16" ship with oversized secondaries. I was able to swap out my secondary giving me 19% obstruction. Vince, it maybe be cheaper to build a secondary holder to suit and go for a DG1 otherwise you'll have to buy another secondary mirror + holder. Let me have a think about it. I'll be home over the next few days we can have a chat and see what we can come up with. Rob