[nswnra] Re: Dew Heaters
- From: "Corona Australis" <corona_australis@xxxxxxxxxxxxxx>
- To: <nswnra@xxxxxxxxxxxxx>
- Date: Tue, 31 Aug 2010 22:54:40 +1000
>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
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