Hi all
I am thinking about getting a smaller secondary mirror for my dob.It is a 10 inch f5 and the secondary is 63 mm.Would a 50 mm be to small ??? any thoughts !!!
john
I don't understand the idea of stepping down in size. You will then have less light gathering capacity as you will lose light passing the outside of the secondary.
What's wrong with the set up you have?
Last edited by ballaratdragons; 27-03-2005 at 12:10 AM.
Hi John, using a smaller secondary will give you a gain in contrast this is good for the planets but as Ken says you will lose some light
Try this free program its for designing your own scope http://home.att.net/~dale.keller/atm...t/newtsoft.htm
this will give you a better idea of what the effects will be if you do change to a smaller secondary.
cheers
David.
John , technically you could do it and still obtain a large enough 100% and 75% illuminated field.
But you would need an ultra low profile focuser and a change in the mounting of both the diagonal and focuser to accommodate the change in diagonal.
As it is on a standard scope I would assume you have a rack and pinion focuser and about a 300mm diameter tube your 100 % fully illuminated field would be some ware from 4-8mm in size with a 50mm diagonal.
This would be O.K. for a ToUcam and short focal length eyepieces but no good for long focal EP's---eg deep sky.
There are a few variables that will change this and I am just guessing on those sizes.
A Newtonian optimised for a 50mm diagonal will give about a 10-15 mm fully illuminated field which would be enough to illuminate most CCd chips and eyepieces.
The 75% field will illuminate any low power eyepieces.
The thing is the reduction from your current 25% obstruction down to a 20% with the 50mm is not worth the expense or hassle .
You would most probably not notice the contrast increase of 5%.
A planetary optimised Newtonian has to be designed from the start to get all distances correct to get the most from a smaller diagonal.
I have a planetary 12.5" f6 Newtonian with a 16% obstruction ratio and 20mm diameter 100% F.I.F. but could have gone smaller at the expense of field size but it is really not worth it that much once you go under 20% , the differences are not noticeable.
Just be happy with the fact that your 25% is way better than a 35-40% of some SC scopes and if your after a performance boost for planetary work may be look at a better quality diagonal if need be and make sure the tube is well blackened inside , also that the tube is long enough past the focuser .
Hope this helps?
I considered doing this myself a short while ago and calculated the numbers for my GSO 10inch dob.
Standard secondary 63mm
fully illuminated field 16.2mm , 0.75 degrees
75% illuminated 42.4mm , 1.95 degrees
Next smallest standard size is 2.14inch (54.35mm)
fully illuminated field 3mm, 0.25 degrees
70% illuminated 18.5mm 1.49 degrees
To reduce the secondary size here will make a visible difference with widest field eyepieces.
Unless you live in a place where the seeing is great and planetary is your main interest, its not worth the expense of the change.
As others have said its not worth the time and effort for the minimal gain involved.
You need to get the secondary obstruction under 20% and ideally down to 16% to 18% to notice any worthwhile gains in contrast for lunar and planetary observing. To do this you need to change the focuser, reposition it and reposition the spider or move the primary down in the tube, all to hard. As Mark Hodgson has said these are design considerations that need to be thought of prior to building the scope, not an afterthought as a bandaid fix. That 50mm secondary would improve planetary contrast by 3% to 5% but would reduce the size of the 100% and 75% FIF and would affect the scopes performance on DSO's. The 3% gain in planetary contrast would really only be detectable under the very best of observing conditions.
In short a complete WOFTAM IMO. If you want a specialist planetary scope get someone to build you a 10"/F8 equatorially mounted newt with a 12% CO, then you notice a difference.
secondary mirror
L.
