Quest for contrast (GSO dob)

[Starkler (Geoff)]

As a spider decided to make my telescope its home, it was time to clean the mirrors and do some routine maintenance on the ole gso dob.

One aspect of the gso dob that has bothered me for a while is the reflectivity of the mirror cell, being a grey hammertone gloss finish.

You can see what I mean in this pic if you look closely.
There is a lot of light reflected from the area between the mirror clips and the tube wall. This light is seen at the eyepiece and will degrade contrast.

http://gar.customer.netspace.net.au/scope2/100_0226.jpg

This is the dissassembled mirror cell with the outer ring sitting on top. See its quite shiny and reflective.

http://gar.customer.netspace.net.au/scope2/shiny.JPG


The solution I used was to use a can of matt black spray enamel paint and I used left over flocking material to line the visible flat surface of the outer ring of the mirror cell.

This is the result. Compare to the shiny outermost part which actually sits on the outside of the tube when the scope is reassembled. I could have got it darker had I been bothered to sand it all down to a rough surface.

http://gar.customer.netspace.net.au/scope2/painted.JPG

Scope back in one piece again pointed almost right at the sun.
Compare this to the top picture. I assure you that the non flocked parts are darker having been painted.

http://gar.customer.netspace.net.au/.../installed.JPG

The ultimate solution is to construct an appropriately blackened baffle to sit above the mirror, but this would necessitate installing a fan as air currents would then intrude into the light path.
Maybe next time

[wavelandscott (Scott)]

Your pictures and descriptions are great...I haven't monkeyed much with my scope but, you are giving me ideas and courage!

Keep it up!

[gbeal]

Good work Starkler.
I built my 10" up from optics, and used a card tube, (sonotube/formatube call it what you like).
I cut a series (from memory about 8) thin ring baffles, and affixed them within the tube, evenly spaced. This reduces, or hopefully eliminates any reflections from the interior.
I also cut and installed a series of baffles opposite the focuser, in an arc. This means that the portion surrounding your 45 degree diagonal is nice and dark too.
Result? It works well.
Worth the effort? Possibly. but I am happier so it must be good.
Gary

[Dave47tuc (David)]

Good work Geoff
Look foward to having a peek

Best

[mch62 (Mark)]

Have you tried to extend the tube at the focuser end , as from a 12" GSO I have tried this is where most of the scattered light is coming from. The diagonal is to close to the end of the tube.
If you look down the focuser and can see outside the tube than you may be getting stray light from the opposite side from the focuser coming up the focuser.
A general rule of thumb when making a Newt is to have the focuser cage length about 1.5 x the tube diameter with the focuser in the middle of this.
For portability they tend to be to short.
My Newt is a very open truss design with a long focuser cage and i don't have a problem with grazing light on the mirror , even with a full moon. I do how ever have a black cloth shroud for around the mirror for dew prevention.
Even my mirror cell and truss tubes are unpainted aluminium.
There is a lot of debate over baffles in a newt , but I won't go into it here but there is a great web site (can't rebember it-do a search if your interested) on thermal effects in a Newt with and with out baffles fans and flocking ect.(it has footage of the effects) Unless you have enough mirror clearance baffles can create more problems than there worth , and with excess clearance comes a smaller illuminated field.
P.S. most probable stirred up a can of worms with this , as it has in the past with posts on CN .
Pps get some descent optical black , not just matt black. It makes a lot of difference.

Here is a link for improving a newt http://home.digitalexp.com/~suiterhr/TM/Topten.htm#01

Mark

[Starkler (Geoff)]

Originally posted by mhodson
If you look down the focuser and can see outside the tube than you may be getting stray light from the opposite side from the focuser coming up the focuser.
Mark

I get around this with a baffle in the end of my focuser draw tube

The last time I observed I didnt have it in and I noticed a big difference !

[mch62 (Mark)]

Yes a baffle or 2 in the focuser is a way to avoid this but have you ray traced the optical path to make sure you are not intruding on the light path.
A lot of 2" focusers especially in short focal ratio newts won't have a lot of room for allowing baffles.

Do you have or use Newt 2.5 ?
It is a help in designing and determining the light path of a Newtonian.

Mark

[ving (David)]

i am baffled by these baffles...

[Roger Davis]

The best matt black you can get is KRYLON available from some retailers but not all. Do a web search to find a supplier near you. Flocking is good, you may be able to get away with a patch on the opposite side to your focuser. When you look through you focuser (without an eyepiece) at the opposite inner side of the tube that is the bit that will be flocked. You can spray the rest of the inside of your tube with the paint.
As Mark said, see if you can look past the end of your tube when you look through the focuser, if you can, this needs an extension to block light from accessing the focuser.