I've gone off axis mask crazy!

[Moonshane]

I had some old split OOUK tube caps (they are always splitting so I make end caps from elasticated cloth caps (like a shower cap)) lay around the place so I decided on a whim to make an aperture mask for both my 12" f4 and my 6" f11 dobs (I already have one for my 16" f4).

I make them by cutting a circle of plastic large enough to be put inside the tube and rest on the spider. I then cut out 1, 2 or three holes of various sizes and make a revolving masking piece to cover the two not being used. These holes must be round and fit within the spider vanes.

For the 16" f4 (actually f4.5 with the paracorr) I have holes of 170mm (f11), 130mm (f14) and 100mm (f18)

For the 12" f4 (actually f4.5 with the paracorr) I cut a 115mm (f12) and 85mm (f16)

For the 6" I cut just a 60mm (f26)

Read on as I am not quite as daft as I sound at this point.

I obviously use full aperture when looking at faint objects but for the two fast scopes, I find that doubles and planets are greatly enhanced by stopping down the aperture and creating a smaller but unobstructed slower scope. I think a reason for this is the reduced exit pupil. For planetary and lunar observing a 1-2mm exit pupil is ideal according to several articles I have read and to get this with my fast scopes means (assuming f4.5) an eyepiece of 4.5mm -9mm (magnification of 408x/204x in the 16" and 307x/153x in the 12") and such magnifications are not usually possible on planets anyway.

Double stars are even worse as ideals are 0.5mm-1mm.

Using the 16" as an example and at the 170mm aperture. This equates to a f11 scope and therefore a 1-2mm exit pupil is with a 11-22mm eyepiece (167x - 83x). I had often wondered why I like my 11mm TV Plossl so much in all scopes for planetary use; I have now established why

I may not ever use the 60mm f26 aperture on my planetary dob but it's fun to try and what this method does is allow you to change the format of the scope at will and carry effectively four scopes, three scopes and two scopes in one. Whilst I am not suggesting you all go and buy a big dob (well, OK I probably am), I'd suggest that a 100mm f15, or a 130mm f12 or even a 170mm f10 refractor would all be as unwieldy a tool as a my 16" dob but I have the advantage of a mount that weighs about 25 pounds and a scope that can be used to see very deep into space with the removal of the mask.

I'd honestly urge anyone with a newtonian of 10" or more to give this a go, it will surprise you and for the cost of a sheet of card, cardboard, hardboard or even funky foam, it's one of the best mods you'll ever do.

[Ohmless]

I suspect your impression might have been caused by seeing limitations. here is a cool site that describes optimal apertures for different quality of seeing. http://www.telescope-optics.net/images/MAG3.PNG

i think there is value in doing this in order to get a more pleasing and stable view when seeing is limiting the performance of your aperture(in FUN units.)

[nicks90]

I suspect your impression might have been caused by seeing limitations. here is a cool site that describes optimal apertures for different quality of seeing. http://www.telescope-optics.net/images/MAG3.PNG

i think there is value in doing this in order to get a more pleasing and stable view when seeing is limiting the performance of your aperture(in FUN units.)

i didnt understand that link one tiny little bit!

but as an owner of a SW 12" dob I am very interested in making an aperture mask for planetary viewing, as although I have had a couple of stunning views of jupiter in the past month (yes, it has stopped raining twice!), the big aperture and rubbish seeing has made everything a bit wibbly wobbly except for the very occassional 5 second spells that have blown me away.

From my understanding stopping it down to maybe 100mm F15 would bring back some wow and allow me to use some of my classic orthos to give super contrasty views.

I also found a link that said you could make a twin holed aperture mask - eg two 100mm holes giving an effective aperture of 150mm F10 and it also acts as a brilliant focusing tool. As long as the holes are perfectly symetrical to each other by 180deg and equidistant from the centre, you get 2 images out of focus, but they come together and perfectly overlay each other when the scope is in focus.

Has anyone tried this method? As I am thinking this is a cracking way of having 3 focal lengths, native F5, F15 with one hole and F10 with both.

[Moonshane]

I agree with your first point and that when seeing is poor you should get a slightly dimmer but more contrasty view which will be more stable than the full aperture. re the two hole approach, I am not sure this would work but as a mask is so cheap to make, give it a go and report back   

[Mr Spock]

Is some of the effect due to there being no central obstruction to degrade contrast?

[swamp thing]

Might give this a go on my 20" at some point for a giggle. I don't really do planets but I'll have a bash at this and see how the views compare to some observing buddies scopes (Maks and Fracs). Like you say it's only a piece of cardboard, so cheap as

[Moonshane]

Is some of the effect due to there being no central obstruction to degrade contrast?

yes, partly but the main difference is that I get either a 16" f4 scope or a 6.5" f11 scope so the exit pupil is a lot smaller with the latter, creating a dimmer image at the same magnification and therefore detail has more easily obtained contrast.

[estwing]

Maskin my dob with the 170mm gives me a F12...I may go smaller just for curiosity!...

[nightfisher]

Its not only the dob mob that mask of.....i sometimes stop my Tal down to 80mm f12.5, and i had the antares 105 stopped down to 80mm f19, this seems to bring up the "depth of focus"