Aperture masks for 10 inch Dobsonian

[bluesilver]

Hi,  I am sure this is a basic question,  but so far after doing some research i am still a little lost on them.

From what i think i have understood,  you can use a smaller hole in front of the primary mirror to basically make the viewing image of the planets stand out a little more and not be as bright,  plus helps with the fuzzy haze a little also,  is correct or am i way off?

On my 10 inch skywatcher dobsonian,  the primary mirror has a plastic cap,  on this cap it has two round caps,  one is moulded on and the other you can take the cap off revealing a smaller hole ( not very detailed explanation i know )

Can this small hole be used as an aperture mask?  basically just put the plastic dust cap on and take of the small cap?

I have heard of people making their own aperture masks by making up to 3 holes of different sizes, 

Is there an ideal size to make the holes for my 10 inch dobsonian?

 

Very interesting to try this out,  but just not sure where to start on hole sizes or even if that plastic dust cap is one.

Any advice or help would be appreciated.

Thanks.

 

 

 

 

[Stu]

The basic idea is as you describe. Using the hole in the cap may work, it depends where it is positioned. The hole needs to fit between the vanes of the scope's secondary support, that way you get the full benefit of a clear aperture, no diffraction spikes.

You could draw out the spikes to full scale then work out what is the biggest aperture which fits between them, that should work.

[Alan64]

What's the diameter of the built-in stopped-down?  Ideally, you can make a separate aperture-mask, and have at least an 80mm unobstructed aperture, if not a 90mm...

[John]

This approach does work - I use a 100mm aperture mask with my 12 inch dobsonian from time to time. The result is visual performance virtually identical to my 4 inch refractors. As I have a couple of very nice 4 inch refractors I don't do this often though.

The plastic dust cap apertures do work and give you something around a 50mm aperture I seem to recall. A rather big 2 inch scope !

 

 

[bluesilver]

Sorry for the long delay in response,

Thanks for the replies,  appreciated.

I might have answered this next question already myself,  but just want to make sure i am thinking correctly here

My Dobsonian is the collapsible version.

I take it the aperture mask actually goes right up front of the dobsonian,  not in front of the primary mirror,  ( this is where my plastic dust cap is on my collapsible version).

I am thinking that i am suppose to take this plastic dust cap aperture and place it right at the end of the scope in front of the spider vanes and secondary mirror.

Is this correct?

I am guessing it is very basic and now sounds like a silly question when comparing it to a solid tube version.

 

 

[Stu]

That's right, the aperture needs to be at the top of the dob, ahead of the focuser and secondary spider, same as it would be on a solid tube dob.

[cletrac1922]

On 04/07/2019 at 19:27, bluesilver said:

Hi,  I am sure this is a basic question,  but so far after doing some research i am still a little lost on them.

From what i think i have understood,  you can use a smaller hole in front of the primary mirror to basically make the viewing image of the planets stand out a little more and not be as bright,  plus helps with the fuzzy haze a little also,  is correct or am i way off?

On my 10 inch skywatcher dobsonian,  the primary mirror has a plastic cap,  on this cap it has two round caps,  one is moulded on and the other you can take the cap off revealing a smaller hole ( not very detailed explanation i know )

Can this small hole be used as an aperture mask?  basically just put the plastic dust cap on and take of the small cap?

I have heard of people making their own aperture masks by making up to 3 holes of different sizes, 

Is there an ideal size to make the holes for my 10 inch dobsonian?

 

Very interesting to try this out,  but just not sure where to start on hole sizes or even if that plastic dust cap is one.

Any advice or help would be appreciated.

Thanks.

 

 

 

 

The non removable plastic cap, is designed to hold the removable one

For solar viewing, sticky tape some visual baader solar film, underside of small hole in cap

Remember to leave off finderscope

John 

[Geoff Lister]

 

On ‎07‎/‎07‎/‎2019 at 10:14, bluesilver said:

My Dobsonian is the collapsible version.

I have not tried my collapsible Dobs. for solar, but I would expect to need a very good shroud between the two main tube sections.

Geoff

[KevinOBrien]

I have a 10 inch collapsible skywatcher dobsonian and the hole in the cover works just fine as an aperture mask in its customary position, between the primary and secondary reflectors.  Here's a shot I took yesterday.

 

[KevinOBrien]

If I made a mask with two holes, would it give me split image focus confirmation?

 

[Cornelius Varley]

27 minutes ago, KevinOBrien said:

If I made a mask with two holes, would it give me split image focus confirmation?

 

Yes it would. That type of focussing mask is known as a Hartmann mask.

http://astro.i-net.hu/node/56