Is Bhatinov mask size critical?

[Gasconman]

Someone has offered to give me a Bhatinov mask that she no longer needs. The total diameter of the mask is 14 cms and the section with the cut outs is 10 cms in diameter. The diameter of my OTA is 15 cms. I could easily cobble something together to make it fit the end of my scope, but would it still work properly?

I'm not imaging at the moment so I don't have any immediate need for this, but I may do in the future, and you know what they say, "Never look a gift horse in the mouth"

[vlaiv]

Size is completely unimportant and you can use smaller mask on larger scope and vice verse.

What I'm not sure is important is centering - try to center the mask, just in case (don't put it to one side because it is smaller - make it sit in the center of your aperture).

[Erling G-P]

Size and/or shape can affect what you see.

I originally got one large enough for my 200PDS.  Used it a few times on my 130PDS too, as it has adjustable pins, to make it fit various aperture sizes.  I then got a smaller one specifically for the 130PDS, but have only tried it once. Compared to the larger mask, it produced a more flattened view, which was considerably more difficult to adjust after, so I continue to use the larger mask. It's very loose fitting though, so stars near zenit are preferable, to prevent it sliding off.

[ollypenrice]

I use a mask designed for an 85mm refractor on a 106mm refractor and find it works perfectly (checked by also measuring FWHM values at the same time.)

Olly

[gorann]

For my EdgeHD11 I use the mask I cut out from a black plastic sheet to cover my 14" Meade (one hours's work with a box cutter saved me about 50 Euro), and I make a small effort to kind of center it, and it works great. Size does not seem to matter much in this case😉 and I am not sure centering is important either.

Edited September 17, 2020 by gorann

[Gasconman]

11 hours ago, ollypenrice said:

I use a mask designed for an 85mm refractor on a 106mm refractor and find it works perfectly (checked by also measuring FWHM values at the same time.)

Olly

How do you fix it in place? Did you add something to it to make it wide enough?

[ollypenrice]

1 hour ago, Gasconman said:

How do you fix it in place? Did you add something to it to make it wide enough?

No, it still hooks on using the standard two bolts which sit on the dewshield.  I suppose it would be better to mount it on a new card so that it was centred on the lens but since it gives a perfect result as it is I've never bothered.

Olly

[Gasconman]

OK, guys, thanks a lot for the help. I'm going to accept the gift with thanks. I just didn't want to take it from her and deprive someone else of the opportunity if it was going to be of no use to me.