Bahtinov masks & focal lengths

[Astronut1982]

Hi guys,

I'm just wondering - Does a bahtinov have to match the focal length of my scope?

The reason I ask is I Have an 8" astrograph for DSO imaging & an 8" SCT for planetary imaging. I recieved a bahtinov mask with my astrograph & have been using it on both scopes to achieve focus. I have been having trouble getting the focus right on planets & have been puzzled as to how this is.

I have now heard from a member of my observing group that he believes that each mask must match the focal length of the scope. I just wanted to check this before I go & buy another one for my SCT.

Thanks in advance for any advice,

Jeff

[JamesF]

In theory I think they're supposed to be matched to the scope, but in practice I'm not sure it makes a huge difference. You may perhaps get more distinct diffraction spikes if they're properly matched, but as long as they're good enough to do the job does it matter? I'm sure there are stories elsewhere on SGL about Y masks, which are almost the same thing, being made from bits of twig or whatever else someone could find lying about.

James

[Astronut1982]

Thanks James.

I was just wondering of it would make much of a difference or not & whether it was worth buying. I'm just having problems getting perfect focus on Jupiter & when it was mentioned to I wondered if it could possible be the solution to the problem I'm having.

Jeff

[JamesF]

It's worth a go, but you might as well try to make your own Y mask first out of a few bits of card and see how that goes.

James

[Bizibilder]

There is a lot of leeway in the dimensions of a Bahtinov mask. The focal length and slot size of the mask are mathematically related but, as slot sizes generally go up and down in whole millimeters, you need a big change in focal length to require a slot size change. Any sized mask will give the required pattern - its just that a "matched" mask gives the sharpest and most well defined one.

[Freddie]

I use the same mask on my CPC with 2x, 3x and 4x barlows no problems.

[Astronut1982]

Thanks for the replies,

I'll try & make a Y mask as James has recommended but other than that, I'll just stick with what i have for now. I'll have to look elsewhere for the source of my problems as it most likely isn't from the mask.

Jeff

[kirkster501]

You can use anything. I used twigs the other night when I realized I'd left my BM at home. That said, the proper one for your aperture/FL gives clearer, longer spikes and makes it easier for tools like Bahtinov grabber to work.