Turning the Bahtinov mask

[MarkS]

Question of the day regarding Bahtinov masks. 

I did some experiments last night before the clouds rolled in.  I very carefully focused on a star central in the CCD with the Bahtinov mask in place.  I then turned the mask through 90 degrees and it indicated that the same star was definitely out of focus.

This is probably telling me something important - but what?

Could a tilted CCD cause this?  I think I need to draw lots of diagrams to work this out.

[Mike]

Definitely sounds like the CCD is not flat to the light plane.

[Ian]

could be astigmatism somewhere...

[mickw]

What's the Bahtinov mask made of ?

Is it flat and does it sit square on the scope ?

[MarkS]

Made of laser cut plastic.

Yes, it was sitting square on the end of the scope but, strangely enough, they should still work correctly even if they are tilted.

[RobertM]

Very interesting, was the star central?

[Carole]

.....and just when I thought I had cracked it!!!

[MarkS]

Robert, yes the star was dead central in the CCD.

Ian, you're right - astigmatism would certainly cause this effect - I can easily prove this.  However, I don't think the scope is suffering from astigmatism, at least I hope its not!  That's why I wondered if CCD tilt could cause the same behaviour - but I can't work out how (or if) this is the case.

Mark

[Fay]

Well, if anyone can work it out, you can Mark. I have not tried mine yet.

[Rocket Pooch]

Hi Mark,

They are crap (oops), err not brilliant, and you have just found out why, if a scope is in line on a single axis in any way then they don't work off axis, or if the mask is out of line, or if they are clipped on wrong etc.  I rate them from Refractors (sometimes) but nothing else.

All you have done is proved they are accurate to a small %, which is probably enough imagers who don't want 100% perfection, but they are fab to get a quick focus.

Years ago I had two sticks in front of my SCT at 90deg to make a perfect cross, they kinda worked to get an ok focus, but it still needed tweaking, when these masks came out I though about them but decided there would be too many issues.  I still use the ADU, FWHM and MK1EB method of focusing, it works every time, and not extra expense on masks etc.

Chris

[Fay]

Carole, must admit I chuckled when I saw you post.....a classic

[RobertM]

It would be interesting to see if you had the same result with a Hartmann mask or a pair of sticks as Chris has used.  At least then you'd be able to see if it's a problem with mask design or something else.

Robert

[MarkS]

This guy has some interesting observations:
http://deepspaceplace.com/gso8rccollimate.php

[The Thing]

This guy is sort of emulating the way I collimate my SCT with a mask with three slots around the periphery, except he is positioning a star around the periphery as the Bahtinov mask covers the entire aperture. In the three slot mask the diffraction spikes produced are emanating from the extreme edges of the optical system and so provide a measure of the relative positions of each the three collimating screws and thus the tilt of the secondary (it's an SCT). When the points of the spikes meet the scope is collimated, when they cross it's focussed. Could this principle be applied to the banana scope?