Bahtinov mask not working.

[bloodhound31]

I printed a mask off from an online mask generator onto clear projector paper and used it on Jupiter for practice.

As you can see, image 1 is slightly out of focus as the central spike is high.

Image 2 is focused as the spike is centred.

Funny thing is, it works in theory, but when I take the mask off and take an exposure, the stars are not sharp. Why would that be I wonder?

The mask is taped to the very front of the dew shield on the ED120. Photos are very short exposure with the Orion Starshoot Pro

For focusing, I think I will need to stick to the numbers game in MaxIm for now. I get perfect focus every time this way. It takes a bit longer, but at least it's right.

Baz.

[Dennis]

This is probably due to Jupiter being an extended object (disc) and the Bahtinov Mask is really designed for focusing on pin point objects (stars). IIRC, a disc can be seen to be made up of a necklace of “points” and this apparent multitude of points may be confusing the optimal focus?:shrug:

I have successfully focused on the Galilean moons of Jupiter, but only used Jupiter’s disc to obtain coarse focus.

Cheers

Dennis

[bloodhound31]

Good point Dennis. More experimentation is needed then methinks....

Thanks mate.

Baz.

[bloodhound31]

Thinking more, so then what you are saying is the smaller the star (point of light) the sharper, finer result I should be able to get?

Baz.

[Terry B]

Baz.
If you put a clear sheet in the optical train it will change the focus point. You have to cut the slots out so there is only air and not plastic between the lines. This is difficult to do with clear sheeting. I printed a mask in paper, glued it to some 3mm foam board and then cut that out with a craft knife. Took about 30mins but works a treat and cost about $3 for the foam board.

Try a clear sheet vs nothing and you will see a change in the focus. Similar to trying to take an image through a window.

[bloodhound31]

Terry, your logic is impeccable. I will do just that!

Good advice.

Baz.

[dannat]

there is anew mask on CN - it is basically a big Y sideways) you could test it

[Moon]

Baz,

Interesting that you raise this point. I also have some question about the focus results I get with Bahtinov mask. I had a hunch it works better on reflectors due to the absence of chromatic aberration, but I have not had a chance to prove it conclusively. Craig (http://www.stark-labs.com/blog/files/NebBahtinov.php) did a good comparison of the mask to Nebulosity, but it was on a Newt. I'd like to see the same test on a refractor, just to check.

Let us know if you get to perform the test proposed by Terry - that would be the simplest explanation in your case.

[Dennis]

From my limited understanding and reading of diffraction pattern related stuff, you need a point source that effectively has no “extension” or “dimensions”. That is, your optical system should not be able to resolve a disc – it should only see a point.

This is why artificial stars need to be small (for the specific aperture of the ‘scope) and placed sufficiently far away from the ‘scope so that the scope cannot see or resolve the edges of the (pinhole) aperture forming the artificial star.

Cheers

Dennis

[Dennis]

On a side note, I have seen the central bar effectively “move” with poor seeing and also with the position of my eye when using an eyepiece that allows your eye to wander around in the field of view.

So, for visual work, I focus with my eye centred.

Cheers

Dennis

[bloodhound31]

Thanks guys. I have printed off a new one and will paste it onto some foamboard tonight for cutting and let you know how I go as soon as a pinprick in the curtain of night appears.

Baz.

[seeker372011]

you could also try the new "Lord" mask that's very easy to make..

[telecasterguru]

I will be using my Bahtinov mask on my ED127 for the first time tomorrow, fingers crossed, so thanks for the information.

Frank

[Paul Haese]

Baz,I think this has been covered but I am gonna confirm that the mask must be used on a point source only. Stars are a point source, planets are not.

[bloodhound31]

LOL! Thanks for the scoop Paul!;)

I won't make that mistake again.:thumbsup:

Baz.

[Moon]

Sorry for digging up this old thread - but I have an update.
I did some tests with my refractor and the Bahtinov mask with a CCD camera.
It shows clearly the blue filter has a different focus point to the R and G filters - which makes sense for a refractor. In addition - with the blue filter in place, you can see curvature in the diffraction spike, which indicates that the different wavelengths of blue light have different focus points and perfect focus of blue light will never be possible.
By the same token, the Bahtinov mask gives a visual clue to the the width of the pass band width for all filters, include narrowband.
LINK (http://deepspaceplace.com/ed127.php)
It would be great to repeat this test with a few achromats and top end APOs as well.
Overall a very useful tool!

James

[mswhin63]

That is an interesting find and makes you think more about colour imaging through a refractor. It would be interesting to see whether wavelenths of light are affected in a different way with reflector but as they use mirrors should show no effect but what is interesting is that I have seen this effect on my DOB. Maybe it is just my eye playing tricks without my glasses on.

[Octane]

James,

I had the exact same problem with my ED127.

I had no problem using a Bahtinov Mask and the Bahtinov Grabber software on red stars. The focus was spot on. Go to a blue star, and, the focus would induce chromatic aberration on the star. This was OK, as all I did, after the Bahtinov Grabber software said focus was tight, was to just turn the 10:1 reducer back a fraction. This still retained sharpness but removed the fringing.

H

[mswhin63]

Says something for mono imaging

[Tandum]

All filters on my TAK are exactly the same so I guess it's a scope thing. Sounds like a good excuse for robo focus and focusing offsets

[JohnH]

Hi James - I think we had the same discusion on Yahoo! My results agree with yours (and H's)...to find the optimum focus on a Triplet refractor you cannot just use a Bahtinov mask, I think this is because the focus points for RG and B are not identical. The optician can get a zone of good focus with no CA but not a point of perfect focus with no CA. If you go to best focus one of the colours will be out - typically B. I find the FWHM measures in AA4 to be more reliable.

As others have found you can use the mask - after all it gets you close, go to best focus (centralise bar) then rack out 1/4 turn (on the fine focus of the ED 127) and you should be there, sharp stars no colours.

For the very best results with NB you do need to re-focus with each filter change, I am looking at adding Robofocus to do this, I think thats is the only way to get it always perfect.

[troypiggo]

This is a fascinating read. Never realised these differences in focus.

[multiweb]

It is indeed. I always wondered why my blue channel looked like c**p. Might have to review my focusing technique with the ED80. :confuse3: