Camera focus - Bahtinov Mask

[Kenny]

Anyone used one of these before, and particularly with a DSLR (with and without a telescope)?

http://astrojargon.net/MaskGen.aspx

http://www.nightskyimages.co.uk/bahtinov_mask.htm

[mickw]

Yes, most here.
It creates a distortion that should work with any optics, certainly does with DSLR/scope combo.

Get one 

[Rick]

Curiously, there's this: http://forum.orpington-astronomy.org.uk/index.php?topic=3869.0

[Carole]

Yup, use one all the time.  If any-one knows where I can buy one small enough for my DSLR lens I'd love to know, my smallest one (for the 71/72 scope aperture) doesn't work with it.  Meanwhile I've made a Y mask as a stop gap.

Carole

[Kenny]

The link above generates a template to create your own mask. However, while that seems to be effective for a DSLR / telescope combo, I'm not clear whether it would work solely with a DSLR on it's own (and how I would do it for an 18-200mm lens for example).

The following thread raises even more doubt about that hence interested to know whether anyone has had success with the mask just using a DSLR (not with a telescope).

http://www.skyatnightmagazine.com/forum/september-issue-bahtinov-mask-camera-lens-combination-t100593.html

[Mike]

Sometimes people use a cotton thread made into a cross in front of a lens to produce diffraction spikes. When out of focus there are two spikes when in focus only one.

[Kenny]

Sometimes people use a cotton thread made into a cross in front of a lens to produce diffraction spikes. When out of focus there are two spikes when in focus only one.

I'm definitely going to give that a go...

[The Thing]

I cut a Bahtinov mask out of a piece of black plastic to fit in a Cokin P square filter holder. The number of slots and angles are just what I could fit in the space and cut reasonably easily and accurately. Works a treat.

[MarkS]

Cotton diffraction spikes won't be accurate enough and don't bother with Bhatinov masks unless you can analyse the diffraction pattern via computer using e.g. Bhatinov grabber.  But even then you only achieve focus in the centre - remember you want to get good focus both in the centre and at the corners of the image

Using a scale and taking a series of test exposures (or getting the stars to trail during one long exposure whilst you adjust the focus) is easily the most accurate method by a huge margin.

Have as few marks on the scale as possible and make them easy to see in the dark and always turn the focusing ring in the same direction (hence the arrow) to overcome backlash.



If you take a series of test images, zoom right in and (on the Canon) use the knurled knob to inspect the sequence of shots at high magnification to determine the best.  Inspect both the centre and the corners to determine the best compromise setting between them

Mark

[Kenny]

Thanks Mark. If I change the zoom i.e. from 18mm to anywhere up to 200mm on my lens, will the same focus point work or is a refocus required?

[MarkS]

Assume you need to refocus - not many lenses will zoom and retain focus.