Focusing Device: Hartmann Hat

Hi guys. I'm trying to work out how a focusing device used on the
telescope I'm using works. Basically it's a flat disc with two large
circular holes one above the other, which fits onto the front of the
telescope. I've been told this device is called a "Hartmann Hat" but I
can't find any mention of sucha device online.

I'm assuming it has a different name - does anyone know what it might
be? Failing that, any theories on how it works?

To focus the telescope we put this "Hartmann Hat" on the telescope
whilst it is pointing at a 5th magnitude star. When out of focus we get
two distinct blobs from the star. We alter the focuser settings on the
telescope until these two blobs merge and the flux from the star is
maximised. Then the hat is carefully removed and we can begin.

Kind Regards,

Matt

On 16 Jan 2007 08:57:12 -0800, "Matt" wrote:

Hi guys. I'm trying to work out how a focusing device used on the
telescope I'm using works. Basically it's a flat disc with two large
circular holes one above the other, which fits onto the front of the
telescope. I've been told this device is called a "Hartmann Hat" but I
can't find any mention of sucha device online...

I'm assuming it has a different name - does anyone know what it might
be? Failing that, any theories on how it works?

It is much more commonly called a Hartmann mask. Search on that and
you'll get thousands of hits, and many explanations of theory and use.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Matt wrote:
I've been told this device is called a "Hartmann Hat" but I
can't find any mention of sucha device online.
I'm assuming it has a different name - does anyone know what it might
be?

A Hartmann mask for an SLR lens and other focusing techniques
(including star trials and software metrics) are shown on Lodriguss's
focusing method page:
http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM

Lodriguss substantially re-wrote this page since I last looked at it in
the summer of 2005. He has a couple of new techniques for focusing
digital cameras that may warrant reading by advanced lurkers here -
most notably using a live feed of a Canon 20Da camera to a laptop or a
"watchman".

Hissong's how to photos for a Hartmann mask-
http://www.cloudynights.com/item.php?item_id=518

Weasner's variant for a telescope:
http://www.weasner.com/lxd/techtips/hartmann-mask.html

Paul Gitto's variant - two holes and a triangle
http://cometman.com/Mask.html

I have three - one for an SLR lens, one for a 5 1/4 refractor and one
for a 10" Newt.

Hartmann masks cut down aperture and reduce the maximum magnitude you
can reach for focusing. For faint fuzzies, you may have to focus on a
nearby bright star and then slew to the target.

In general, the Hartmann mask will get you in the focusing ballpark for
parafocal or a rough imaging setting. Then you use software metrics
from your astronomical CCD camera to fine tune the setting.

- Canopus56

P.S. - Another mask that I fabricated is mentioned by Lodriguss - a
diffraction spike mask. I used 1/4 wire mesh from a hardware store and
fabricated a padded circle that fits onto the 10 inch Newt. The wire
mesh creates diffraction spikes around the stars, which can be used as
a focusing aid.

canopus56 wrote:

http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM

Lodriguss substantially re-wrote this page since I last looked at it in
the summer of 2005.

Thanks for pointing this out. Reread it and learned some valuable things.