In this article, I hope to explain how to make a very inexpensive Hartmann
mask to aid you in focusing your telescope for imaging.
I obtained a cross-stitch hoop and a black "Foam Sheet" from the
crafts department at Wal-Mart for a total of about $1.60. The foam sheet is
about 3mm thick and about 99% opaque (If I put the foam right up to my eye and
at a bright light, I could barely detect light going through it. For our purposes,
it will be just fine.) I decided on the 9" cross stitch hoop as the scope
is a little bigger than 9" in diameter on the outside of the OTA. So if
you have a 10" scope, you probably need to go to 11".
Materials for the Hartmann Mask.
Cutting the Holes
If you are as fortunate as I am, you have a spouse that is a scrapbooker. I
cannot complain, as I have found useful items from her stockpile of stuff for
my interests. She has a handy device called a Fisker's Shape Cutter. I used
the template that contains various sized circles as that is what I wanted to
use for my mask. There are other templates you can use for other shapes. If
you don't have access to one of these handy devices, you could use the bottom
of a soda can to cut around with a hobby knife.
Using the shape cutter, I was able to cut about half way through the foam.
Using the Fiskers Shape Cutter and circle template.
I had to use a hobby knife to follow the cut made with the shape cutter to
go all the way through the foam. It worked very well.
Finishing the cut through the foam.
Another view of the circles being cut.
Cut the other two holes in a similar fashion. You may have to check your work
by putting the cross-stitch hoop on top of the foam to make sure your circles
are placed appropriately.
Mounting the Mask to the Hoop
I attempted to use a hot glue gun to attach the foam mask to the hoop. However,
I found that it did not work very well. So I settled on using contact cement.
There are probably other methods of attaching it, but I found the contact cement
to work the best. I applied the contact cement to both the foam (around where
the hoop will be) and the top edge of the hoop. You may need to somehow mark
the outline of the hoop on the foam to get it right. I eyeballed it.
Contact cement that will be used to apply the foam mask to the hoop.
Contact cement applied to the foam mask and the hoop.
Let the mask and hoop sit for about 20 minutes to let the contact cement set.
Once it does, set the hoop, contact cement side down, on top of the foam mask,
contact cement side up. Set a phone book or other heavy item on top of the mask-hoop
combination for a while to let the contact cement bond completely.
After the contact cement bonds, trim off the excess foam around the outside
of the hoop. The mask is now ready to use. You could paint the wood hoop black
or apply some black tape around the edge to make it look better, but it does
not really matter.
You could use a plastic cross-stitch hoop as well. It would be water proof
and more durable. There are plastic sheets that you could also buy that would
be adequate to use as a mask. The important thing is to experiment and see what
you come up with.
The Hartmann Mask in Action
Here is a photo of the mask on the front of the telescope.
Here is a focus sequence of the mask in action at an imaging session.
Good luck and happy imaging!