focusing masks, bahtinov/carey

Hi,
I was doing abit of digging around for these masks for my 12" scope and found them abit expensive, I came across the following site which allows you to print out the mask and you cut it out yourself.
astrojargon.net/MaskGenerator.aspx
All you need do is fill in the details for your scope, it generates a .svg file,
If you cant open the file, go to the following site and download inkscape
www.inkscape.org/download/?lang=en
This program will print out the mask to proper scale on an A4 printer,

I did this for the 12" and had to print out to 2 sheets, I then double laminated the sheets making them someway rigid, then then came the very tedious 2-3 hour run at cutting out the slots with a good sharp crafts knife.
i'll be trying it out on the next clear night, I'm considering making one up for the 20" but I'm not sure it'll be able to support its own weight, so I'm looking out for a rigid yet easily cut material, maybe thin black cardboard like the type you'd see in an arts shop.

Another mask worth considering is the Carey mask.
www.btinternet.com/~gfcarey/careymask.htm
although it looks alot harder to cut out!.

For a good rundown on how bahtinov mask work, see
www.btinternet.com/~gfcarey/bahtinov.htm

Anyone seen other masks out there, besides the usual hartmann types.?

I found this site which is UK-based and appears better value for money than other sources that I have found.

www.stayfocused.co.uk/index.php?act=viewCat&catId=17

The Bahtinov mask I made for my 8" reflector was made from cutting a mouse mat - cuts entering through picture side - and framing in a picture frame. The foam on the underside of mouse mats don't cut well but I found that using a candle I could smooth the edges. It warps the mat hence the need for the picture frame. I'm happy with the utility of mine.

Making a mask for my ETX 70 OTA by successfully cutting multiple small gaps in any material with precision has eluded me. I don't think the number and width of strips is anyway critical - though I suspect that they should be consistent. When I have another go I'm thinking of making a Y-shaped frame within some outer frame and threading wire (guitar - or strimmer wire for example). This might be easier for very small and very large scopes.

To add to the diversity of masks there is the chicken wire mask
www.zodiaclight.com/equipment/chickenWireMask.htm

There is also the combined Bahtinov / Hartmann where three triangular apertures are cut with strips such that the strips are at, say 60 degrees, orientation from each other aperture - A quick search didn't find an example of that one but I did find reference to it in the past.

Mark

BTW I don't see the necessity in having a mask that covers the full aperture. I believe that making a mask for a stopped down scope will work just as well - providing that operating aperture provides enough light to see the difraction spikes. My mouse mat is a tadge smaller that full aperture - I just block off the unmasked aperture. It was this experience that made my think of this.

Mark

Hi Mark,
I was wondering about that, I see also that some masks have only a small number of gaps while others seem to have gaps all along the diameter of the mask, I suppose the differance to the eye is a differance in star brightness through the eyepiece.

Sorry Dave - for whatever reason I missed your last post on this thread.

I still believe (and maybe I should say strongly suspect) that a smaller mask than full aperture should work. However, I had to think about this after my post.

My mask is definitly a wee bit smaller than full aperture - and it works fine - as far as I can tell. I've tried it with the view of maximising the quality of captured images but also of optimising my visual experience and I'm happy with what I get. However, I subsequently thought that with smaller aperture one gets greater depth of focus (more tolerance) and therefore maybe this belies the situation when the smaller aperture mask is removed.

I'd love to put this to an objective test - I still have strong suspicion that what I said is true and the basis in this is that I see difraction paterns as being independent of aperture - but I could be wrong in that.

In the absence of other reports to support or deny my suspicion I woud like to experiment with this myself - but as stated earlier I have diffulty in making a smaller Bahtinov mask than the one that I made but if I can confirm or deny I will follow-up.

Mark

Hi Mark,
I'd suspect that making a small mask would be very finicky, I'm not sure I understand why you are going down the route of a smaller mask, do you mean a 6 inch mask on a 8 inch scope of some sort of off-axis setup ?
The image you see using the mask is not affected by seeing from what I can see. Keep in mind that the diffraction spikes are brighter the more light you let into the scope, a reason to go full aperture.
another point to think about is that the size of the bars and gaps are dictated by the size and focal length of the setup you are using (going by the program used to generate them).

Making the mask for the 12" was painfully tedious and took nearly 3 hours to do, what a pain, but was it worth it ?
I tried it tonight and boy oh boy, this thing really works!
With the webcam on the planets, I now consider this thing as an essential piece of equipment, I focused as best I could visually a few times and each time the mask showed I was very slightly off each time, focus of the planet after the mask was used is bang on.
With a canon300d, its not so easy to see it through the view finder and what I do is take a shot then enhance in in photoshop and see whats happening.
Visually through an eyepiece, a 5 year old could focus the scope using this thing, its that easy and obvious!
Here is a pic of Altair with the 300D focused with the mask and enhanced in photoshop so you can see it clearly.

That guy Bahtinov diserves an award !!