Focussing Masks

[MorningMajor]

Thanks to the "wedding" I found myself with an extra day off last week, so, rather than dig out the bunting I decided to try something more practical instead. I've read quite a bit on these boards about the various focussing masks you can make so decided to give them a go, I made them out of sheets of card as prototypes with the intention of making a more permanent one when I worked out which worked the best. Each could be attached to an embroidery frame - top of the picture - (I bought at our local crafts shop) by velcro.

The frame, by the way is an excellent way of attaching the masks as they come in sizes from 4 to 12 inches and with a little bodging, fit well on the front of the OTA.

The 4 I made are pictured below, along with the frame.

The top left is the classic Bhatinov using this template generator - it worked reasonably well, though the diffraction spikes were quite dim - but this was probably due to the poor seeing conditions - but I certainly could focus quite well.

The one, top right, was copied from ASIGN_baz's excellent ASIGN Site

It was pretty easy to make and produced excellent results - when out of focus, you see three distinct stars and just adjust the focussing until they become one - really easy, really effective.

The bottom two were different size versions of the Y mask as I have read so many people praising them - sadly neither worked - no spikes and nothing that helped focus. Now that MUST be down to me as they are so popular - looking at them have I made them wrong - the angle between the arms of the Y are 40 degrees - which I thought I'd read somewhere as being right - but I was wondering is they should be wider - 120 degrees?? Or, are they too thick/thin - the first ones are about 4mm and the second about 9mm

Would love your thoughts/comments/help - so far the one with the triangles (don't know official name) wins hands down - but I feel I'm missing something by not getting the Y mask to work,

Clear skies!

[virtualpilot45]

Just cut out the triangle mask, it's very easy to make, and I can't wait to try it out tonight...

[Weebz]

I had some trouble getting results with the Bahtinov, will try the triangle mask next. Thanks for the tip

[Glider]

A good explanation on this site, Bahtinov mask

[Revs]

I tried a Bahtinov mask for the first time last night and it worked very well indeed. I printed out an image, generated on the site linked above, and simply cut it out with scissors. It just sat nicely in front of the objective.

[Digz]

The 'Y' shape mask looks about right to me. I've yet to try one myself but like you I have seen people rave about them.

I wonder if they may be too thick rather than too thin?

There is some sort of formula you can use, have a read through this thread see if it helps (sorry if youve already done this.)

http://stargazerslounge.com/beginners-help-advice/134838-bhatinov-focus-mask.html

I think the gist is multiple your scopes focal length by 3 and then divide by 150, the resultant number should give you the width of the strips. I think around a couple of mm is usually good enough, but thats just my speculation.

HTH

[Revs]

Not really sure from the pic, but the 'Y' doesn't look quite central to me. I suspect they would need to be perfectly aligned and centered with the tube to work properly.

[Digz]

Im no expert but Im not sure whether the need to be centered or not.

Take a look at the attached. I cant remember where I got it from but its an interesting read.

Fraunhofer-primer.pdf

[MorningMajor]

A good explanation on this site, Bahtinov mask

A VERY interesting article, thanks Glider - sadly I couldn't work out why my mask didn't work - never mind, the triangular one is excellent, so I reckon I'll stick with that

Cheers

[MorningMajor]

Not really sure from the pic, but the 'Y' doesn't look quite central to me. I suspect they would need to be perfectly aligned and centered with the tube to work properly.

You could be right - but I would have expected some sort of diffraction spikes - even if it didn't produxe perfect focus - but I will have another go i and if that fails, as I said the triangular one works really well

[MorningMajor]

Thanks Digz, I'd seen those articles - they are good aren't they - even if they push the limits of my rather non physics like brain!!

I guess experimentation is one of the joys(?) of astronomy, so will try another version - and who knows the skies might clear so i can see the results!!

[drmillrace]

I used a home made Bahtinov on my Skymax 127 earlier this week and was very pleased with how easy it was.

Must try the triangles too. Also the embroidery ring is an excellent idea.

[MorningMajor]

Also the embroidery ring is an excellent idea.

I stumbled on that through something I'd read, so ventured to the crafts shop - which is really not my domain. A rather austere lady asked if she could help, so I calmy asked for a cross stitch frame, she then asked me what kind of cross stitich i was considering!! - Damn, cover blown at the first hurdle, I then proceeded to tell her that I wasn't particularly interested in cross stitch but wanted to mount a Bhatinov mask on my telescope - her eyes took on a strange distant look - I wish I hadn't said that - anyway, she dutifully pointed me in the right direction and I could only find a 5 or 7 inch one - I asked if they did them in 6 inches and she said, rather wearily, that they were on the other stand which held all the even numbered frames - oops - Never mind - perhaps I should get FLO to stock them!!

[Gina]

Very interesting

[JamesF]

Never having tried any mask at all, I made my own "Lord Y" for my Mak using the first order figures. It's pretty well bodged together, but it certainly does work -- I get a flattened cross diffraction pattern with a horizontal line that moves up and down as the focus is moved in and out. The diffraction pattern isn't very bright, especially when used with my SPC900, but I'm guessing that it could be brighter than it would be if I used a Bhatinov mask which would obscure more of the tube.

I have been meaning to try a third order version of the same mask, but haven't got around to making that yet. For the sake of completeness I really ought to make a Bhatinov too. If I get that far I'll try to post comparative webcam shots of the diffraction patterns from all three.

James

[glowjet]

Hi Sim, if you want to calculate the dims for your mask, simply divide your FL by 150 and 200 (Bhatinov`s formula ) this will give you a max and min width tolerance for a space plus bar, whatever dim you decide to use the result must again be halved to give the actual bar width. if the result is to small to make for all practical purposes then you can resort to what is called a third order mask by multiplying your FL x3 and repeating the maths, It must be remembered and pointed to all who are contemplating a mask, that the original intention of the design and the informative paper written by Chris Lord on the Y mask, is intended for imagers in assisting to bring a webcam or digital SLR to focus on a star, the best results will always be seen in the live view of an SLR or resulting web cam test images. Perhaps I should also add that a steel straight edge and the use of some black plastic material like a document folder should perform better than coloured card chopped out with scissors, I did happen to mention embroidery frames some time ago as these are an excellent way to attach you finished product HTH

John.

[Psychobilly]

The triangle one is a variant of the Hartman mask.. the straight edges of the triangles generate diffraction spikes that you wouldn't get with the circles of the traditional Hartman...

For imaging use I prefer the precision of the Bahtinov/Y mask...

Billy...

[MorningMajor]

Perhaps I should also add that a steel straight edge and the use of some black plastic material like a document folder should perform better than coloured card chopped out with scissors, I did happen to mention embroidery frames some time ago as these are an excellent way to attach you finished product HTH

Thanks John, I did point out in my original post that these were protypes and I was going to construct a more permanent version of the most effective mask. As I also said I had read somewhere that the embroidery masks were an excellent way of attaching the mask to the OTA - it must have been from one of your posts - apologies for not giving the credit

[chemtom24]

can someone help me by telling me what exactly you look through when focussing with one of these masks? i understand they are for focussing before attaching your dslr yes? but what ep should you use to focus with, or shouldnt you, because surely the focus will change once you remove the ep and put the dslr on? just a little confused on this...

[Digz]

Have a look at the attachments I made above, they show what you would see using various mask types.

The whole point, however, is to use them with the DSLR because if you focus with, say a 10mm EP the focal point will be different than what your DSLR needs. As you say above.

HTH

[Glider]

I look at the camera Liveview if it has it or take short exposures and view them on a PC using AstroPhotography Tool.

[chemtom24]

thankyou. so with a 400d without live view, i will need to take a shot, download to pc, have a look, then adjust as necessary? is it possible to have the 400d connected to pc whilst using it, to have 'live view' on the pc if you will?

[Glider]

No, the 400D doesn't have Liveview. AstroPhotography Tool will let you sequence short and long exposures and download to a PC view viewing.

[chemtom24]

wow, so you can adjust settings on ur camera by using the astro tool, then use the tool take a pic? i have a remote fro the dlsr, so will i not need it in conjunction with the prog?