517 replies to this topic

[Gus_Smedstad]

I like the plastic lid idea, though I doubt I'm going to find a 9.25" diameter lid (outside diameter of my 8" OTA).

I did discover that while a Dremmel will cut through foam board very easily, it's also almost impossible to control.

- Gus

[Nocturnal]

I think that constructing a mask like this using strips of opaque material like wood or plastic is a realistic option. Hobby stores probably have thin wooden strips for model building in all kinds of widths. Use a stapler to instantly connect the strips to a central support. Glue works too of course but that takes time to dry. With staples you can space the strips with a spare piece. If you absolutely gotta have aluminum you can get strips of aluminum of various widths at hardware stores. Stapling in out and I think drilling and riveting might not be accurate enough unless you're skilled. Self drilling screws are an option too but the strips need to be clamped firmly.

Options options. Mostly because it seems the gap size really isn't that important.

[cvedeler]

I did discover that while a Dremmel will cut through foam board very easily, it's also almost impossible to control.

- Gus

A Dremmel is WAY overkill for foam board. I can just picture the foam flying everywhere! The easiest way to cut foam board is with a razor blade. My ex-wife does quilting and for fabric they have a rotary cutter. Basically a circular razor blade you can run along a straight edge. Makes perfectly straight cuts on fabric and foam board. I used hers to make my light box out of foam board. The only tricky part would be the ends of the cuts. Perhaps you could cut it to within an 1/8th of an inch and finish the cut with a utility razor blade.

Lots of great ideas! I may be looking for my next generation using foam board. Hmmmm.

[jgw12936]

Adam,

I'll let you know how it goes, I have everything set up and ready to go, and my clear sky clock looks perfect!
Jim

[Joe G]

Does anyone think that you can print this out on that clear transparency paper (plastic) that teachers use for overhead projectors?

That way you wouldn't need to make any cuts. Just print it out and tape it to the end of the scope. Would the clear plastic cause any problems?

[John Wunderlin]

Does anyone think that you can print this out on that clear transparency paper

Back a few pages, I posted that I tried this approach. You can see the spike pattern, but it's distorted a bit. When I added the CCD imager to it, it was unusable. However, I was just hand-holding it. If it's mounted in a rigid frame it may work better. Unless the optical quality of the material is better than what I used, you really need to cut out the clear parts.

[Joe G]

Thanks. It's a long thread making it tempting to skim through much.

[Old Dinosaur]

I'd think it would work best if whatever the material, it was finished in flat black. And if, at least the sides, of all the slots were as smooth and straight as possible for the sharpest diffraction pattern.
I know all the masks I've made over the years worked best with smooth sharp edges for any of the hole patterns I've tried.

[raf1]

Hey Jim - very nice design and I noticed that you're also from the Lone Star state. What is it that gives us Texas boys that flair for elegant design and cutting edge vision for manufacturing? I dunno - just blessed I guess. As you can see, I've raised the construction bar a bit so don't take offense at this new entry in the Bahtinov mask design contest. Introducing the “South Texas Chinet Focus Mask”. That's right folks, that cellulose fixture at all Fourth of July horse shoe throwin', washer pitchin', dominoe shufflin', barbeque eatin', Shiner Bock drinkin' family tradition get-togethers! The Chinet paper (actually cardboard 'cause nothin’s too good for the family) picnic plate. Ok, I know you're dyin' for the details so here we go. Downloaded Inkscape and used Adam's mask file (thanks Adam!). Printed it out and Scotch taped it to the aforementioned construction material. I then used the sharpest skinnin' knife I had and cut out (actually chopped out) the mask using Adam's template.

As you can see it is a quick and dirty prototype so I’m really pushing the limit on tolerances required to achieve successful focus. I’ll be reporting back on my "first light" results as we are expecting some pretty good local weather this weekend. Seriously though, many thanks to Pavel for the idea, Dennis for the news and Adam for the scalable vector drawing. This has really been a thoughtful thread and I’ve learned a lot from you guys so “thanks” to you as well.

Ron

Attached Thumbnails

• 

[Nocturnal]

I'm curious if instead of slats you could use wire spaced at the appropriate interval. I've always been amazed at the diffraction pattern you get from fishing line in a cross. There's going to be some weather the next few days so I won't be able to try this out. If someone else does, let us know how it goes

I imagine a frame from foam board would be rigid enough to allow enough tension on the wires to keep them straight.

[Gus_Smedstad]

Mine, I'm afraid, is remarkably ugly. It's not just that I can't cut a straight line, it's that I printed out the template on a full-sheet label, and for whatever reason the black toner just didn't fuse properly. So I've got this beaten-to-hell look on a brand-new mask.

The circular area is 208mm in diameter, slightly larger than my aperture, and the slats / bars are 3mm each (since S is 5-6.7mm for me). The entire thing is made out of foam board in a 9.25" square, since my outside diameter is 9.25". I have some vague plan of gluing tabs on the edges to keep it in place.

I didn't see any point in making a circle to match my central obstruction.



- Gus

[cvedeler]

Gus, it looks better than mine!!! It isn't so much how it looks but how it works.

You could always shoot some black spray paint over it.

[Gus_Smedstad]

This is the template I created in Photoshop - it's designed for a 1000mm f/4.9 telescope.

In retrospect, it might be smarter to reverse that, so the parts to be cut out are black and the remainder is white.

[Mark Sibole]

I took 1 of the images posted and resized it for an 80 mm.
I printed it out and laminated it then cut it out.
This was just a test to see how well it would work.
Focus is very easy with it and it worked very very good.
Eventually when it wears out Ill redo it in plastic.
Thanks for starting this topic.

Regards

Mark

[jgw12936]

Adam,

The refractor template works perfectly, took about 20 seconds to focus! Thanks for the template.
Jim

[snowdragonusa]

Adam,

The refractor template works perfectly, took about 20 seconds to focus! Thanks for the template.
Jim

AWESOME! Thanks for the feedback Jim. Glad it worked out for you.

[raf1]

Update on the Chinet mask - absolutely awesome! I worked for about 15 minutes to focus the old-fashioned way and thought I was really close - I wasn't. The "spikes" were very clear with the mask and I could easily see movement toward focus. So, for those of you who want to just whip out a Q&D mask to check this out, don't worry about getting the cuts straight or the correct spacing initially. The mask design is very robust and will overcome deviations from perfection quite well. If you look at my mask in a previous post, you'll see what I'm talking about. Looks like I'll be searching for someone to create acrylic masks for my refractors. Again, thanks to all who have contributed to this thread. BTW, I used Alpheratz as my focus star (2.06mag)

Ron

[raf1]

Adam - your template worked great. Thanks again.

Ron

[snowdragonusa]

Great Ron! Glad the Chinet Experiment worked (wasn't that a movie?)! DOn't forget to keep that plate away from the others. I'd hate for someone to mistake it for a regular plate and have their BBQ ribs go sloppin on to their lap!

I just received my two masks from the cutters and they are beautiful! I had come up with the refractor modification after I had placed my order so there was no changing. I had them cut some tabs for me as well which I am in the process of bonding. Here are a couple of pictures.

The little paper mask was my demo to see just how well focusing would turn out as I mentioned in a previous post.

Attached Thumbnails

• 

[snowdragonusa]

This picture shows how the tabs allow the mask to fit over the scope's dew shield. I plan on adding some small pieces of felt to the tabs as well for a firm fit and scope protection.

Attached Thumbnails

• 

[raf1]

Adam, those look great! I still can't believe how well the Bahtinov mask works. Heck, mine looks like it was made with a chainsaw and it still works great. I wish we had discovered this years ago. All those images to do over!

P.S. - pretty close on that movie title - China Syndrome. And I'll only let folk use the mask for dipping beans out of the pot - darn, I wonder how that would work. Gotta go and file that paperwork to protect my rights to the Chinet Focus Mask and Bean Strainer. Now that's cool.

Have a great weekend.

Ron

[Mark Sibole]

Here is an image using the focus mask.
Focus is to easy with it.

Regards

Mark



http://astronomy.qte...1-SiiHaOiii.jpg

[Joe F Gafford]

All, be wary of using foamboard for use outside. It may warp on you. I have a piece of black foamboard I use as a light block on the back of my 10" Newtonian. It has warped over time due to heat of the day and the dewing at night. Foamboard is plastic foam with paper coverings. The paper and adhesive are the culprits here, especially if one side is more to the weather than the other. The curves that form are permanent. As with my light block, creases were formed when I readjusted it to fit the back of the scope. No problem there, but may be a concern with this kind of mask.
Joe.

[jgw12936]

Ron,

It must be something about Southeast Texas, we see something that looks good and we just have to try it for ourselves, I have to admit, your reverse engineering with the highly technical Chinette plate was genious! Glad it worked, and as Adam said, watch out for the BBQ
Jim

[btieman]

I have a 2800mm fl scope. I often image with a 2x barlow and I sometimes image with a 3.3 fr.

924mm ~= 4-6mm
2800mm ~= 14-18mm
5600mm ~= 28-37mm

I need 3 different masks right? I don't understand the angle relationship--do the 3 need different angles as well?

Anyone starting to get an idea of how far off your focal length can be before problems start?

Brian