focusing masks, bahtinov/carey

Dave

I'm not advocating that having less than full-aperture has any benefit per se apart from availability of materials. My mask is produced from the same software that you refer to - but I'm not particuarly good at any level of DIY so it took me a while to find something of approximate size to cut that I would feel happy cutting that would have some rigidty and would be durable - and a mouse mat worked for me. As it happens length wise was a good match and the short dimension was a tad small but I suspected that it didn't really matter. I also factored up the size of my bars and gaps by a factor (three I think and I got that number from the same website as the tool to generate the mask). I did this because I didn't want to be cutting too many slots - and it works fine.

You're right about it being finicky doing small masks - I've tried a few times for my 70mm scope and haven't had success yet.

I'm glad yours worked out I find it works very well - I think its magic.

Mark

Hey Mark
From my understanding of diffraction i believe you are correct in that the diffraction patterns are independent of aperture, however the intensity of energy spread out into the spikes and the size of the airy disk will be affected by aperture and focal length. IMHO I would expect that this has little effect on focusing with the Bahtinov Mask.

Mike

I suppose the proof is in the pudding.
I took an avi sequence on sep19 using the bahtinov mask for focusing and this is what I got.
farm3.static.flickr.com/2482/3976815840_cae2345fc8_o.jpg

12"LX200 + 2xbarlow + toucam pro
None of the individual frames looked anything special yet the resultant image was alot better then I thought it would be
Its not up there with the real planetary camera shots, but it'll do for now.
I took a similar image a few days before without the mask and it doesnt compare to it at all.

Now I'm interested in these mask again. (actually before the jupiter test, but even more since. I have a few good pics of Saturn, and one ace pic (8" LX200, web cam) I always put the ace one down to an exceptional seeing and transparency night (which was true). I now wonder if that night also allowed me better than usual focus.

Anyway: I want 254mm, 127mm, 80mm, 54mm masks.
The issue of materials has concerned me. For the 254 and 127 masks the black plastic students folder covers seem a good choice (will confirm later, metric will be successful mask + finger cuts) as a balence of rigidity and ease of cutting. {This is not my idea but one from John Murphy}

However for the smaller slits it looks too hard to cut. I'm given to wondering if some flimsy very thin material will do if it is held in a rigid frame. I'll try later and report back.

Not much else to do this week-end

Steve

Hi Steve,
i used normal A4 paper and double laminated them, that made them rigid to then use a crafts knike to cut it out.
It takes a few hours to do it carefully, but like you say the wetaher is bad and there little else to do anyway.

Dave

That's a good sharp image - these masks work well.

One observation - there appears to be a bit of a venetian effect in your image - a bit like a raster scan effect. I don't think its pixelation as it appears to be in one dimension only and the toucam pro has square pixels. Do you know what's causing this?

I think it was Carl O'Beirnes, in an older thread, suggested that venetian effects can happen when using Registax and if the gain was too high during capture. I haven't come across this phenomenon personally and don't know if your image is a manifestation of this - but maybe something to think about. If I recall Carl suggested setting gain to about 60%.

Mark