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M13 1200 taken on 9th/10th Aug 2011 from my back garden in Southampton in less than ideal conditions. Bahtinov mask has been ordered ;-o


LDUNN1


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M13

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Bahtinov mask has been ordered :)

In the meantime, check out the Hartmann Mask - I find it a LOT easier to use (and construct for oneself!)

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;-) it does look like that doesn't it! I am amazed on how '3d' it looks, to my eyes it really does look like a ball of diamonds.

Just wish I had nailed the focus, USM can only get you so far......still in a week or two I should have the focus mask, so that will help, I have also ordered the SW electric focuser, don't know if I'll be able to modify it to work on my 'scope, but it seemed like it was worth a shot.....scope is hard to focus as image wobbles like jelly when I touch the focus knobs.

Last night was really just meant as a 'shake down test' of my kit after some major re-arrangements due to the issues I've been having, so any imaging was just a bonus!

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Cheers Martyn - I had not heard of that type of Mask b4......i just googled it & found:-

"As the images merge as you get close to focus, the exact point of best focus can sometimes be difficult to determine."

"As the aperture is stopped down with the mask, the focal ratio increases. A 4 inch f/8 scope will become a 2 inch f/16 scope with a Hartmann mask with 2 inch holes in it. As the focal ratio increases, so does the depth of focus. So focusing with the Hartmann mask is less critical than the actual focus when the mask is removed. This can lead to a false sense of security and accuracy. "

"I don't really recommend using a Hartmann mask anymore since the development of the Bahtinov mask, which is superior for helping attain focus."

........interesting, as I had never heard of this b4, but I am glad I'm going with the Bahtinov!

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Bahtinov masks aren't that difficult to make; plastic from A4 "wallet/folder" (WHS), sharp knife/scalpel, and Robert's your mother's brother. Might do until you get a bought one.

I did use a Hartmann years ago, but gave it up in favour of finding a star visible among the branches/leaves of a tree (just merge the images as you do with a Hartmann) -- much better, and free!

Nice thing about your out-of-focus star is that it proves you have a triplet apo -- the bright star looks like an old RAF roundel, as opposed to the "apple & plum" colours you get with a doublet. :-)

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"Nice thing about your out-of-focus star is that it proves you have a triplet apo"

......ahhh, my secrets out, the out of focus was intentional ;-)

......It has proved a useful test actually, apart from the OOF issue, the full image shows only a little vignetting around the sides/corners......& I suspect that most of this is due to the adapter used to join camera to scope. The full image was very usable except for the very outer edges, & this is one reason I went for this type of scope - it is one of the few that can fully illuminate a full 35mm sized negative (it actually can do better than that). I have mailed the 'scope manufacturers as they do special adapters for Canon Eos cameras that should eliminate any vignetting due to the adapter I am currently using. They also do a telecompressor (0.75x) & field flattener......The compressor will come in handy, as the scope is relatively slow/long at f9, but I don't know if the field flattener is worth going for yet......although it does seem to me that there are some mis-shaped stars towards the RHS of the image.....but that might be indicative of a collimation issue????

It is funny that you saw RAF roundel's, I saw donuts ;-)

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I'm still a fan of the Hartmann. Everything quoted re its faults is no doubt true but bang for buck I like it best. Purely personal, but I have made Bahtinov's and cannot get on with them. However I AM using a 5D mk1 (no live view) and ordinary lenses exclusively, which may explain why I can't focus with one! Plus I wear glasses and my old eyes probably just can't cope!

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.......oh yeah, btw Martyn, I DO have liveview on my camera, & I STILL can't get focus right ;-)

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"but I don't know if the field flattener is worth going for yet......although it does seem to me that there are some mis-shaped stars towards the RHS of the image.....but that might be indicative of a collimation issue????"

Do you want to put a nice bright star in that region, focus it as well as you can, defocus it so you get lots of diffraction rings, then defocus the same amount the other way, take an image of each, and then we can see what the patterns tell us.

"It is funny that you saw RAF roundel's, I saw donuts ;-)"

Very funny indeed, especially considering that you're an engineer and I'm overweight! :)

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......I'll wait till I have a bahtinov mask before I do that test....just to ensure I get the infocus one right!

......I will do this though, & would really appreciate the analysis - if indeed there is an issue.

......your closing line made me laugh out loud - I hadn't thought of it that way!

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