CCD Focusing

[pjphilli]

Hi - I use the Images Plus Canon ED DSLR Control for focusing my Canon 400D camera using the Half Flux Diameter and manual focuser adjustment method. I would like to use a similar arrangement for focusing my CCD camera (Meade DSI2). Is there any general purpose software available to perform this function with a CCD camera? - Cheers Peter

[[1ponders]]

Hi Peter, the quickest and simplest answer is to get a Bahtinov Mask. Doesn't the software that comes with the Meade able do a similar thing?

[pjphilli]

Hi Paul - I have tried a Bahtinov mask but I found that was not much better than my present techniques and a fairly bright star is required. Yes, the DSI2 Envisage software does have a facility whereby the focusing star is shown as a three dimensional "hill". I don't find this of much use and prefer to use the x5 screen magnification to focus on small dim stars. It is just that I am impressed by the Images Plus method of focusing with a Canon ED and hoped that maybe similar software would be available for application with the DSI2. I have found similar general use software but this is intended for controlling motor drive focusers.
Cheers Peter

[[1ponders]]

I used to use Images plus and DSLR Focus (the main one) to focus my DSLR using the same process you are. I found using the FWHM method a bit of a pain with twiddling back and forward to get that sweet spot. I then ignored all those graphs and started using faint stars magnified 6X. Which I found to be more reliable in the longrun, though I would still use the graphs initially until I was confident with the process.

But after using a BT mask for only a short while, the only thing I use DSLR Focus now for, is to display the downloaded image. Yes a bright star is needed or longer exposure lengths, I tend to use about 10 sec but it is soooo much quicker than my previous methods. And I feel just as, if not more accurate.

I did a bit of an experiment one night using DSLR Focus and then the BT Mask. I've been using DSLR Focus since the days of the Canon 300D so I'm well experienced with it. Given that fact and that I know my scope/camera combo pretty well, the BT mask method was still much quicker, even with the longer exposures and as I said I think did a better job of focusing. Remember though I'm talking manual focus, not automated.

Just my experience though. You might have a look at Nebulosity from Stark Labs. I've not used it but I would be surprised if it didn't support the DSI and have the functionality you want.

[TrevorW]

Bahtinov Mask works

tried Maxim, DSLR Focus etc

[Tandum]

Another vote for the bart mask here. Just zoom in on a star and up the exposure time to focus with any filter.

[pjphilli]

Thank you all for your advice. It appears that the Bahtinov Mask gets a unanimous vote so I will give it another try along the lines you have suggested. I have downloaded Nebulosity (which supports DSI) and also give Starks focusing tool a try. Cheers Peter

[JohnH]

Well not quite - I will offer another view - the mask works well for reflectors but is not so good for refractors due to the residual chromatic focus shift. Focus with the mask and bar central may not give the best colour-balanced focus compromise. Not an issue with mono camears of course and easy to compensate for.

[gbeal]

Jiggered if I could use one though, although that is no slur on the actual mask, as operator error was the likely cause.
I went through a stage thinking I would use my Sony DSLR, but the major and really only stumbling block was focus. Long exposure was easy, I own a programable remote, but focus, accurate focus was an issue I couldn't resolve.
I made a Bart Mask, and tried it, but in the end went back to trying to eyeball best focus with a selection of downloaded images. If I could nail this with the B Mask I would look at this again.
In comparison to programs like Maxim, AstroArt, even Nebulosity where you use the FWHM, the B Mask was to me less than ideal. But, I see nearly everyone expounding their virtues, so guess I am the weak link here.
Gary

[bmitchell82]

ill be another bone in the 455 :D

I found the bat mask got me close but not 100% and that the FWHM method was always dead on without any hint of error. you knew when you went past the sweet spot quickly. I am a very scientific kind of person, if i cannot repeat the value then i don't really like it.

For the bat mask you need programs to show you where the exact middle is of the central spike, and you can be off just by a little bit and be way out. Using nice reflectors that have f ratios up around the late 5s and on the bat mask works a charm. but when you start looking at sub F5 ratios your CFZ (critical focus zone) is under 50 micron... as you can see one small step off the line shoots everything. So this is my work flow.

Open up nebulosity (I find it has the best image reproduction of the progs i use)

take a 1 second preview, find the dimmest star. Fine focus
it will keep taking 1 sec photos for as long as you want it to
giving you a read out of Max intensity, and best FWHM to see when have
A. hit that sweet spot and
B. keeping a record so you know when you have gone past it and then back to it :D
Take a 10 second picture inspect it at 200% zoom and if you have a bright enough star, and your focus and collimation are good the diffraction spikes will have a rainbow effect. the sharper and brighter this is the better you are! (that's for the newt owners though :D)


that generally takes me about 3 minutes from start to finish. and a wee bit longer if i want to fiddle a bit more.

[asimov]

Yes, I've been thinking if you have perfect looking spikes you'd be pretty well at perfect focus Brendan. Your post kind of confirms it in my mind anyhow..

I'm also thinking that by making your own 'spike mask' (fishing line/thin copper wire) for a refractor will not only add diffraction spikes for those that just love diffraction spikes in images, but double as a focussing device. Thoughts?

[bmitchell82]

well of course, if you understand how diffraction spikes work they can be utilised to your advantage just like the bat mask.

with diffraction spikes like on a 4 vane secondary holder, it all most looks like a hash key when coming in, those spikes get closer until they visually form one spike. But for me its the colours that i look for that tells me more about my collimation than anything else. When you are spot on then you get red green blue red green blue of course if you have diffraction spikes from one side of the screen to the other that is best :)

[Hagar]

Bahtinov grabber does it for me and when you use it with Bahtinov Grabber http://www.njnoordhoek.com/?p=325 it is the most accurate tool I have found yet.

[Tandum]

I'll have to try this, it's been around for a while. Can you make it big doug? So you can see the numbers from 5 meters away?

[Hagar]

You need to get yourself a motor driven Focuser Robin. It's life on easy street in comparison to doing it by hand.

[Tandum]

Yes I know but the CEO wants a trip to Paris. Astro toys have to take a back seat :(