APT Bahtinov Aid Focus Problems

[groberts]

Previously I've just used a Bahtinov mask to get focus with DSLR DSO imaging and the Canon Utilities capture software imaging. Having recently purchased the excellent ZWO 1600MM-Cool camera I'm now using Astro Photography Tool software (APT) and after initially fining the stadard Bahtinov mask + visual technique for focusing did not seem to achieve the focusing accuracy required for this camera, on the advice of this Forum I have been using the APT Bahtinov Aid with mixed success.  Even when I exactly match the two circles that are supposed to indicate good focus, the outcome often still seems to be just off focus at times; I also note that when the two circles are aligned the x3-way Bahtinov 'Cross' is not as it should be.

I'm wondering what are other's experience of using the APT Bahtinov Aid?

Is the 'Circle Method' accurate and reliable enough or is there a better approach?

Thanks, Graham

          

[spillage]

I have had the same issues as yourself. My way of dealing with this was to go back to using the canon live view at 200% then open up apt. I am on the look out for a auto focuser to see if this makes any difference.

You do need to make sure you keep clicking the recalculate button as it seems to loose track a bit.

cheers

Spill.

[groberts]

Thanks Spill but would not think the Canon Utilities will work with the ZWO1600. I know what you mean about recalculating and already do this - according to the result + circles the focus should be spot on but it isn't always.  In time I also will look into auto focus.

The $64,000 question is - does APT Bahtinov Aid work or is the original manual + visual adjustment better?

[AndyUK]

I'm not sure if this will help either of you (I've not used APT, and I don't use a DSLR), but the original "Bahtinov Grabber" is still available as a separate stand-alone application here

 

[groberts]

Thanks I'll take a look but I thought the APT Bahtinov Aid was based on the Grabber?

What sort of camera do you use the Grabber with + do you gt good focus? 

[AndyUK]

I'm afraid I can't answer the question on the APT Bahtinov aid being based on "Grabber" as I've never used it. However...

For data capture I currently use MaximDL, using a Moravian G2-8300.  To date, I've always used Bahtinov grabber as stand-alone software for focusing (maybe 4 years or so?).  Personally, I've found it exceptionally reliable and I wouldn't be without it, although I know others swear by focusing using FWHM (I did try once, but couldn't get on with it at all!)

In short, Bahtinov Grabber has never let me down... (although when using 3nm filters, I do need to increase the exposure to at least 6s, otherwise the "whisker pattern" breaks up which can cause Bahtinov Grabber to go into a flip-flop paddy)

[steppenwolf]

Although I mainly use an auto-focuser on my telescope, there are occasions when I still use a Bahtinov mask - for example when measuring filter offsets for use with the auto-focuser or when testing equipment that doesn't have an auto-focuser motor. Although I have used the 'grabber' that Andy uses (albeit probably an early version), I find that trusting my eye to determine the correct focus point by examining the two angles of the cross sections against the centre bar and ensuring that they match works perfectly well for me. Much as I like electronic/software aids, in this case, I feel that a manual approach is just as appropriate and accurate. With some filters, as Andy mentions above, the pattern can break up (in my case into 'beads') but I find that these beads can actually work in my favour as they close up the gap between the cross and the bar making it even easier to judge that the two angles are exactly equal.

Sorry that that answer doesn't directly address your APT query but the point I am making is that if it isn't working for you then I am not sure that you are losing anything by doing this manually.

[groberts]

Thanks Steve, you've got me thinking.  

What type of autofocus do you use / would be suitiable for my WO GT81 + can anybody direct me to useful background information on how to use autofocus?

Thanks. 

[steppenwolf]

I use a SharpSky Pro auto-focuser on three different instruments (Esprit 150, WO FLT 98 and a Canon 200L lens) and MaxIm DL software to operate it but SG Pro, FocusMax and many other programs that are ASCOM compliant will operate it and indeed other similar systems. As with all of these systems, you'd have to find your own way of attaching the supplied bracket to your focuser but this is usually fairly trivial.

 

[steppenwolf]

1 hour ago, groberts said:

can anybody direct me to useful background information on how to use autofocus?

Autofocusers work by building up a profile of how the star 'size' changes with focuser movement either side of focus. This profiling produces a 'V' curve showing the Half Flux Diameter (HFD) against focuser position either side of focus - the calculated bottom of the 'V' is the true focus position.

Although this link is specific to MaxIm DL, it will give a hint of what I am waffling on about!

[groberts]

Many thanks Steve, looks promising.

[SonnyE]

I don't use my DSLR very much with my telescope. I do use a CCD OSC camera, and for that a Bahtinov mask.

My Camera's program has a zoom setting in it that operates in 100%, 200%, 300%, 400%, and 800%.

So I'm wondering if the ZWO ASI 1600 MM has that in it's software? Zooming capabilities.

I too, am very interested in learning more about Auto-focusing for my Borg-a-scope. I currently have a copy of Tekky Daves focuser that I operate wirelessly. It operates via stepper motor drive and is amazingly accurate, but manual.

But would relish having it run in an auto-focusing mode... But not at that price.

Know of anything else? (All I want is the Hubble, for free.)

I did find this, and wondered if anyone else has seen this Auto-focusing method/software? http://www.optecinc.com/astronomy/catalog/focuslock/

[Wirral man]

sorry to restart this but did you find a way of getting apt to focus as i have just got the 1600mm and am having the same issue with apt

[Yoddha]

1 hour ago, Wirral man said:

sorry to restart this but did you find a way of getting apt to focus as i have just got the 1600mm and am having the same issue with apt

Do you have the image in Preview Mode 1:1? Also take a look in Bahtinov Aid if the pixel/focal length/diameter are set correctly.

 

[Astrobug]

I have this problem when the spikes of my secondary are in the same axis as the bahtinov spikes. Maybe you can give it a try to rotate your mask a bit?

[Jonesdee]

I am also experiencing this issue with an ASI 1600MM-Pro camera with the ZWO mini filter wheel on my 80mm refractor.  My Bahtinov mask is labelled as being suitable for 65mm to 110mm - I assume that this is the range of the slotted mounting screws rather than objective size.  The mask cutout is 60mm diameter and I discussed with Ivo on his forum whether I should enter 60mm or 80mm diameter in the Bahtinov aid.  We agreed that 60mm would be the logical choice for this parameter .  The aid works exactly as expected and the image looks pin sharp with the mask in place.  When the mask is removed, however, stars appear as de-focused airy discs.  

I have not had the opportunity to investigate this further due to more pressing mount problems, but will try changing the diameter to 80mm (actual objective size rather than mask diameter) also will try the mask with the dew shield extended and retracted.  I struggle to believe that this is a camera issue, however I wonder if the close proximity of the filter to the sensor (I focus using Luminence) could somehow be influencing the focus process?.  I also have a 2" LP filter mounted in the focuser, several inches away from the filter wheel.

II will update the thread when I have had a chance to experiment further.

Cheers

Dave

[Stu Todd]

I would say if you're having issues with software Bahtinov masks, try making your own for free. 

http://www.deepskywatch.com/Articles/make-bahtinov-mask.html

I get perfect focus with one I made in 15 minutes for a C9.25. There is a generator tool on the page if your lens/scope isn't already listed.

Compare results with this and your APT tool.

Stu

[Yoddha]

If the spikes are perfectly crossed but the star is donut I would say that the mask is not making correct diffraction pattern...

As Stu suggests try to make one for your scope to see if there is difference

[Pompey Monkey]

I have collected a number of bahtinov masks over the last couple of years. A few months back, I took half an hour comparing the numbers (I don't look at the circles) and there is a definite difference in reported focus between masks. 

However, the differences that I saw were small compared to the amount of de-focusing that is in the original post. I was only seeing a change of about 40 to 50% in the FWHM figures in an actual image: sufficient to annoy me as an imager, but certainly not enough to make donuts.

Edit: the donut stars was from a different thread! Elsewise the post stands! ?