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It works and then it doesnt, sound familiar!?


Rustang
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It seems this is the case with lots of aspects of this hobby! I really wish one thing would just stay working so I could concentrate of all the other things to improve on. Anyway I thought it would be easier to start a new thread in regards to this issue. I have since day one of starting out doing astrophotography, used a bahtinov mask to focus. Ok so there might still be room to improve of focus but until now Ive been ok with the focus of my images. I now have a knew scope, WOZ73 with a built in B mask in the lens hood. First light on Friday just gone, took my time with the mask to focus, got focus and started imaging (see image below). Because I have always done this and its been fine, the stars appeard the size I'm used to seeing so I just carried on. Its not until you start to zoom in you can see that its not in focus. I took an image straight after the mask was removed and that is out of focus when you zoom in and contiunes to be that way through all of my data. So why? should I be worried about Collimation of the new scope? what else could it be? I know lots of people use a mask and get on just fine. Valv has mentioned about using the FWHM of HFD values on the APT focus aid, from his help and info I know the figures fluctuate and they need to be small but I dont know how small to know I'm in good focus. I have attached below 3 subs that I opened in APT and opened the focused aid on them, they have different sets of numbers so are any of them showing good focus or not? if they are then that's probably not good I guess and could still mean an issue with the scope!?

 

 

 

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Edited by Rustang
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Did you eye ball the bahtinov mask focus? To my eye the centre line looks too high (but very close) but if you used the Bahtinov Aid in Apt then it's my eyeball at fault 😉

When not using auto focus I've found the Bahtinov Aid in Apt to produce good results but perhaps not as consistent as a good autofocus routine. 

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Posted (edited)

Thanks guys, purhaps my eye isant as good as I thought if two of you think it's out. I used Polaris, just been lazy but I've used it many times and my images have been sharper. I don't have the option to zoom in anymore as it's a CCD camera. I've not used the APT bahtinov mask so could try that to improve on. It's still strange as I've always just eye balled it, but I guess I've got it wrong this time I guess, maybe I should book an appointment at Spec Savers! 😁

Edited by Rustang
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Posted (edited)
17 minutes ago, Astro Noodles said:

Hi Rustang. Which star did you use for focusing? It wasn't a double star was it?

Nope, Polaris! I should probably use a brighter star to see the spikes better! 

Edited by Rustang
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5 minutes ago, Rustang said:

I don't have the option to zoom in anymore as it's a CCD camera. I've not used the APT bahtinov mask so could try that to improve on.

Apt has another tool that lets you put a box over an area and get x3, x5 and maybe something higher (I think I have that right but am going off memory)

The Bahtinov Aid is worth using and those spikes look clear. The WO masks with the clear material do seem to create bright spikes. 

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17 minutes ago, geeklee said:

Apt has another tool that lets you put a box over an area and get x3, x5 and maybe something higher (I think I have that right but am going off memory)

The Bahtinov Aid is worth using and those spikes look clear. The WO masks with the clear material do seem to create bright spikes. 

yeah it was nice to use, I will give it another go using any APT aids I can and try and get it better. Do you know what values mentioned above are low enough for good focus so I can use that also to help check through the night?

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37 minutes ago, Rustang said:

Do you know what values mentioned above are low enough for good focus so I can use that also to help check through the night?

Sorry, I don't.  It will only be testing out of focus, in focus, out of focus and back will you see the best values.  One thing I noticed is the screen view and peak value looking like 8bit, in the Apt manual it says you have to switch to 1:1 preview mode to use the raw FITS data for calculation.  Just double click the image to get this 1:1 view.

https://astrophotography.app/usersguide/focusing_aid.htm

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I gave up on APT's focus on tools and now use the standalone Bahtinov Grabber tool which, although APT's version is based on it, is actually much easier to use and more reliable. 

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1 hour ago, BrendanC said:

I gave up on APT's focus on tools and now use the standalone Bahtinov Grabber tool which, although APT's version is based on it, is actually much easier to use and more reliable. 

Thanks, il look into that. 

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Posted (edited)
1 hour ago, geeklee said:

Sorry, I don't.  It will only be testing out of focus, in focus, out of focus and back will you see the best values.  One thing I noticed is the screen view and peak value looking like 8bit, in the Apt manual it says you have to switch to 1:1 preview mode to use the raw FITS data for calculation.  Just double click the image to get this 1:1 view.

https://astrophotography.app/usersguide/focusing_aid.htm

No worries, l have a try. I would rather be getting on with improving other aspects of the hobby but never mind. I'm a bit embarrassed it's looking like user error though after all this time but strange that I've always just eye balled it and been ok, still I would rather that then it be an issue with the scope. 

Edited by Rustang
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If I’m not mistaken isn’t Polaris actually a double star ? , I’ve never had a problem using the bahtinov aid in APT hover the cross hair over the centre and press calculate that will tell you how close to focus you are  , maybe next outing verify on a different bright star , if you get round to an autofocuser the autofocus routine in Apt is very accurate .

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10 hours ago, Rustang said:

I would rather be getting on with improving other aspects of the hobby but never mind. I'm a bit embarrassed it's looking like user error though after all this time but strange that I've always just eye balled it and been ok

Yeah, it's frustrating and I sympathise.  When I saw your thread title, I knew exactly what you meant!  I find the lack of clear nights exacerbates that feeling of urgency/frustration for issues that require testing.  All you can do next time out is have a plan - try the tools suggested in Apt first, maybe get it bang on according to the Bahtinov Aid, then check using the Focus Aid as a reference, take a frame or two and check them zoomed in etc.  If you're happier all good, if not, start to tweak the focus through the zone and keep checking the frames.  Ensure to check back with temp changes or just every 30/60 mins maybe (I like to focus on filter change too)

Edited by geeklee
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Posted (edited)
17 minutes ago, geeklee said:

Yeah, it's frustrating and I sympathise.  When I saw your thread title, I knew exactly what you meant!  I find the lack of clear nights exacerbates that feeling of urgency/frustration for issues that require testing.  All you can do next time out is have a plan - try the tools suggested in Apt first, maybe get it bang on according to the Bahtinov Aid, then check using the Focus Aid as a reference, take a frame or two and check them zoomed in etc.  If you're happier all good, if not, start to tweak the focus through the zone and keep checking the frames.  Ensure to check back with temp changes or just every 30/60 mins maybe (I like to focus on filter change too)

Yeah it's a frustrating struggle, I hate that I've lost a good set of data as well but I now need to be a bit more thorough and don't just assume things are ok, we live and learn 👍

Edited by Rustang
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14 hours ago, Rustang said:

focused aid on them

Hi

No theory, just hands on...

We don't use apt but have found that using a focusing mask (FM) doesn't give you best focus when using a camera lens or -particularly the cheaper doublet- refractors. E.g. on our old Takumar and Tair lenses, the green and blue agree and focus well, leaving the red out of focus. A FM will therefore give a red halo around stars. Our 72ed is different in that it has the red and green in sync, giving blue halos.

Conclusion, the FM will give you the strongest two out of three colours.

The trick is to compromise. Use the FM mask to get close, then using a mid brightness white star, look -use your eyes- at the halo around the star. Focus manually until whatever halo you have disappears. It's a tiny amount.

Reflectors fare better and the FM gives perfect focus; mirrors are perfect apochromats.

Worth a try?

Cheers and HTH

Edited by alacant
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3 minutes ago, alacant said:

Hi

Some hands on.

We don't use apt but have found is that using a focusing mask (FM) doesn't give you best focus using a camera lens or -particularly the cheaper doublet- refractors. E.g. on our old Takumar and Tair lenses, the green and blue agree and focus well, leaving the red out of focus. A FM will therefore give a red halo around stars. Our 72ed is different in that it has the red and green in sync, giving blue halos.

Conclusion, the FM will give you the strongest two out of three colours.

The trick is to compromise. Use the FM mask to get close, then using a mid brightness white star, look -use your eyes- at the halo around the star. Focus manually until whatever halo you have disappears. It's a tiny amount.

Reflectors fare better and the FM gives perfect focus; mirrors are perfect apochromats.

Worth a try?

Cheers and HTH

It seems that my SW80 ED DS Pro was Ok with my mask focusing but the WO Z73 not so much so will try all that's been mentioned and see how I get on when ever the next clear night is. On the plus side I'm really pleased with the results now I'm taking 20min subs so still looking forward to future results. 

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