Bahtinov Mask and Display Histogram Stretch

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turfpit
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Bahtinov Mask and Display Histogram Stretch

Post by turfpit » Tue Sep 25, 2018 12:21 pm

bahtinov-mask-with-display-histogram-stretch.jpg
bahtinov-mask-with-display-histogram-stretch.jpg (76.17 KiB) Viewed 1189 times
Stuggling to see the diffraction pattern when focusing with a Bahtinov Mask?

Set Zoom to 100 - 125% and apply a Display Histogram Stretch.

Dave

cloudymoon
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Joined: Thu Apr 26, 2018 12:16 pm

Re: Bahtinov Mask and Display Histogram Stretch

Post by cloudymoon » Wed May 29, 2019 11:17 am

Hello Dave, I tried the Bahtinov focus routine in Sharpcap last night and it made no sense to me. I just had a complete screen with a red overlay and a replication of the three spikes. Everytime I tried to alter the focus the lines appeared at random places on the screen. I didn't see a picture of a star just the red overlay and the coloured spikes which never altered their pattern . I have a feeling that should I pick a star first before I use the Bahtinov focus routine with Sharpcap?

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turfpit
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Re: Bahtinov Mask and Display Histogram Stretch

Post by turfpit » Wed May 29, 2019 6:16 pm

I slew to a star, get approximate focus, attach the Bahtinov Mask, then use the diffraction pattern to achieve accurate focus.

Things that help are
- zoom in to over 100% to see detail
- use the Display Histogram Stretch to see detail and allow the exposure to be kept short

When I think I have focus, I lock the focuser then remove the stretch and increase exposure to around 15s to check.

I only consider good focus has been attained when as well as the line through the intersection of the X, I can see the 3 parallel lines as well.

When selecting a star for focusing I tend not to use the brightest stars available but rather a medium brightness star. I also use a focus star near to the DSO I am interested in. For example with M13 I could use Arcturus or Alpheratz for focus but choose the latter as there is less ‘twinkle’. For M5I I would focus using Alkaid in Ursa Major.

I don’t use a star on the opposite side of the meridian to the object of interest - consider flexure during a meridian flip.

I also check focus during a long session as focus can be affected by temperature drops or an object being at a higher altitude.

Good focus is essential and worth spending a few minutes to get right.

Hope this helps.

BTW, I don’t use the focus routine in SharpCap as sometimes I image with a Linux laptop. This means I keep to lowest common denominator i.e. mask + zoom + display stretch.

Dave

nexusjeep
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Re: Bahtinov Mask and Display Histogram Stretch

Post by nexusjeep » Fri May 31, 2019 4:17 pm

I have had the issue with the red colouration it is the mask in Sharpcap what I usually have to do is use the FWHM focus tool so that I can resize the selection box/mask. If I then swap to the bahtinov focus assistant I can then resize the selection area, if I just go straight to the bahtinov focus aid I do not appear to be able to resize the selection as it appears to automatically select the whole screen.

Cheers
Nick

BlackWikkett
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Re: Bahtinov Mask and Display Histogram Stretch

Post by BlackWikkett » Sun Jun 02, 2019 9:38 pm

When you start the Bahtinov Mask focus tool and the entire screen is red. Try clicking the selection box icon on the top tool bar. This should bring up the scalable ROI selection box. I use this tool every time I use SC and when you first start the tool the selection box is not enabled.

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