First timer, focusing (?) problem

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
psy1280
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Re: First timer, focusing (?) problem

Post by psy1280 » Thu Oct 10, 2019 10:05 pm

Much thanks chongo,

I did, in fact, play with the histogram (for hours)...I'm going to try your suggestion of a longer exposure, etc.. So, you don't think my "halo" is a photographic artifact?

Joe

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turfpit
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Re: First timer, focusing (?) problem

Post by turfpit » Fri Oct 11, 2019 10:21 am

Joe

M31 is a huge object. The following shows what can be expected to be 'seen' by various telescope/camera combinations. The diagram from the centre outwards:
  • an SCT 8" with focal length 2032 and CCTV camera with a 720x576 capture area
  • refractor 81mm with focal length 625mm with CCTV camera
  • refractor 66mm with focal length 400mm with the CCTV camera
  • refractor 66mm with focal length 400mm and ~$1000 CMOS camera with a ~4500x3500 capture area


M31-FOV.JPG
M31-FOV.JPG (43.96 KiB) Viewed 853 times

key.JPG
key.JPG (44.93 KiB) Viewed 853 times


The image was generated via the Astronomy Tools Field Of View Calculator https://astronomy.tools/calculators/field_of_view/

astronomy-tools-FOV-calculator.JPG
astronomy-tools-FOV-calculator.JPG (90.73 KiB) Viewed 853 times


The SCT with the CCTV camera will only be able to capture the core of Andromeda. Adding a 0.5x reducer will not really improve things.

M31-0.5x-reducer.JPG
M31-0.5x-reducer.JPG (39.44 KiB) Viewed 853 times


I would consider M27 & M15 more realistic targets for the SCT + CCTV + 0.5x reducer combination.

M15-0.5x-reducer.JPG
M15-0.5x-reducer.JPG (41.37 KiB) Viewed 853 times

M27-0.5x-reducer.JPG
M27-0.5x-reducer.JPG (35.48 KiB) Viewed 853 times


Using the Astronomy Tools FOV Calculator during observation session planning to identify suitable objects for various equipment combinations can save lots of wasted time and disappointment.

From my previous post in this thread
to achieve results will require hard work, discipline, attention to detail and patience
Dave

psy1280
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Re: First timer, focusing (?) problem

Post by psy1280 » Tue Nov 26, 2019 7:01 pm

After years of trying to get to the point where I actually captured a deep sky image, I finally managed to capture Orion. I realize this image has a lot to be desired and I'm hoping to get some feedback as to how to proceed (i.e., better focus, etc., etc.). But, like your first child, this image means a lot to me. All suggestions will be greatly appreciated.

FYI: This was captured under an almost full moon

Thank you all
Joe
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turfpit
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Re: First timer, focusing (?) problem

Post by turfpit » Tue Nov 26, 2019 10:19 pm

Joe
hoping to get some feedback as to how to proceed (i.e., better focus,
To improve focus, buy a Bahtinov Mask and have a look at viewtopic.php?f=18&t=1151. Focus on a star near the object you want to image, put the mask over the end of the scope, zoom in to at least 100%, enable Display Histogram Stretch then adjust focus until you achieve the pattern in the link above. Note - I would suggest you don't use the Bahtinov focusing routine in SharpCap, as that will just introduce complexity you do not need at the moment.

My early M42 attempts looked like this, once I obtained a Bahtinov Mask things improved rapidly - best £20 I ever spent on this hobby.

M42.jpg
M42.jpg (22.57 KiB) Viewed 687 times

Dave

psy1280
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Re: First timer, focusing (?) problem

Post by psy1280 » Tue Nov 26, 2019 10:33 pm

Thank you Dave,

Not clear on one thing. You say to zoom in 100% and to stretch the histogram then adjust the focus with the mask, but then you say do not focus in SharpCap. I only have a histogram in SharpCap…can you clarify.

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turfpit
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Re: First timer, focusing (?) problem

Post by turfpit » Wed Nov 27, 2019 12:01 am

Joe

focus-with-Bahtinov.JPG
focus-with-Bahtinov.JPG (98.59 KiB) Viewed 683 times

In the attached image,

1) to set the Zoom to at least 100%
2) to turn on the Display Histogram Stretch

Clarification: In Sharpcap, under Tools > Focus Assistant you will find an entry for Bahtinov Mask. I am suggesting you don't use that as it will introduce complexity that I feel you do not need at this stage.

Once you have done 1) & 2) then just use the focuser on the telescope to achieve the pattern. You will need to carry out all the steps with the camera attached to the scope. Choose a medium brightness star rather than a very bright one.

It takes practise and may take a few minutes to achieve good focus. Without good focus the whole imaging session is just a waste of time.

It might be worth posting back a screenshot showing your focus with the mask.

Dave

psy1280
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Post by psy1280 » Wed Nov 27, 2019 12:56 am

Dave,
Wow! You've made this crystal clear! Thanks so much for taking the time to present all this. I will definitely check back with a followup image (skies permitting).

Yours,
Joe

psy1280
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Re: First timer, focusing (?) problem

Post by psy1280 » Wed Nov 27, 2019 1:01 am

Dave
Forgot to ask this. You say turn on the display histogram to apply stretch. Why do this? Do I need to manipulate the histogram? Looking at the histogram in your last post, the curve is on the left side...not sure what the histogram is showing. I guess what I'm saying is what should I be doing with the histogram and why. Thank you in advance.

Joe

psy1280
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Re: First timer, focusing (?) problem

Post by psy1280 » Wed Nov 27, 2019 4:51 pm

Dave,
One more question. Since I'm interested in post-processing of deepsky images, is there an advantage to using capture vs lifestack? I guess I'm not clear what the difference is other than Livestack showing a live stacked image. If you have a minute, I'd love to hear what you have to say.

Thanks in advance,
Joe

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turfpit
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Re: First timer, focusing (?) problem

Post by turfpit » Wed Nov 27, 2019 6:14 pm

Joe
Do I need to manipulate the histogram
No - just click the button with the lightening symbol. This will brighten any objects in the display.
apply stretch. Why do this
Brightening the objects in display makes the diffraction pattern (the X plus the line) easier to see. It also makes the 3 parallel lines visible - if these are not visible, then good focus has not been achieved. The alternate way to brighten is to increase the exposure (and/or gain). The exposure I was using was 1s (1000ms) if you look at the capture screen. To achieve the same effect as the Display Histogram Stretch might mean the exposure has to be increased to 15s. Trying to adjust focus and then wait 15s to see the result is going to mean getting good focus might take many minutes.

The Display Histogram Stretch is useful for seeing if faint targets are centred - a better option than increasing the exposure to 60s or 120s and waiting around. See the 2 images below, which are a capture in progress of M13 using 30s exposures. In the first one the Display Histogram is reset (yellow line is straight) and there is only 1 star visible. We don't even know if we are on target. In the second one the Display Histogram Stretch is enabled (yellow line is curved). The cluster is clearly visible. This action only affects the display, the data being captured remains unchanged. The alternative to using the stretch would be to increase to exposure to 60s, 90s or even 120s and wait around. This was because the camera in use was an Atik 314E CCD which only has 1 control - exposure.

M13-capture-in-progress.jpg
M13-capture-in-progress.jpg (67.99 KiB) Viewed 675 times

M13-capture-with-display-histogram-stretch.jpg
M13-capture-with-display-histogram-stretch.jpg (87.1 KiB) Viewed 675 times

Hope this clarifies things. Sometimes you just have to try things to progress.

Dave

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