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Help needed - bleaching in centre of disc


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While I try to tinker with the settings and sort out the problem by myself, I have a few newbie questions - just incase there is an easy answer out there.

The Coronado PST / SV305 2mp colour camera / on an AZ mount is giving reasonable results for an amateurs slightly out of focus first attempt at solar imaging.

  • The main issue is the bleaching at centre of frame, with darker edges to the sun = is there an easy fix, or a better way to capture. I am using Starcap, around 50ms exposure rate, with a medium gain adjusted to give contrast & detail, 200 frame avi video, 20x best frames kept, PIPP centering, registax, then picture sharpening & colour enhancing for contrast.
  • The left hand side of the image was slightly out of focus when captured - I don't understand.
  • Filters - I assume that an infra red / UV cut filter will be of no benfit to a double stacked <0.5 angstrom solar scope (as the signal will have been filtered by the scope already)

Thanks in advance - a search of the forum topics failed to uncover any similar problems.


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Best to under expose on capture rather than over expose , once a pixel is saturated there's no recovering it , but an under exposed frame can be stretched to achieve the results you seek.

The disc naturally dims towards the limb , 

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Thanks Steve,

Reducing the exposure should bring the prominence's into focus, and sharpen up the left hand trailing edge as the sun moves left to right. (maybe due to pixel burn and slow response)

Tuning the exposure may end up solving all 3x problems. - Cheers

Edited by Reeny
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  • 4 weeks later...

3-weeks on and the clouds have cleared, and I have had some spare time today to play with the settings.

For some strange reason the 200 frame videos came out blue (the Sharpcap input may have been set to monochrome) / PIPP selected the best 8x pictures / sharpened / stacked / then converted to negative to obtain the orange colour.

With Steve's help and advice above, I am more than happy with the results from the SV305 camera on my second attempt.

My main problems appeared to be: A) not enough tilt on the double stack filter, causing a ghost image and difficulty with the focus / B the etalon needed to be re-tuned for the camera, not left adjusted for the best visual contrast / c) the lower exposure level of 25ms works well, I tuned the settings for the best view of the prominences - there will be even more improvement to come next time.




Edited by Reeny
Thanks Steve
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Not too sure on sharpcap's value settings but 25 ms in firecapture would be far too much..  to me your focus seems off.... What's sharpcap FPS registering?  

For solar, you need a fast FPS, which is a balance of exposure and gain, the lower the exposure and higher gain will give you higher FPS..l

 And as you said the best you can get the etalon tuned will balance out the uniformity, although you will always have a brighter center than the edge of the disc

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Thanks, I needed to get a clear picture on the laptop screen in bright sunlight. And the focus was targeted on getting rid of the blur at the edge of the disc, with a potential loss of focus in the centre of the image.

I will try to adjust the exposure down a bit. I assume the process is focus and adjust contrast first, then tune down the exposure to a lower level.

If I reduce the exposure rate down to around 12m/s, I will need to increase the gain to see the target. Yesterday the increased gain seemed to disturb the contrast.

More tinkering and adjustment will be required to get a happy balance.

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Hi Reeny

I had a little play with PhotoFiltre7 image handling software.
Just extra gamma and contrast. Cropped and Resized to 700 pixels for the forum.
You have captured quite a lot of detail.
The offset detail/brightness may be a tilting problem somewhere.
Or, the etalon misbehaving. 
If you are using a standard PST it could be the focusing prism.
They have a bad reputation for getting out of alignment.
Checking this would involve removing the side of the black box.

Solar15-9-22Neg1.jpg.8a6a76512d212179464528e6cbba926e rsz 700.jpg

P1310423 rsz pst prism.JPG

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Thanks everyone, I will play around with the tilt & etalon adjustments over the next few weeks. My tuning adjustments are likely to be the problem.

If the problem remains, I can investigate the tuning prism as a last resort via the manufacturers warranty.

The PST is a discounted customer return, so the "as new" scope has had a few trips with the courier before it reached me.


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Just for clarity: The prism is not for tuning. It is ONLY for focusing the PST.
It is designed to fold the optical path, inside the prism, to make the telescope more compact.
Its support and movement mechanism is crude and a well known problem.

The "discounted return" issue raises possible existing faults within this PST.
Only with the permission of your vendor I would start with the prism.
If the prism is badly aligned it could be the culprit for the image asymmetry.
Dismantling the PST without their written permission [as proof of agreement] will void any warranty.

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Thanks Rusty,

To simplify the process - I should be able to check the operation of the focus prism first, before the double stack etalon is attached. Then re-check with the double stack etalon fitted.

I was surprised at the decent quality of the single stack optical view when setting up the PST last time, so I assume the scope & prism are OK, and my adjustments are to blame.

No way would I consider opening up the side of a brand-new scope - The warranty will be used to return to the supplier if I have evidence of a mechanical fault.


By chance, I used to operate spectrophotometer equipment in a chemical analysis laboratory 25-35 years ago - so I am (was) familiar with the tuning of diffraction gratings to single wavelengths, and I know it can be a fine a balance to get it spot on.

I am happy with the results as a newbie with only 2x serious attempts so far, and know there is lots more to learn. Luckily I am in the right place.



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  • 3 weeks later...

I think the problem has been cracked with your help. Thank you to all the contributors.

Focus prisms appear to be OK and will be left alone. The issue is likely to be the small-sweet spot, causing darkness and out-of-focus regions at the edge of the field of view (for visual and for imaging) - as described by InFINNityDeck above.


A 2-week break in Scotland gave me at least one sunny day to test the basic set-up of the scope.

Test = 24th Sept / exposure time reduced to around 6ms / focus adjusted for prominences / PST only, with no blocking filter / SV305 turned 180 degrees in the eyepiece holder to check for camera faults / eyepiece visual check for problems around the field of view.

Outcome = single frames of the quick & dirty test videos were good enough to process without stacking / blocking filter not required for imaging / camera darkens and blurs the focus at the frame edges (left side & right side) / eyepiece viewing darkens and blurs the focus all around the edge circumference of the field of view / focus and brightness are good in centre of the field of view (or frame) / bleaching is tolerable if the camera image is taken in the middle of the frame (at the sweet spot).

I found a review on AstronomySource-dot-com which sums up the problems with the relatively small diameter of the PST when used for imaging.

Quote: the sweet spot would be smaller than the field of view, meaning the images would have a banded appearance.


Note: the attached photos were a test to push the boundaries - not a final result.

Hopefully the pictures will improve as I learn more.




Edited by Reeny
spelling of prominences
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