How do I take Flats with a HyperStar?

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bbeckman4
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How do I take Flats with a HyperStar?

Post by bbeckman4 » Sun Aug 19, 2018 10:56 pm

I love SharpCap but how do I take Flats? I'm working with a C9.25, ZWO ASI071MC-Cool and a Hyperstar. I can't use the T shirt method with what I have.

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Re: How do I take Flats with a HyperStar?

Post by admin » Mon Aug 20, 2018 5:51 am

I don't have a hyperstar myself, but if you had a dew shield on your C9.25, could you then use the t-shirt method?

cheers,

Robin

padraigh
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Re: How do I take Flats with a HyperStar?

Post by padraigh » Sat Oct 19, 2019 7:44 pm

I believe you can take flats by imaging the twilight sky.

BlackWikkett
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Re: How do I take Flats with a HyperStar?

Post by BlackWikkett » Sun Oct 20, 2019 3:48 am

If you don't want to use sky flat method you can add a dew shield to the OTA. This will allow you to use an artificial light source and diffuser mounted to the dew shield clearing the the hyperstar and camera on the end of your OTA. An other method is to mount a large diffused light source on a wall or fixture and point the OTA at this light source to capture the flat.

umasscrew39
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Re: How do I take Flats with a HyperStar?

Post by umasscrew39 » Sun Oct 20, 2019 9:31 am

I have a dew shield on my C11" with the hyperstar and place a white t-shirt over it and aim at the clear blue morning sky. For example, with my ZWOASI 294MC Pro Cool, gain is around 6 ms to get the peaks in the middle of the histogram using my Optolong L-eNhance filter.

Frankly, when using a hyperstar you:

- do not need darks
- do not need flats
- do not even need to be very polar aligned

I now take flats and darks with my cooled cameras but that is only out of habit when not using the hyperstar.

Bruce

clouzot
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Re: How do I take Flats with a HyperStar?

Post by clouzot » Tue Oct 22, 2019 8:39 pm

umasscrew39 wrote:
Sun Oct 20, 2019 9:31 am

Frankly, when using a hyperstar you:

- do not need darks
- do not need flats
- do not even need to be very polar aligned
While your third point is definitely true (f/2 is very forgiving and you never take long exposures), I definitely have to use flats with my Hyperstarred C9, as the vignetting is substantial. As for darks, well... depends on the quality and make of your camera (my ASI183 does need darks, sadly...)

umasscrew39
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Re: How do I take Flats with a HyperStar?

Post by umasscrew39 » Wed Oct 23, 2019 10:32 pm

clouzot wrote:
Tue Oct 22, 2019 8:39 pm
umasscrew39 wrote:
Sun Oct 20, 2019 9:31 am

Frankly, when using a hyperstar you:

- do not need darks
- do not need flats
- do not even need to be very polar aligned
While your third point is definitely true (f/2 is very forgiving and you never take long exposures), I definitely have to use flats with my Hyperstarred C9, as the vignetting is substantial. As for darks, well... depends on the quality and make of your camera (my ASI183 does need darks, sadly...)
Wow- that's too bad about needing flats and darks. Sell the ASI183 and get a C11".......just kidding. Weird how some of the equipment combinations don't always work as advertised.

padraigh
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Re: How do I take Flats with a HyperStar?

Post by padraigh » Tue Nov 05, 2019 7:06 pm

FWIW

I found it's very easy to take flats using a the twilight sky. I obsessed a little over the issue until I found out how easy it was. I even tried creating images using a vignetting formula but ultimately, that turned into a headache worrying about conversion between various formats and bit depths etc.

To create a flat, simply point your telescope away from the sunset/sunrise position to a place where there are no bright stars if doing it before sunrise. At dusk you wont see the stars yet so it doesn't really matter.

With the camera active, watch the histogram display; at short exposures, if it's too bright, the histogram will be centered near the 100% level.

As the sky darkens, the histogram will move to lower intensities; when near the 50% level, save some frames.
(Watch out for bright stars beginning to appear - they will not seriously impact your results IMHO, but move the field to avoid them.)

Try different gain/exposure combos.

Infinitely easier than trying to hang a teeshirt over the telescope with or without hyperstar.

Since, you're not doing research there's no point in obsessing over perfection. Also, since hyperstar creates such short focal ratios, there will be significant vignetting and any flat will be a huge improvement.

I suspect it should be possible to use flats taken by others as long as they are using similar cameras and optics.

PH

clouzot
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Re: How do I take Flats with a HyperStar?

Post by clouzot » Tue Nov 05, 2019 7:19 pm

Taking flats using the twilight sky is indeed my method of choice with the Hyperstar (no flat panel involved...)

But you have to make sure your Master Flat doesn't contain stars :mrgreen:

Here are the results if it does: blackish streaks. I had the issue this summer when I took flats a bit too late in the evening.

Image

padraigh
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Re: How do I take Flats with a HyperStar?

Post by padraigh » Wed Nov 06, 2019 9:26 pm

Nice example of why you don't want stars in your flats.

If your flats do have stars, you should be able to remove them by applying a median filter (aka median blur in GIMP)

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