Flats

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Mac
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Flats

#1

Post by Mac »

Like every good AP'r I am trying to learn imaging properly which would include taking flats. I've not used any in my processing thus far.

Are 20 frames like this suitable for flats?
Screen Shot 2020-01-03 at 8.03.58 PM.png

Taken at iso1250 with my iPad as a light source. No t-shirt around the end of the scope.

_JU97077.jpg
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Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
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Re: Flats

#2

Post by Mac »

Posting the above flat image on here clearly shows the type of end result I am getting with the lighter barrel-scope towards the center.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#3

Post by JayTee »

That is a textbook looking histogram for a flat and it should work quite well. Nice job.

Out of curiosity, how you acquiring your flat images? In other words what image acquisition program are you using?

Cheers,
JT
∞ Scopes: Celestron CPC1100 #2 Scope: 8" f/7.5 Dob AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scopes: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs
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Re: Flats

#4

Post by Baskevo »

They say your histogram should be between 1/3 and 1/2 of the histogram. People can get really picky and say it has to be 51% of the histogram. The only thing you can do is try it, and see what works, unless you want to get technical, which works just as well.

I take 50 flats because there's so much debate about the right now, I figured I would rather spend the little bit of extra time taking the extra frames, and then I have the option to stack more if I want :)

People also recommend shooting at least 1 to 2 seconds for flats, to avoid the shutter darkening the frames. but again, just try it and see if it works, and if it doesn't, try increasing the histogram to 50% and the exposure time to above 1 second. If that doesn't work, then you can try getting technical and going through this website, which helped me a ton (along with @Stuart)

https://www.myastroscience.com/proper-flats-with-dslr
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
Mount: EQ6-R Pro
Cameras: ZWO ASI1600mm Pro (Cooled) | Canon DSLR EOS T7i
Auto-guiding: ZWO ASI120mm-Mini + Astromania 50mm Guidescope

Filters: ZWO 31mm Ha/Oiii/Sii 7nm + LRGB | Orion 2" Skyglow Filter
Accessories: Explore Scientific 2" Field Flattener, ZWO EFW 8 Position
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Re: Flats

#5

Post by Mac »

JayTee wrote:
Sat Jan 04, 2020 3:05 am
That is a textbook looking histogram for a flat and it should work quite well. Nice job.

Out of curiosity, how you acquiring your flat images? In other words what image acquisition program are you using?

Cheers,
JT
Thanks JT

This was from my DSLR's SD card
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#6

Post by Mac »

Baskevo wrote:
Sat Jan 04, 2020 8:00 am
They say your histogram should be between 1/3 and 1/2 of the histogram. People can get really picky and say it has to be 51% of the histogram. The only thing you can do is try it, and see what works, unless you want to get technical, which works just as well.

I take 50 flats because there's so much debate about the right now, I figured I would rather spend the little bit of extra time taking the extra frames, and then I have the option to stack more if I want :)

People also recommend shooting at least 1 to 2 seconds for flats, to avoid the shutter darkening the frames. but again, just try it and see if it works, and if it doesn't, try increasing the histogram to 50% and the exposure time to above 1 second. If that doesn't work, then you can try getting technical and going through this website, which helped me a ton (along with @Stuart)

https://www.myastroscience.com/proper-flats-with-dslr
Thanks James I won't be able to get this to 1 to 2 seconds from the current shutter speed and iso. Putting the t-shirt method on would enable me to lengthen it some but... I have no white t-shirts. :D

I will give this method a try on my next imaging session and use them in Pixinsight to hopefully get rid of the barrel effect and round circles in the final images.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#7

Post by Mac »

from https://www.myastroscience.com/proper-flats-with-dslr

" There is assumption among Astrophotographers that Flats have to be taken at the same ISO as the Lights, when truth is that they don't."

This was my assumption as well and I used the same iso as my lights

and then there was this
"Very important to remember. When you do your flats at different ISO than your Lights, you have to calibrate your Flats with matched ISO dark-flats or BIAS frames !!! Keep that always in mind."

I am not using Bias frames of dark-flats ( new to me )
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
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Re: Flats

#8

Post by Baskevo »

So, it turns out your t-shirt does not actually have to be white. I use a white dress shirt, but at first I was using printer paper. But you can use anything, the color does not matter. As long as it shows a bright screen with your dust motes and vignetting, and an evenly illuminated field. A dark shirt probably would not work though because it would not allow the light to get through. Also, make sure there is nothing visible in the frame that you can make out, like a logo on a t-shirt or tag. It has to be even across the field

You don't haver to shoot at the same ISO, but the bias frames have to be at the same ISO as the flats, and it is easier to remember (at least for me) if I just do them all at the same ISO.

I would definitely take darks and bias frames with a DSLR. It helps a ton! You definitely need bias frames with the flats though. They are very simple.

Dark frames: same exact setting as lights, but with the dust cap on so no light is getting through. (same ISO, exposure time, and etc.) This includes temperature, so you want to try to take darks in between or after your light frames. Some people do them before the session, but usually it gets colder throughout the night so I like to do them after my sessions.
Flat frames: generally 50% of the histogram, although this can be wrong sometimes. Your camera's histogram does not show you the actual histogram of the image in the linear state, I think it does like a little stretch on it. So try it, and if it does not do anything to your images or makes it worse, then let the forum know and we can help you figure out the right exposure time. The easiest way is to use A/V mode on your DSLR, and it will find the right exposure time for you. The reason people put white t-shirts over the scope is to get the exposure time up.
Bias frames: Put the dust cap on and shoot at the fastest speed possible (for most DSLR's, that's 1/4000s)

Astrobackyard has some good tutorials on calibration frames. Here is one for darks: https://astrobackyard.com/how-to-take-dark-frames/

The hardest one to get right are the flat frames. It took me almost a month! They make a really awesome difference though and are totally worth it!
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
Mount: EQ6-R Pro
Cameras: ZWO ASI1600mm Pro (Cooled) | Canon DSLR EOS T7i
Auto-guiding: ZWO ASI120mm-Mini + Astromania 50mm Guidescope

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Re: Flats

#9

Post by Mac »

I guess that dark frames are the same as dark-flats? Anyways, I did take 10 darks and used them in my latest processing in PI.

I've watched many of Trevors videos but until now, never applied the techniques. I've been working on getting the guiding to the point where I can do 4-5 minute exposures and no star trails. If I couldn't do that, the rest didn't matter.

Thank you James for your help!
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#10

Post by Baskevo »

Dark frames are not the same as dark flats. Dark flats are similar to bias, mostly used for CMOS and CCD cameras. They are used instead of bias because some cameras don't work well with bias I think.

Why such long exposure lengths? You are going the opposite direction lol you should try to get the calibration frames and other basics down before moving on to 5 minute exposures... I've seen really amazing images with 30 second stacked images, but those only come with calibration frames. Without calibration frames, it doesn't matter if you have 10 second or 5 minute exposures, your images will be plagued with noise, dust motes, and vignetting.

A lot of people argue that it's actually better to take 500 30 second exposures rather than so many 5 minute exposures because you lose less frames (losing 1 frame from a satellite is worse in a 5 minute exposure than 30 sec). I have found that the difference between 120 stacked 30 second exposures and 12 stacked 5 minute exposures is minuscule, and you can get just as much detail! :) I'm sure some will disagree, but I don't think anyone will disagree that you should make sure you have calibration frames down before you push your exposure length :)
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
Mount: EQ6-R Pro
Cameras: ZWO ASI1600mm Pro (Cooled) | Canon DSLR EOS T7i
Auto-guiding: ZWO ASI120mm-Mini + Astromania 50mm Guidescope

Filters: ZWO 31mm Ha/Oiii/Sii 7nm + LRGB | Orion 2" Skyglow Filter
Accessories: Explore Scientific 2" Field Flattener, ZWO EFW 8 Position
Software: APT, SharpCap Pro, PHD2, CPWI | PixInsight, DeepSkyStacker, Photoshop

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Re: Flats

#11

Post by Mac »

Baskevo wrote:
Sat Jan 04, 2020 3:24 pm
Dark frames are not the same as dark flats. Dark flats are similar to bias, mostly used for CMOS and CCD cameras. They are used instead of bias because some cameras don't work well with bias I think.

Why such long exposure lengths? You are going the opposite direction lol you should try to get the calibration frames and other basics down before moving on to 5 minute exposures... I've seen really amazing images with 30 second stacked images, but those only come with calibration frames. Without calibration frames, it doesn't matter if you have 10 second or 5 minute exposures, your images will be plagued with noise, dust motes, and vignetting.

A lot of people argue that it's actually better to take 500 30 second exposures rather than so many 5 minute exposures because you lose less frames (losing 1 frame from a satellite is worse in a 5 minute exposure than 30 sec). I have found that the difference between 120 stacked 30 second exposures and 12 stacked 5 minute exposures is minuscule, and you can get just as much detail! :) I'm sure some will disagree, but I don't think anyone will disagree that you should make sure you have calibration frames down before you push your exposure length :)
Fair enough…

I agree that calibration frames are very important as is good guiding. However, I don't see it as going in the opposite direction, as you say. I want to be able to achieve both. If there are star trails, no amount of good darks, flats or bias will fix it. I see many images where they are exposing for longer than 5 minutes and wanted to be able to achieve it with my equipment, not that the majority of exposures will be that long. With the light pollution I have, 5 minutes is never going to happen here.

Three nights ago I did the 20 - 60 second exposures and compared them to 7 - 180 second ones and while not equal in total integration, the noise was less in the later and the final image was better.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#12

Post by Baskevo »

mactech wrote:
Sat Jan 04, 2020 3:40 pm
Baskevo wrote:
Sat Jan 04, 2020 3:24 pm
Dark frames are not the same as dark flats. Dark flats are similar to bias, mostly used for CMOS and CCD cameras. They are used instead of bias because some cameras don't work well with bias I think.

Why such long exposure lengths? You are going the opposite direction lol you should try to get the calibration frames and other basics down before moving on to 5 minute exposures... I've seen really amazing images with 30 second stacked images, but those only come with calibration frames. Without calibration frames, it doesn't matter if you have 10 second or 5 minute exposures, your images will be plagued with noise, dust motes, and vignetting.

A lot of people argue that it's actually better to take 500 30 second exposures rather than so many 5 minute exposures because you lose less frames (losing 1 frame from a satellite is worse in a 5 minute exposure than 30 sec). I have found that the difference between 120 stacked 30 second exposures and 12 stacked 5 minute exposures is minuscule, and you can get just as much detail! :) I'm sure some will disagree, but I don't think anyone will disagree that you should make sure you have calibration frames down before you push your exposure length :)
Fair enough…

I agree that calibration frames are very important as is good guiding. However, I don't see it as going in the opposite direction, as you say. I want to be able to achieve both. If there are star trails, no amount of good darks, flats or bias will fix it. I see many images where they are exposing for longer than 5 minutes and wanted to be able to achieve it with my equipment, not that the majority of exposures will be that long. With the light pollution I have, 5 minutes is never going to happen here.

Three nights ago I did the 20 - 60 second exposures and compared them to 7 - 180 second ones and while not equal in total integration, the noise was less in the later and the final image was better.
Sorry I didn't mean for that to sound rude, my tone was more "that's impressive!" than "you're doing it wrong" when I typed that you are doing it backwards lol of course you can do it however you want! Whatever works for you man! I'm just saying that I think you would get a lot more out of your images with calibration frames than trying to max out your guiding so far :)

There are many variables that go into what makes an image look better from night to night. It could be better seeing, it could be more integration time, etc. Of course the longer exposure on each image the better your Signal-to-noise ratio, but you reach a point where you have diminishing marginal return, was my point... I don't shoot over 120 seconds except with narrowband because I live in such heavy light pollution! but even if I didn't, I probably would want more frames than longer exposed subframes, because then I could probably have a longer integration time with more images :)

I still think that calibration frames are a lot more important than good guiding though, because technically you don't need good guiding to get really great images :) You will see a difference from 30 to 60 sec, from 60 to 120, from 120 to 180, and so on... but the higher you go the SNR doesn't get that much better, if I understand it correctly. Someone can correct me here if I'm wrong
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
Mount: EQ6-R Pro
Cameras: ZWO ASI1600mm Pro (Cooled) | Canon DSLR EOS T7i
Auto-guiding: ZWO ASI120mm-Mini + Astromania 50mm Guidescope

Filters: ZWO 31mm Ha/Oiii/Sii 7nm + LRGB | Orion 2" Skyglow Filter
Accessories: Explore Scientific 2" Field Flattener, ZWO EFW 8 Position
Software: APT, SharpCap Pro, PHD2, CPWI | PixInsight, DeepSkyStacker, Photoshop

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Re: Flats

#13

Post by Mac »

Baskevo wrote:
Sat Jan 04, 2020 10:31 pm

Sorry I didn't mean for that to sound rude, my tone was more "that's impressive!" than "you're doing it wrong" when I typed that you are doing it backwards lol of course you can do it however you want! Whatever works for you man! I'm just saying that I think you would get a lot more out of your images with calibration frames than trying to max out your guiding so far :)

There are many variables that go into what makes an image look better from night to night. It could be better seeing, it could be more integration time, etc. Of course the longer exposure on each image the better your Signal-to-noise ratio, but you reach a point where you have diminishing marginal return, was my point... I don't shoot over 120 seconds except with narrowband because I live in such heavy light pollution! but even if I didn't, I probably would want more frames than longer exposed subframes, because then I could probably have a longer integration time with more images :)

I still think that calibration frames are a lot more important than good guiding though, because technically you don't need good guiding to get really great images :) You will see a difference from 30 to 60 sec, from 60 to 120, from 120 to 180, and so on... but the higher you go the SNR doesn't get that much better, if I understand it correctly. Someone can correct me here if I'm wrong
No worries, and I get what you're saying and agree.

I've been working on my processing, while improving the guiding and being too pedantic about it but certainly not at the expense of proper calibration. I've just got to the point where my lights are of any quality with nice round stars and I'm here asking how to improve using the techniques you espouse. I've been working only in Photoshop/Lightroom and just purchased Pixinsight to make that calibration process better.

I appreciate your advice.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#14

Post by Mac »

My SO donated a white pillow case to my flat calibrations.

Going to drop the shutter speed and the brightness of the iPad as suggested as see what I can come up with. iso1250 is too high for my lights, the D7500 has a decent sensor, so I will try at 400.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Re: Flats

#15

Post by Mac »

Here is .5 secs at iso400 with the white pillowcase. I just had to turn down the brightness of the iPad to get to that exposure time.

Screen Shot 2020-01-04 at 6.31.08 PM.png
_JU97081.jpg
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Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
Image Processing : PixInsight - LightRoom - Photoshop - macOS - Windows 10

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Re: Flats

#16

Post by JayTee »

It's time to experiment with the flats you have taken to see what you get. Spinning your wheels here is not getting you anything. I'm very interested in your results. Please let us know.

Cheers,
JT
∞ Scopes: Celestron CPC1100 #2 Scope: 8" f/7.5 Dob AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scopes: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs
∞ Mounts: iOptron CEM60, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
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Re: Flats

#17

Post by Baskevo »

JayTee wrote:
Sun Jan 05, 2020 12:36 am
It's time to experiment with the flats you have taken to see what you get. Spinning your wheels here is not getting you anything. I'm very interested in your results. Please let us know.

Cheers,
JT
+1
The best thing you can do is try it :)

Because of my light pollution, I never shot above 800 ISO, so you should be fine there!

Let us know how your new flats work, and just remember to take them after the imaging session, before moving the camera, change the framing, or anything. This is to make sure that the dust motes and vignetting are in the exact same position on your flats as on your lights. :)
Good luck!
-James W.

Telescope: Explore Scientific 80mm FCD100 Triplet APO Refractor
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Mac
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Re: Flats

#18

Post by Mac »

JayTee wrote:
Sun Jan 05, 2020 12:36 am
It's time to experiment with the flats you have taken to see what you get. Spinning your wheels here is not getting you anything. I'm very interested in your results. Please let us know.

Cheers,
JT
Spinning my wheels trying to get advice?

Had I not inquired in my first post, I wouldn't have know to use slower shutter speed.

Can't take pictures in a snow storm.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
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Mac
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Re: Flats

#19

Post by Mac »

Baskevo wrote:
Sun Jan 05, 2020 12:48 am
JayTee wrote:
Sun Jan 05, 2020 12:36 am
It's time to experiment with the flats you have taken to see what you get. Spinning your wheels here is not getting you anything. I'm very interested in your results. Please let us know.

Cheers,
JT
+1
The best thing you can do is try it :)

Because of my light pollution, I never shot above 800 ISO, so you should be fine there!

Let us know how your new flats work, and just remember to take them after the imaging session, before moving the camera, change the framing, or anything. This is to make sure that the dust motes and vignetting are in the exact same position on your flats as on your lights. :)
Good luck!
thanks James...

I intend on trying as soon as the skies clear, maybe next week. Next time I will ask less for advice and just go shoot and see what happens.
Steve

Scopes : Explore Scientific ED102 Triplet APO - Celestron 6" SCT - Orion 50mm
Mount : AVX EQ | Software : KStars - EKOS - Stellar OS | Cameras : ZWO ASI533MC ASI1600MM Nikon D7500 ZWO ASI120MM-mini
CPU : iMac 27" - iPad Pro - Ubuntu/Debian Raspberry PI3/4 | Misc : Thousand Oaks dew controller - DewNot straps - Optolong L-enhance - Orion .8 reducer - ZWO EAF
Image Processing : PixInsight - LightRoom - Photoshop - macOS - Windows 10

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Re: Flats

#20

Post by Juno16 »

Another good thread concerning flats.

I am still just shooting them at mid histogram using the same iso as the lights with the ipad. Most of mine are about 1/80 second. Seems to do a pretty good job with dust motes, but every now and then I have to correct for uneven corner lighting (vignetting) in the Startools Wipe module.

Guess that I should try a lower shutter speed too.

Another interesting thread. Thanks Steve.

Thanks,
Jim
Jim

Scopes: Explore Scientific ED102 Triplet APO, Celestron Nexstar 130 SLT.
Mounts: Celestron AVX with Orion MM Autoguider, SLT;
Binoculars: Bushnell 10X50
Stuff: ASI EAF Focus Motor, Stellarview FF/FR
Camera / Software: Nikon D5300 (Ha mod), IDAS LPS D-1 Filter, Astrophotography Tool, PHD2, SharpCap v3.2, StarTools 1.6 alpha, Adobe Photoshop 21.1.3
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