Rokinon 135mm Lens issue [Deep Sky] Acquisition techniques · Chris Parfett · ... · 62 · 1410 · 21

Wes Schwarz:
Also, make sure you are using large structure rejection when you integrate your flats. 

The table is on line 39
https://pixinsight.com/doc/tools/ImageIntegration/ImageIntegration.html

andrea tasselli:
I normally never use it (and shan't be used) for flats, especially taken with flat panels. Although I doubt it makes much of a difference since flats, by definition, do no move around. This said, if you where to take dusk flats bright stars might show up and assuming you're not tracking (which is what you should be doing) they would leave a streak, thus the feature is handy to remove them.

Jon Rista:

andrea tasselli:
I normally never use it (and shan't be used) for flats, especially taken with flat panels. Although I doubt it makes much of a difference since flats, by definition, do no move around. This said, if you where to take dusk flats bright stars might show up and assuming you're not tracking (which is what you should be doing) they would leave a streak, thus the feature is handy to remove them.

Ah, for star rejection.

FWIW, most of the time, I use  bright clear blue sky for my sky flats. Long time ago I tried doing them at dusk, and yes ran into some problems with stars. Switching to daytime blue sky as my light source, I've never had any more of those issues. If you need to you can add a diffuser to the front aperture, and with that you could also use lightly overcast days as well, where the sky is still quite bright and uniform.

Jon Rista:

Wes Schwarz:

I think its actually a Rokinon, but, basically the same thing as a Samyang.

Jon Rista:

Wes Schwarz:

Jon Rista:

andrea tasselli:
I normally never use it (and shan't be used) for flats, especially taken with flat panels. Although I doubt it makes much of a difference since flats, by definition, do no move around. This said, if you where to take dusk flats bright stars might show up and assuming you're not tracking (which is what you should be doing) they would leave a streak, thus the feature is handy to remove them.

Ah, for star rejection.

FWIW, most of the time, I use  bright clear blue sky for my sky flats. Long time ago I tried doing them at dusk, and yes ran into some problems with stars. Switching to daytime blue sky as my light source, I've never had any more of those issues. If you need to you can add a diffuser to the front aperture, and with that you could also use lightly overcast days as well, where the sky is still quite bright and uniform.

Interesting, you got good results with sky flats? Was this with the samyang 135?   I had an RC8 for a while and sky flats worked on that scope pretty well, but it was an F8 aperture, the Samyang 135 I bought around the same time was brutally unforgiving.  I would love to know a recipe for success though, its kind of a pain to come home and re set up to shoot flats after a session when I could just shoot them before sunset.   
In your technique, are you shooting mid day with a blue sky light source or in the evening?

I think its actually a Rokinon, but, basically the same thing as a Samyang. For OSC sensors, blue sky flats aren't necessarily 100% perfect, but they seem to do a good enough job. For mono, blue sky flats have always been wonderful. Its a super even, flat light source. Its quite bright. Exposures can be pretty short, so you only need to use a master bias for calibration (no need for dark flats.) 

The way I usually did it, was right after my imaging sessions. At home, I just waited till the sky was clear and wasn't changing in brightness a lot. At a dark site, I would usually start earlier, and I would start with the flats a little darker than optimal, then kept exposing them till they were a little brighter than optimal, and the flux normalization would neutralize the differences in the stack. Some times, I was not able to get the flats when I was imaging at a dark site. Sometimes it was a problem, and I would try to get some at home, other times, I would just end up being unable to flat calibrate that set of lights. I usually acquire a lot of data, so the uncalibrated frames would average out ok in the end, and pixel rejection usually took care of the rest...and if it did not, well, I would just discard that subset of frames. 

One of my key techniques, no matter what imaging system I am using, was to always try and keep the scope and image strain assembled. I always try to never disassemble, that way if I need to get flats later, I can. I bought and built some pretty large transport cases for my telescopes and mount, so I can keep them all pre-configured as much as possible when transporting them anywhere. I have this very large case for my scope, which I filled with some foam sheets about 2" thick, that I can put my whole scope in with everything attached, so that I don't have to disassemble anything. Its been a real life saver doing that, and usually allows flats to be taken later if necessary. The only real risk, is if I screw up the image train orientation or anything like that, while putting the scope into or taking it out of said case.

Jon Rista:

Wes Schwarz:
I'll have to give the sky flats another visit and give this a try. I usually leave my stuff assembled like you, in fact I have a piece of painters tape on the samyang 135 and I stick it over the split between focuser and the front lens barrel so focus cannot move and I can do flats later. I will say its alot easier to shoot flats with a lens on a tv than a big scope. Im honestly more so interested in sky flats with my Newtonian, that would be a big time game changer.

Just to make sure its clear...I do "blue sky flats" and I use that  term in order to differentiate from the more general term "sky flats" which are often (most often) done in the evening, where you can potentially run into stars, gradients, and intensity issues. 

Blue sky flats done during the day, with a totally clear blue sky, have been very effective for me. I've done them both without any diffusion (if the sky is just totally clear and no clouds at all) as well as with diffusion (in the event there is anything in the sky that might be an issue to flatness.) Sometimes, I've been unable to get flats right away, and I'll end up with some kind of change in my train, and then I'm simply out of luck for said data set. So I always try to get them promptly the next day. Overcast skies can work with good diffusion, but, unlike blue sky flats, you can experience intensity differences frame to frame, which isn't entirely ideal. Pure, clear, empty blue sky is the best, IMO. ;)