Comparing flats take at f5 v's f15

[Merlin66]

A question....
Have you ever compared your flats, taken at fast f ratios to flats obtained at higher f ratios???

I'm cleaning my cameras, ASI 1600MM and ASI 174MM....
Usually used with ED80 (f7.5) and sometimes with x2.5 PM (f19) for solar imaging.
I find a "good" flat taken at f7.5 looks terrible (!!!) when taken at f19.
The slower f ratio seems to show up ANY dust/ motes on the sensor/ cover plate.
Have you found the same?????

[PRejto]

Maybe this isn't too surprising given the CFZ is so much larger at higher f ratios.....meaning, dust that would be blurry and hard to see at f4 would be much crisper at f15. My guess.

Peter

[Merlin66]

ZWO recommend using a Barlow to do flats......
Are users seeing the difference???

[Atmos]

It is something I’ve noticed in landscape photography. Shoot at F/5.6 and the image is clean, shoot at F/16 and the sky is littered with little dust spots.
At Peter suggests, there is more within the CFZ.

[Merlin66]

Yeah but....
If the chip is well focused then the dust on the sensor should also be in focus whatever the f ratio?????
I assume we agree, that to ensure the camera is "clean" we need to check with flats taken at high(er) f ratios.
How many members actually do this??

[The_bluester]

I can't see how you can successfully use flats taken at a different F ratio. any dust motes on surfaces not specific to the camera will be different and more to the point, any vignetting will be different too. For my SCT in particular, the fall off in light level is starkly different between F10 and F7 so a flat from one configuration would look awful used with lights from the other, you would end up with either bright corners or dark ones depending which way you went.

[Merlin66]

Paul,
These would not be "conventional flats", but flats taken to establish the cleanliness of the camera only. The front cover plate and the sensor surface.

I feel it's necessary to at least ensure the camera itself is as clean as possible, the other "flat" issues from front mounted filters, vignetting etc. etc. only add to any residual camera dust/ motes etc.

[multiweb]

Always had to do longer flats with longer FL and a lot more of them or they're grainy. Apart from that longer FL fov is almost always better illuminated than shorter FL. The exceptions to that would be if you have a large imaging circle (like an FSQ) or a small one (like a C11). I also found that flats shot with an OSC might not correct every color the same way. It might work for some channels but over/under correct for others. So in practice I'd shoot flats for each channel with different durations then debayer to channels respectively. But it's a lot of work.

[Merlin66]

Marc,
I'd stress again, I'm considering these flats to address dust etc within the camera, not to be confused with "normal conventional flats".
Maybe most don't worry to much about these dust spots???

[multiweb]

That's cool but dust motes will still show if you don't have the right levels in your flats in my experience. If the spots are small enough you can get away by dithering only. Drizzle integration helps as well.

[Merlin66]

Marc,
""but dust motes will still show if you don't have the right levels in your flats""

I'm using mono cameras and get the histogram up around 85% when imaging these flats.
Is this what you mean? (I assume levels may apply to OSC?)

[Merlin66]

Here's the set-up I've been using to check the camera's cleanliness.
An ST80 covered by a clear plastic bag pointed to a bright sky, x2.5 PM to give f12.5.
This presents a good even flat which shows any/ all small dust particles on the sensor and the small "dust doughnuts" from dust on the camera cover plate.

[multiweb]

Mono or bayer doesn't make any difference. What I mean is that if you have a dark dust donut and flatfield it with the wrong flat it might still show as a dark(er) blob or lighter spot in your calibrated sub.

Are you trying to flat field primarily to get rid of specs and dust, but not worrying too much about uneven illumination? I'll dig some flats from various scopes I still have on file to show what I mean.

The sharp small specs on the sensor, just wipe them out and be done with it. The bigger ones, well they can be anywhere on correctors, filters, optics but they're easier to get rid off with flat fielding. The little ones close to the imaging planes are the pesky ones to process out IMHO. Easier to wipe.

[Merlin66]

Marc,
Yes, only to clean the camera....not looking for vignetting or "large" doughnuts etc..

I'm seeing sensor dust which is not evident in a flat at f5 very obvious at f15.