Flat frames

[scotty38]

3 hours ago, Ouroboros said:

I have wondered whether crossed polarisers might be a way to adjust down the light level of led panels. 

Putting a polarising filter into the optical path you mean? Not a chance that's a good idea I'd say 🙂

[tooth_dr]

5 hours ago, ninjageezer said:

the back of the panel is white as well and no where near as bright and this can be used without paper I believe.

Never thought of that. Good idea!  I use some Perspex sheets - either grey or opal white, depending on filter being used.  

[Ouroboros]

7 hours ago, scotty38 said:

Putting a polarising filter into the optical path you mean? Not a chance that's a good idea I'd say 🙂

It should be fine as long as there are no polarising components in the optical train.  People use tablets as light screens and the light from these is polarised. 

[Lee_P]

+1 for tracing panels from Amazon, or if you want to go down the DIY route then this is an option.

[scotty38]

2 hours ago, Ouroboros said:

It should be fine as long as there are no polarising components in the optical train.  People use tablets as light screens and the light from these is polarised. 

Ok fair enough, I imagined it doing funky stuff that could not be replicated from one session to the next but if that's not the case then great.....

[Ouroboros]

If worried it’s a problem then the addition of a thin layer of cloth or some other diffuser would probably scramble polarisation anyway.

The only reason I mention angled (not crossed) polarisers, and the fine control those might provide, is that I often find it quite difficult to find the ideal brightness for flats.  I usually use the following morning’s sky, and several layers of white cloth over the telescope, having kept the optical train exactly as it was during the imaging session.  That’s fine for OSC but maybe not if using different filters and slightly different focus positions.

I use an ASIair too, which is great for getting the flat exposure times set up. Trouble is I understand that the exposure must be more than 3 seconds for the ASI2600 camera.  I also have it in my head that for the ASI air the flat exposure time must be less than ten seconds. Is that right or just a myth? Anyway, I’m forever juggling layers of cloth to hit the sweet spot of 50% level with exposures between 3 and 10 seconds. I have always succeeded, and my flats have worked fine, but it is a bit of a faff. 

[ONIKKINEN]

1 hour ago, Ouroboros said:

I use an ASIair too, which is great for getting the flat exposure times set up. Trouble is I understand that the exposure must be more than 3 seconds for the ASI2600 camera.  I also have it in my head that for the ASI air the flat exposure time must be less than ten seconds.

Definitely not true this one. This myth comes from some of the other, more tricky to calibrate cameras like 294, 183, maybe others but has no foothold at all with the newest gen of cameras of 533/571 chipped cameras.

Have taken 50ms, 136ms, and 205ms flats and never had issues from the exposure time. Trying to dim my panel down so that flats take 3s to expose sounds kind of silly.

[tooth_dr]

23 minutes ago, ONIKKINEN said:

Definitely not true this one. This myth comes from some of the other, more tricky to calibrate cameras like 294, 183, maybe others but has no foothold at all with the newest gen of cameras of 533/571 chipped cameras.

Have taken 50ms, 136ms, and 205ms flats and never had issues from the exposure time. Trying to dim my panel down so that flats take 3s to expose sounds kind of silly.

I take 3+ second exposures with my 2600MC if I’m using a narrow band filter when I use an opaque acrylic sheet.  I just take whatever exposure recommended by the soft ware to get the correct ADUs  

I used to have a KAF8300 camera with a mechanical shutter, and it really did 3 second flats to about shutter shadow.  Maybe this is where the 3s rule came from.  

[Ouroboros]

Thanks @ONIKKINEN and @tooth_dr.   That will certainly make taking flats simpler.  I don't know where I got the 3 second rule from, and now of course I can’t find it on line. Just shows you shouldn’t follow all the advice  you read on forums.  

[jacko61]

21 hours ago, ninjageezer said:

I've just bought one B4 size so it fits my 8 inch edgehd as well .the good thing is its its got a short built in lead with a 5.5mm socket on so hope it lasts better ,very common fault on most led light pads the loose socket. I think it was someone on here who recommended the one I got.

the back of the panel is white as well and no where near as bright and this can be used without paper I believe. the front goes super bright !

https://www.amazon.co.uk/gp/product/B07WQXFZN5/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

 

That looks a much better set up.  Think it'll be on my xmas list.

Graeme

[ONIKKINEN]

2 hours ago, Ouroboros said:

Thanks @ONIKKINEN and @tooth_dr.   That will certainly make taking flats simpler.  I don't know where I got the 3 second rule from, and now of course I can’t find it on line. Just shows you shouldn’t follow all the advice  you read on forums.  

I think its that tehcnology evolves at a rapid pace at the moment and so some advice that was very much good a few years ago is not today. I mean the long flats thing is still a good idea for many models of camera (and narrowband, but there it is out of necessity because the exposure is much darker), but unnecessary for the newest CMOS ones. In a few years there will be a new shiny toy in the market and some new rule of best practice comes out and we are all clueless then.

[ninjageezer]

On 14/11/2022 at 18:16, ONIKKINEN said:

I think its that tehcnology evolves at a rapid pace at the moment and so some advice that was very much good a few years ago is not today. I mean the long flats thing is still a good idea for many models of camera (and narrowband, but there it is out of necessity because the exposure is much darker), but unnecessary for the newest CMOS ones. In a few years there will be a new shiny toy in the market and some new rule of best practice comes out and we are all clueless then.

I spent today in pixinsight processing some data I took on sat .I honestly could see no difference in the final image with no calibration frames at all...I tried all different combinations inc dark dark flats and flats and left them all out but could see real difference.,,,,,

[ONIKKINEN]

1 hour ago, ninjageezer said:

I spent today in pixinsight processing some data I took on sat .I honestly could see no difference in the final image with no calibration frames at all...I tried all different combinations inc dark dark flats and flats and left them all out but could see real difference.,,,,,

If you dont have any vignetting or dust on optical surfaces (very unlikely) then you dont really need flats. Well, flats also correct uneven pixel responses somewhat, but with a 571 or a 533 (or another new camera) you have linearity to 99% so even this is deep into nitpick territory. If you do have vignetting and dust, which applies to 95% of imagers with normal f/ratio scopes (Rasa and the like excluded) then you do need to shoot flats, and in order for the flats to work they need darkflats and your lights need darks. So almost all will benefit from calibration frames which is why its good practice to take them.

Even if you dont have dust now, you can have it later even if you dont open up your imaging train. Dust is stuck somewhere inside your imaging train at all times and one day it will shift by itself and land onto one of your optical surfaces close to the camera and cause dust bunnies.

[900SL]

I have a 533 and I always take flats, mainly for dust. It's a nightmare trying to remove spots on stretched images.

I've used an old LG pad with a lightbox app, an A4 dimmable tracing pad, and once-and-never-again the stretched T shirt method

Exposure time doesn't seem to matter unless you have a slow screen refresh rate, which can cause issues

I find my powerbank switches off if I dim the tracing pad too much, as it isn't drawing enough current, so I'll probably get an opaque perspex sheet at some point