Flats essential

[ollypenrice]

On 19/04/2019 at 22:16, alacant said:

Hi. Software moves on. If you have a clean sensor, I'd say it does a better job correcting what a few years ago could only be tackled by flat frames. The best we've found is StarTools' wipe module; it doesn't care what orientation or focus your camera was at after taking the light frames.

HTH

Try this test. Take a flat (if you don't have one I'll be happy to send you one) and run it through your software. See how flat the result is. Then take a flat and divide it by another flat from the same run. See how flat that is. Which will win?

Here's a stretched flat from our widefield rig:

 

All that uneven illumination is present in every image captured by this instrument. Now here is this flat corrected by the application of another flat.

 

Are flats essential? I think they are.

Olly

 

 

 

[Adam J]

On 18/04/2019 at 15:25, Fieldsy said:

Hi guys 

I understand dark frames but flat and bias frames ? .im not over keen on leaving equipment out all night to do flats the next day ( do u have to leave settings and focus same as imaging session) and then wrap a white cloth over scope? And bios isnt that just like a dark?. I’m using dedicated cam not DSLR.

Just in case some else has not said this already, dont use bias frames with CMOS use Dark Flats to calibrate your flat frames. These are dark frames taken at the same settings as your flat frames.

[iapa]

On 26/04/2019 at 19:48, ollypenrice said:

Try this test. Take a flat (if you don't have one I'll be happy to send you one) and run it through your software. See how flat the result is. Then take a flat and divide it by another flat from the same run. See how flat that is. Which will win?

Here's a stretched flat from our widefield rig:

 

All that uneven illumination is present in every image captured by this instrument. Now here is this flat corrected by the application of another flat.

 

Are flats essential? I think they are.

Olly

 

 

 

so, for the feeble minded, use the 1st flat as a light, and apply the 2nd as a flat to multiply out the different pixel intensities?

So, do we take a bunch of flats, and stack as for lights, then apply the same flats as fast to the stacked image? The final results being our master flat?

[ollypenrice]

1 hour ago, iapa said:

so, for the feeble minded, use the 1st flat as a light, and apply the 2nd as a flat to multiply out the different pixel intensities?

So, do we take a bunch of flats, and stack as for lights, then apply the same flats as fast to the stacked image? The final results being our master flat?

Well... no, not entirely.

My aim here was to show what flats do and how they work. I had a set of about 40 flats. I made a master flat 'A' using the first half of that set. I then loaded the second half of the set (20 x Flats)  into my stacking software (AstroArt 5) as if they were lights and stacked them using masterflat A. What came out was a perfectly flat field. This perfectly flat field is totally useless outside this conversation. I cannot use it for any purpose other than to demonstrate how flats flatten a not-flat image. The perfectly flat field simply shows how well flats will flatten your lights. I don't believe any software can acheive a comparable result. Such software simply does not have the information it needs to flatten an image properly.

Olly

 

[carastro]

FVIW, I recall some years ago going to an overnight session at some-one else's garden in a dark location.  The following morning I was packing away and 3/4 of the way through packing I suddenly realised I hadn't yet taken any flats.   

Since time was of the essence I didn't have time to reassemble everything again (cables, laptop table and chair etc and cart it all back from where I had parked the car), but luckily I hadn't yet removed the camera from the scope.   So I placed the scope with camera still attached and unmoved very carefully into my car and when I got home I simply rested the telescope with camera attached on something suitable and pointed it at the window with a white cloth over it and took the flats.

So just mentioning this for the benefit of those who have to disassemble each night, that it could be possible to take the flats the following day if you are able to keep the camera attached (still in focus).

Carole 

[iapa]

15 hours ago, ollypenrice said:

Well... no, not entirely.

My aim here was to show what flats do and how they work. I had a set of about 40 flats. I made a master flat 'A' using the first half of that set. I then loaded the second half of the set (20 x Flats)  into my stacking software (AstroArt 5) as if they were lights and stacked them using masterflat A. What came out was a perfectly flat field. This perfectly flat field is totally useless outside this conversation. I cannot use it for any purpose other than to demonstrate how flats flatten a not-flat image. The perfectly flat field simply shows how well flats will flatten your lights. I don't believe any software can acheive a comparable result. Such software simply does not have the information it needs to flatten an image properly.

Olly

 

Thanks - I was obviously off on a completely wrong tangent.