Mono Calibration Frames

[Startrek (Martin)]

After just over 3 years using the 2600MC OSC I’ve decided to move onto Mono imaging with the 2600MM ( gear arrives next week )

A question regarding calibration frames -

When shooting Broadband should I take Flats for each filter (LRGB) or can you get away with just taking Flats for Luminance filter and reuse for RGB filters ?

When shooting Narrowband should I take Flats for each filter ( Ha , Oiii , Sii ) or can you get away with just taking Flats for Ha and reuse for Oii and Sii ?

Im thinking its best practice to take Flats for all filters but just putting it out there to see if any folk take short cuts which work ok

Thanks

Martin

PS: Man this site is slow , should we suggest an upgrade of some kind ( if that’s the issue ) and where costs are involved , I’d be happy to pitch in with a co operative member contribution to get it “permanently fixed” moving forward into the future for current members and future members.

[gregmc (Greg)]

You should take flats for ANY time you alter the image train as flats are meant to show up differences and anything you do will change that image path.

That includes any rotation of any part, filter change or sudden movement causing any dust particles to move. This also means flats for anything you use in that session.

Flat darks are also a good idea.
Script in NINA will help automate the process to an extent depending on the capabilities of the light panel.

[Startrek (Martin)]

Thanks Greg,

Good point about changes in the image train.

I use APT and its Flats Aid Tool is pretty good ( I use Flats and Bias for the 2600MC so will do the same for the new 2600MM ( similar cameras as far as flats go )

Most folk take their flats at the end of an image session which I currently do with OSC but what happens if you have micro changes in focus between filters on a nights imaging run. "Theoretically" you should take flats for each filter between filters ?? This would take a significant amount of time as you would have to move your scope back to park and point it to zenith ( I use a flat panel ) this seems totally impractical ?

My new Antlia LRGB and Ha Oiii and Sii Mono filters are suppose to be Parfocal but as we know focus can shift slightly through the night ( even through one filter )

Your thoughts and experience

Cheers

Martin

[Addos (Adam)]

Hi Martin


Yes, you do need separate flats for each filter. Not best practice - required.



Each one lets a slightly different amount of light through and refracts the light slightly differently. You'll see this in your flat wizard - no two filters should have exactly the same exposure time (assuming you're using a dynamic flat with a static light panel).


Focus doesnt matter. Leave it as is after your imaging session.



People often go overboard with flats. You may not need to take flats after every single imaging session if you're imaging setup is reasonably static. With my refractor and some precautions, I sometimes only need to take flats every month or so.



You are correct that the IMX571 likes bias calibration - keep doing that with the mono version. No need to take flat darks on this sensor.

[Startrek (Martin)]

Originally Posted by Addos

Hi Martin


Yes, you do need separate flats for each filter. Not best practice - required.



Each one lets a slightly different amount of light through and refracts the light slightly differently. You'll see this in your flat wizard - no two filters should have exactly the same exposure time (assuming you're using a dynamic flat with a static light panel).


Focus doesnt matter. Leave it as is after your imaging session.



People often go overboard with flats. You may not need to take flats after every single imaging session if you're imaging setup is reasonably static. With my refractor and some precautions, I sometimes only need to take flats every month or so.



You are correct that the IMX571 likes bias calibration - keep doing that with the mono version. No need to take flat darks on this sensor.

Thanks Adam,

Appreciate your comments

I have to take flats for each image as I image with Newts and my TS GPU 4 element wizz bang coma corrector introduces vignetting into the optical train (one of the trade offs for perfect pin point stars to edge of field )

Flats are no issue , Ive taken hundreds of them with my OSC 2600MC , so the 2600MM will just take a tad longer

Looking forward to Mono NB imaging especially when in Sydney under Bortle 7/8 skies ( SQM 18.50 )

Down at my south coast Obs ( Bortle 3 SQM 21.80 ) my existing 2600MC is excellent for Broadband and Narrowband , the new 2600MM hopefully will be mindboggling especially narrowband.

Cheers
Martin

[Addos (Adam)]

Originally Posted by Startrek

Thanks Adam,

Appreciate your comments

I have to take flats for each image as I image with Newts and my TS GPU 4 element wizz bang coma corrector introduces vignetting into the optical train (one of the trade offs for perfect pin point stars to edge of field )

Flats are no issue , Ive taken hundreds of them with my OSC 2600MC , so the 2600MM will just take a tad longer

Looking forward to Mono NB imaging especially when in Sydney under Bortle 7/8 skies ( SQM 18.50 )

Down at my south coast Obs ( Bortle 3 SQM 21.80 ) my existing 2600MC is excellent for Broadband and Narrowband , the new 2600MM hopefully will be mindboggling especially narrowband.

Cheers
Martin

Fair enough, a reflector does introduce a lot more potential for something to shift between imaging sessions especially if you're pulling it down every night. All I was saying is, you dont necessarily need to if you're confident nothings shifted / changed.


Im sure you'll feel rewarded with the shift to mono. Having gone the same path, I can vouch for the improved resolution and snr you get with a mono sensor. I've got the same filter set, and can confirm they're *close* to parfocal. I run filter offsets in Nina and I get a +-10 focus step difference across the set using luminance as the base. Good enough IMO. I think true parfocal probably only exists at the Chroma / Astrodon level



Im in ~19.2sqm bortle 6 and the 3nm cuts through it like butter. Sure you'll have similar success in 7/8. Interestingly moonglow on the Oiii is more prevalent than light pollution. Nothing a good Graxpert run cant fix!



Have fun with the next step of your journey!

[Startrek (Martin)]

Adam,
Thanks for your advice, much appreciated
Should be a challenging and exciting venture
I know a lot of Mono folk would disagree but I’m expecting to require more clear sky time to complete a project compared to my OSC , even with a better SNR and resolution.
My OSC is still an incredible camera so I’m fortunate to have best of both worlds

Cheers
Martin

[AdamJL]

On your camera (same sensor as my 268M) you'll need just flats and bias frames. Bias can be taken anytime. I have a master file for bias. So just take your lights and flats when you shoot.

Good luck, Martin. Looking forward to seeing what you can do with your new camera!

[Startrek (Martin)]

Originally Posted by AdamJL

On your camera (same sensor as my 268M) you'll need just flats and bias frames. Bias can be taken anytime. I have a master file for bias. So just take your lights and flats when you shoot.

Good luck, Martin. Looking forward to seeing what you can do with your new camera!

Thanks Adam,
Yeh I’ve used my 2600MC for over 3 years now and use Flats and Bias , Darks are not required and Dark Flats didn’t work as well as Bias
The new 2600MM will be no different, so Flats and Bias
The 2600MC and 2600MM have the same sensor
Cheers
Martin

[The_bluester (Paul)]

Reiterating what people have said above, shoot flats for all filters as they will tend to produce slightly different vignetting even if you can keep them perfectly clean to avoid dust bunnies. I have found that the flats are better shot at gain 0. Shooting flats on gain 100 with my 2600's tended to produce some posterisation effects that showed in the lights. Per what others have said and the same as your MC, a master bias is good to calibrate flats, can be re-used for months and allows varying flat sub lengths to be used. I have not shot a dark-flat in a couple of years.

You may find that darks will improve the images as well. I have found, particularly with the newt that if I am stretching the data for really faint stuff, flats are absolutely required but if you use them without a master dark matching your light frames the corners tend to show brightening. The longer the subs the more pronounced the effect (I assume it is what little dark current there is on these sensors rearing it's head and being divided into visibility by the master flat)

If you are setting up and knocking down every session you might need to do flats per session but I leave the camera mounted and the flats are stable for a long time (While using the same pointing angle, I show a little asymmetrical vignetting from the secondary so if I rotate the camera I shoot new flats) so long as I clean the scope side lens of the Paracorr before shooting the flats and keep it clean. Anything further away from the sensor than that is so far out of focus that it is not noticeable.

[Startrek (Martin)]

Originally Posted by The_bluester

Reiterating what people have said above, shoot flats for all filters as they will tend to produce slightly different vignetting even if you can keep them perfectly clean to avoid dust bunnies. I have found that the flats are better shot at gain 0. Shooting flats on gain 100 with my 2600's tended to produce some posterisation effects that showed in the lights. Per what others have said and the same as your MC, a master bias is good to calibrate flats, can be re-used for months and allows varying flat sub lengths to be used. I have not shot a dark-flat in a couple of years.

You may find that darks will improve the images as well. I have found, particularly with the newt that if I am stretching the data for really faint stuff, flats are absolutely required but if you use them without a master dark matching your light frames the corners tend to show brightening. The longer the subs the more pronounced the effect (I assume it is what little dark current there is on these sensors rearing it's head and being divided into visibility by the master flat)

If you are setting up and knocking down every session you might need to do flats per session but I leave the camera mounted and the flats are stable for a long time (While using the same pointing angle, I show a little asymmetrical vignetting from the secondary so if I rotate the camera I shoot new flats) so long as I clean the scope side lens of the Paracorr before shooting the flats and keep it clean. Anything further away from the sensor than that is so far out of focus that it is not noticeable.

Thanks Paul,
I’ve always matched my flats Gain 100 with lights Gain 100. I’ll try your suggestion and try Gain 0 for Flats.
My TS GPU coma corrector is excellent and provides round pin point Star to edge of field but due to its 4 element design , the focal plane is shifted about 20mm and therefore moves light cone, which results in Vignetting in the corners. I do have TV Paracorr but only use it for visual work in my 12” Dob. The Paracorr is a heavy beast but a quality piece of optics.

Cheers
Martin

[The_bluester (Paul)]

Obviously try both ways and in the end, do whatever you think best, but I use gain 0 for flats with both my 2600 cams, regardless of what gain I used for the light frames. I have found that 20+ flats per filter produces a niec master flat.

I have only ever used the Paracorr with my newt as that is what came with it.