Best acquisition strategy for low altitude objects

[lucchett]

Hi,
M20 is quite low at my latitude (+42), only about 20-25 degrees.
my nominal resolution is 1.83 arc/sec pixel, binning 1x1.

I can estimate that the FWHM won't be better than 3.5, even 4, at this altitude. (tipically is about 2.5 from 50 deg but I am only 1200 feet above see level)

I usually take my frames R,G,B,Ha in binning 1x1.

In this case, the idea would be to follow this schema:

R,G,B: bin 2x2
Ha: 1x1

The low resolution in bin 2x2 (3.85arcsec/pixel) will be less critical due to the seeing.
Undersampling could be addressed with drizzling.

telescope : tec 140
ccd: kaf 16803
This will allow me to collect much more data/signal in just a few days.

what do you think?
I would be really grateful if you can give me your thoughts on this.


thanks a lot,
Andrea

[mschuster]

IMHO worth trying if you are going for dimmest structures: especially if expected FWHM > bin 2x2 resolution, then bin 2x2 Ha also for better SNR.

Mike

[RickS]

I wouldn't bother with hardware binning on a KAF-16803.  There's little benefit in reduced read noise with this sensor.  Bin in software later if needed.

Cheers,
Rick.

[lucchett]

Thank you Mike & Rick.


Rick , I am not sure I got your point right.
I understand the read noise can be similar.
But binning can give me much more signal in the same time window (object signal vs sky noise).
In addition Drizzling can help to recover the undersampling.

Are you saying that downsampling Bin 1x1 frames has the same effect?

Thanks,
Andrea

[RickS]

Rick , I am not sure I got your point right.
I understand the read noise can be similar.
But binning can give me much more signal in the same time window (object signal vs sky noise).
In addition Drizzling can help to recover the undersampling.

Are you saying that downsampling Bin 1x1 frames has the same effect?

Andrea,

The only benefit of hardware binning is that you may get less read noise.  Take the example of 2x2 binning.  If your read noise for a 2x2 super-pixel is less than 4 times the read noise for a single pixel then you may gain some SNR improvement if you are read noise limited.  If you are not read noise limited then you gain very little.  If your sensor doesn't give you that 4 times improvement in read noise (like most of the Kodak/TrueSense sensors) then you don't gain much either.

The alternative to hardware binning is software binning.  If you resample your data in PI with IntegerResample and halve the resolution then you'll get the same SNR benefit except for the effect of the read noise.  This is a more flexible approach since you can decide to do it after capture time.  You can also use the data in different ways - if you capture Lum at 1x1 and RGB at 2x2 then you can't use the RGB to generate synthetic luminance to enhance the captured Lum like you could have if you got all the data at 1x1.

Drizzle is not a magic free lunch.  If you upsample with Drizzle then you gain that extra resolution at a cost of extra noise.

Hope that helps to explain my position.  I don't see any reason to do hardware binning unless you have a sensor which supports it well and you are also capturing data that is read noise limited (rarely true except for narrowband data unless you are taking subs that are too short.)

Cheers,
Rick.