Calibrating in Tycho Tracker using ASTAP masters

[Bill Tschumy]

I'm a little unclear how to set up calibration in Tycho Tracker when using master calibration files created in ASTAP. It appears ASTAP creates a master Dark that does not subtract the Bias. The master Flat does have the Bias subtracted. ASTAP does not create a master Bias. Is this all correct?

I've attached the Calibration dialog in Tycho. Since the Flat has the Bias subtracted I assume I leave the Calibrate unchecked? But this implies the Dark has beee subtracted which it hasn't

No clear what to do for the Master Dark.

It sort of seems like Tycho is set up to use masters in a way that is different than ASTAP produces.

Thoughts?

[han.k]

Hi Bill,

This subtracting a bias from a master dark doesn't make any sense.

The principle of using a dark/bias is very simple:

You expose a frame with light and you do the same again for second frame without light. Then subtract the second frame from the first to get a clean light signal.

So it is

1) light - dark
2) flat - flatdark
3) Correct then the (light-dark) with the (flat-darkflat) to compensate for the less light in the corners.

A bias is equivalent to a dark only with an incorrrect exposure time.

I would suggest you contact the Tycho author to clarify this. For me this menu is born out of an misunderstanding.

Cheers, Han

[Bill Tschumy]

Han,

Thanks for the response. I had always assumed flat darks were the same as biases, Is that not the case? In ASTAP where ou have "Flat darks" I always put my bias frames. Have I been doing this wrong?

I will ask the Tycho author about what you said about subtracting the bias from a master dark.

[han.k]

You could use a bias instead of a dark. This is because the dark doesn't change much by the exposure time. With a bias you capture only the read noise. With a dark you capture the read noise and dark current. The dark current is very small so you could use a bias instead. The dark current influence grows by the time. So for longer exposures a dark is better then a bias. So using a dark is better then using a bias.

Share the Tycho author response with me. Maybe he is doing something special, but I don't see it at this moment.

Cheers, Han