Question on guiding

[AstroMuni]

I have a basic question on guiding. I have read about guiding and the options; the options below are taken from https://agenaastro.c...hotography.html

 -   A separate guide scope and guide camera
 -   An off-axis guider that splits off light collected by the main telescope, but which directs the light to a separate guide camera that detects a guide star somewhere in the same field of view as the main object that's being imaged
 -   A dual-chip CCD camera on the main scope with one chip for imaging the main subject and one for the guide star
 -   A specialized dual-sampling CCD with some pixels designated for guiding and some for imaging the main target.

My question is why do they need multiple sources to achieve this? And why cant the image from the main camera be used as the source for the guiding.

Edited June 9, 2020 by AstroMuni

[Skipper Billy]

Guide scope and cam are usually used for short focallength scope - up to about 1000mm

OAG comes into its own with focal lengths over about 1000mm

The guide cam needs to take an image of about 2 - 5 seconds constantly - the imaging cam cant do two things at once - eg during a 600 second exposure it cant be clattering away taking guiding images 150 times during the long exposure.

Hope that helps!!

[vlaiv]

It is actually related to read noise of sensor.

With modern low read noise sensors, we are approaching moment when no additional guiding system will be required.

As has been pointed out, at the moment, there is discrepancy between imaging exposure and guiding exposure - a few magnitudes of a difference. Imaging exposures tend to be in hundreds of seconds while guiding exposures are in seconds.

Since difference between long exposure vs short exposure image quality depends only on read noise (or more precisely - it's relation to other noise sources) - we still need to keep our imaging exposures at least few minutes long. With advent of very low read noise sensors - this time will reduce and at some point - exposure lengths will match and then you'll be able to guide on imaging exposure.

In fact - something like that is already partially possible in what is called EEVA - short exposure live stacking where exposure lengths are tens of seconds long. I don't think software is yet capable of doing it, but in practice EEVA software would benefit from such guiding as it would be able to dither and improve results.

[AstroMuni]

4 hours ago, Skipper Billy said:

Guide scope and cam are usually used for short focallength scope - up to about 1000mm

OAG comes into its own with focal lengths over about 1000mm

The guide cam needs to take an image of about 2 - 5 seconds constantly - the imaging cam cant do two things at once - eg during a 600 second exposure it cant be clattering away taking guiding images 150 times during the long exposure.

Hope that helps!!

So the guide camera needs to take samples at less frequent intervals compared to the main camera?

Correction: I think its the other way around 🙂

Edited June 9, 2020 by AstroMuni

[vlaiv]

1 hour ago, AstroMuni said:

So the guide camera needs to take samples at less frequent intervals compared to the main camera?

Correction: I think its the other way around 🙂

Yes, other way around.

It really depends on mount used. Higher quality and better performing mounts don't need as frequent corrections. Many people use 1-2s exposures for guide cycle. That is often fast as seeing influence can be quite big on those scales. Better mounts tolerate 4-8s guide cycle, while top level mounts can go over 10s of seconds for single correction - mount simply stays on target that long.

Guide exposure depends on several factors - one is seeing, you need long enough exposure to average seeing effects. Other is quality of polar alignment. There is a tool that will calculate DEC drift rate depending on polar alignment error. In most cases, this rate is something like 1-2 arc seconds per minute or less. From that you can calculate guide exposure length - just choose what is maximum offset in DEC you will tolerate for single guide exposure.

Similarly, another factor, acting in RA is periodic error. This can be as much as 30 arc seconds peak to peak in one worm cycle or as low as few arc seconds. Depending on how smooth it is (for example, pure sine wave is probably the smoothest form you can have - but it is rarely so), you can again calculate max drift rate in RA. Based on that and wanted max correction - you can calculate guide exposure.

Ideally - you want longer guide exposures as that means that seeing effects will be reduced, your polar alignment is good enough and your periodic error is low enough and smooth to be able to use longer guide exposures.

Sometimes in the future it can even happen that imaging exposures become shorter than guide exposures ( less read noise and better mounts requiring corrections less often) - but that is easily handled by "summing" (stacking) multiple short imaging exposures to form single longer guide exposure - thus still being able to both guide and image with single camera.