Autoguiding and averaging seeing effects

[Wall-E]

Hi all, 

This is my first post (apart from the one in the welcome section). If my question is not cristal clear, please tell me and i'll turn this differently.

Does anybody know about PHD 2's logic to autoguide? Indeed, considering it is often advised not to send corrections too rapidly, e.g. at the same frequency as those high fps camera, because we would end up autoguiding on seeing disturbances and not actual mount errors (e.g. low freq P.E.). So, when setting the correction interval to say, every 1s, what does happen in those autoguiding softwares? I currently foresee two cases:

1) do they calculate the centroid position of the star only on those very shots (every 1s)?

2) do they still calculate the centroid position of the star at the freq. of the camera (say, for 50ms exposure, we'd have an image every ~50ms) and average this over 1s to send the correction every 1s (again, here, 1s is just an example)?

In PhD2, the only setting related to this is the "time lapse" in the camera setting. The name of this mode tend to suggest the former case, while I would hope that it would do the latter. Does anyone know what it actually does? And is there any other software that uses the method in case 2)? (which I would prefer)

Thanks

[Freddie]

When you set the duration, you are setting the exposure time. Therefore if you set 3 seconds, the cam will expose for 3 secs and then PHD will analyse that frame to determine the movements needed, it will then repeat this when the next 3 sec exposure is complete. The length of exposure is therefore "averaging" the seeing over the exposure period.

[Wall-E]

Freddie, thank you for your reply. It's not quite what i was getting at though

It is true that exposure time can be used to average seeing, but only as long as you don't saturate, otherwise this falses the centroid detection (PHD2 has a warning for this). Another method is to defocus the star a bit. But what i'm asking is whether there's a software that uses the frames taken at higher fps than the correction rate to calculate the seeing-averaged centroid of all the intermediate frames and thus send out a correction that is less impaired by seeing. 

Imagine a 50ms exposure (which is what I typically use with my guiding camera,  ZWO ASI 220) and correction interval of 1s. There's 20 frames for which the software can calculate a centroid position on non-saturated star, and average this series to get a more accurate pointing correction.I sn't that implemented in any software?

[Freddie]

Not that I know of. Can't say I've looked though as PHD works just fine for me with a 3 sec exposure. The way PHD works, a 50ms exposure would be way too short.

[Wall-E]

Ok, 50 ms is typical in my case with PHD 2, but it's less about software, but more about sensor sensitivity, optics, camera gain... with an 80/600, and a ZWO ASI 120 MM-S, 3s is just way too long on any star visible by eye. 
So the current state of autoguiding implementations to average seeing effects seems to consist in finding a star that is faint enough to allow the photographer to set an exposure time that is long enough (say 1 to 3s) to average the seeing without saturating the star. Do I get that right? Don't you sometimes have trouble on a field with only bright stars that does not allow you to set your exposure long enough? Or do you just reduce the gain of your camera so you can pose for a longer time? 

[Thalestris24]

Hi

You should use the phd2 star profile tool to help select a suitable guide star. There are usually plenty of stars available in a typical guide scope fov that will expose nicely in the 2-3 sec range. You definitely don't want to use a bright star that will saturate.

Louise

[Wall-E]

21 minutes ago, Thalestris24 said:

Hi

You should use the phd2 star profile tool to help select a suitable guide star. There are usually plenty of stars available in a typical guide scope fov that will expose nicely in the 2-3 sec range. You definitely don't want to use a bright star that will saturate.

Louise

Hi Louise, I haven't tried the star profile of PHD2 yet, will do. If you always find a star that fits the 2-3s range, then it's fine. In my tests I just plugged my camera in my main scope, and so, things were just too bright, forcing the fast exposures, but this will be less the case in an actual guide scope. Nonetheless, I'll be guiding with a Lacerta MGEN-ii, so I won't even be using PHD for guiding. In the MGEN, I wonder what are the different modes that can be used to track a star and average seeing, apart from the longer exposure. For example, I didn't have much success with PHD2 when defocusing the star a bit, while it worked fine years ago with PHD V1 even with fast exposures and high cadence, and a webcam (Philips toucam pro). So I wonder if the embedded software of the MGEN can handle the defocusing method. 

[Ken82]

You cant/wont over saturate a star by selecting a higher (camera exposure duration) in the main phd2 software face. Indeed this is only to account for different focal lengths and pixel ratios (guiding with a longer focal length requires a longer exposure and possibly camera binning) to compensate for seeing conditions. If you select auto camera exposure under the brain icon this is the true camera exposure and will make automatic  adjustments to keep your desired signal to noise ratio. In the star profile tool you should select a star with a nice peak but not with a flat top which indicates an over saturated star.

[Wall-E]

42 minutes ago, Ken82 said:

You cant/wont over saturate a star by selecting a higher (camera exposure duration) in the main phd2 software face. Indeed this is only to account for different focal lengths and pixel ratios (guiding with a longer focal length requires a longer exposure and possibly camera binning) to compensate for seeing conditions. If you select auto camera exposure under the brain icon this is the true camera exposure and will make automatic  adjustments to keep your desired signal to noise ratio. In the star profile tool you should select a star with a nice peak but not with a flat top which indicates an over saturated star.

We eventually saturate stars if the exposure is too long (non Auto). Do you mean that I'll never saturate the star ONLY IF the camera exposure is set to auto? I'll try this auto mode and see what happens. 

[Wall-E]

After a quick test, this Auto mode actually does not address my original problem. Since it adjusts the exposure to reach the desired SNR, I lose control over how much time i'd like the seeing to be averaged. Louise's and Freddie's method seems to better address the problem here, by selecting a star faint enough to average the seeing. 

No comment on the defocusing method? Has anyone tried it with success? 

[Thalestris24]

48 minutes ago, Wall-E said:

After a quick test, this Auto mode actually does not address my original problem. Since it adjusts the exposure to reach the desired SNR, I lose control over how much time i'd like the seeing to be averaged. Louise's and Freddie's method seems to better address the problem here, by selecting a star faint enough to average the seeing. 

No comment on the defocusing method? Has anyone tried it with success? 

Hi

I think maybe you are overthinking the selection and use of a suitable guide star.  A star that allows an exposure in the 2-3 sec range without saturating will do the job! It will even out the seeing and give you sub-pixel guiding  Reasonably good focus will give you a good snr. Guiding should operate as expected.

Louise

[Wall-E]

6 hours ago, Thalestris24 said:

Hi

I think maybe you are overthinking the selection and use of a suitable guide star.  A star that allows an exposure in the 2-3 sec range without saturating will do the job! It will even out the seeing and give you sub-pixel guiding  Reasonably good focus will give you a good snr. Guiding should operate as expected.

Louise

Louise, as i said above, i'm fine with your  method. 

I'll try to implement also case 2) (1st post) in my dev project though, as it should work on any star (see "LightDrops" in Github). 

Thanks.