Seeing Vs PHD2 guiding ?

[knobby]

Quick question for the PHD gurus ... I'm getting mixed success with my AZ-GTI, some nights just under 1 arc second but others hovering around  the 2 arc second mark .

I recently noticed the seeing index on Meteoblue and it rarely ever goes below 2 arc seconds for my location. 

Question is, is my guiding limited by the seeing ?

On the bad nights I've tried longer guide exposures ( normally 1 second and tried up to 5 ) but no real improvement.

[vlaiv]

I would say that you are probably mount limited rather than seeing limited. With AzGTI, I would be surprised if it could do less than 1" RMS. In fact I think that stable guiding below 2" RMS is very good achievement with it. It's just a small mount and not very precise. According to this:

http://eq-mod.sourceforge.net/prerequisites.html

It has about 1/5 of precision of Heq5/Eq6 class mounts. Those have 9024000 stepper ticks per revolution - which makes their stepper motors have resolution of 1296000 / 9024000 = ~0.143617" per micro step (1296000 being number of arc seconds in full circle or 360 x 60 x 60).

If AzGti has 2073600 ticks per revolution, then resolution of motors is 0.625" per micro step. You can't really expect mount to have below 1" RMS if it can't keep DEC precision below 0.625", and two to three times this value is more realistic or 1.5"-2" RMS.

Btw, resolution in above link for AzGti is given to be 0.0625 - which is less than 0.143617 that of Heq5 - which I believe is a typo with 0 added as is clearly shown by above calculation.

[knobby]

Cheers Vlaiv, I must say that I'm really impressed with the little mount (so far) and the 1.5 to 1.8 figure seems about right as I'd say that's probably where it normally sits. Just wondered why / how I'd managed below 1 on a few occasions ... 

[vlaiv]

53 minutes ago, knobby said:

Cheers Vlaiv, I must say that I'm really impressed with the little mount (so far) and the 1.5 to 1.8 figure seems about right as I'd say that's probably where it normally sits. Just wondered why / how I'd managed below 1 on a few occasions ... 

I think that such mount can do below 1" RMS for a limited amount of time if there are no significant outside influences and seeing is really good. Seeing needs to be good enough not to cause corrections in position that are larger than single step of stepper motor, and 0.6" shift of guide star in longer exposure - like 1-2 seconds requires really poor seeing. If there is no wind and vibrations are low - it can keep the mount at about 1".

You also need to have good guide resolution to be able to properly measure 1" and below RMS error. I would say that you need to have at least 4"/px guide resolution for something like that. With camera like ASI120 or ASI224 - that would mean at least 190mm of focal length in a guide scope.

[knobby]

Makes sense, I'm using finder guider and ZWO 120 mm which works out at 4.3 "/pixel . Imaging at 335 mm ZWO 1600 pro  at 2.34 "/pixel.

[vlaiv]

I also have this mount and at some point want to mess around with it and do some light imaging.

I purchased it to be moon gazing mount because it is light weight and tracking and I have Mak 102 sitting on it, but will do some EEVA at some point with it. Still need to get all the bits needed to do EQ conversion - wedge and counterweight ...

If you can manage 1"/px semi regularly (which would be great) then that would be perfect for 2.34"/px. I think that even with 1.5" RMS you will be ok, but at 2" your images might start to feel a bit soft because of mount.