Considering going for OAG - please advise on guiding!

[Gib007]

Sounds like you'll have yourself a quality OAG setup there.

Hehe, I do hope so, after all I've spent on it! Obviously I will start again with PHD Guiding settings. I will try the default settings and see how they work out and tweak from there. 

[kookoo_gr]

FYI in order to calcultae the min. motion for the phd settings there is a formula, it was posted a few years ago from another member of this forum but i kept a copy of the post at my pc. I have confirmed the calculations and they work, you only need to adjust the aggresion settings (mine are at 2000 for dec and ra). The calculations are made for a Lodestar camera and a 50mm finderscope, but you can enter the SSAG/rc 8" data:

Firstly the formula for the min.motion:-
I've found a formula to work out the resolution of a camera and scope in arcseconds per pixel. From that you can work out the minimum pixel movement the camera can make out. Here it is:-
as/pixel = (206.3 * ccd_pixel_size) / focal_length.
    (206.3*5.4)/1625=0.68

The Loadstar has 8.2um * 8.4um pixels and you have a focal length of 210mm so:-
(206.3 * 8.2) / 210 = 8.05 as/pixel.
(206.3*8.2)/1625=1.04

We want pixels per arcsecond so I flip it:-
1/8.05 = 0.124 pixels/arcsecond.
1/1.04=0.96

As PHD wants sub-pixel movements I divide that number by half a pixel:-
0.124 / 2 = 0.06. 

So as you said 0.05 is just about right.

[Gib007]

FYI in order to calcultae the min. motion for the phd settings there is a formula, it was posted a few years ago from another member of this forum but i kept a copy of the post at my pc. I have confirmed the calculations and they work, you only need to adjust the aggresion settings (mine are at 2000 for dec and ra). The calculations are made for a Lodestar camera and a 50mm finderscope, but you can enter the SSAG/rc 8" data:

Firstly the formula for the min.motion:-

I've found a formula to work out the resolution of a camera and scope in arcseconds per pixel. From that you can work out the minimum pixel movement the camera can make out. Here it is:-

as/pixel = (206.3 * ccd_pixel_size) / focal_length.

    (206.3*5.4)/1625=0.68

The Loadstar has 8.2um * 8.4um pixels and you have a focal length of 210mm so:-

(206.3 * 8.2) / 210 = 8.05 as/pixel.

(206.3*8.2)/1625=1.04

We want pixels per arcsecond so I flip it:-

1/8.05 = 0.124 pixels/arcsecond.

1/1.04=0.96

As PHD wants sub-pixel movements I divide that number by half a pixel:-

0.124 / 2 = 0.06. 

So as you said 0.05 is just about right.

I love it, thank you. I always love being given mathematics WITH a methodology. That 206.3 factor in the first equation is something I had to derive myself before I believed it, hahaha. Many thanks for this. I will use it together with my imaging focal length with the OAG to work out the best Min. Motion setting. As for RA Duration and Dec Duration, I'm not 100% sure what to use for an imaging focal length of about 1100mm (my Altair Astro 8" RC) and then for my wide-field setup with 355mm (Borg 77EDII F4.6). I have a good idea of what each setting in PHD Guiding does however so I know more or less what to look for and where to start. 

[Gib007]

Something interesting to add actually. The Astro-Physics CCDT67 performs its focal reduction depending on distance from lens. If the OAG sits directly in front of the imaging CCD camera, in effect the OAG is seeing a slightly different focal length to the imaging CCD camera. My imaging CCD camera sits at a distance to give it 1092mm of focal length. Assuming I can keep this exactly the same (it will change a little with the addition of the OAG and modifying of spacers used), the OAG will be roughly 17.5mm in front of my imaging CCD sensor, simply because 17.5mm is my imaging CCD camera's optical backfocus (distance between front T2 thread and CCD sensor). This means the guiding camera would have 1169mm of focal length. Therefore:

Imaging Camera (~1092mm focal length) = (206.2648 x 5.4) / 1092 = 1.02 arcsec/pixel

Guiding Camera (~1169mm focal length) = (206.2648 x 8.2) / 1169 = 1.45 arcsec/pixel

Therefore guiding camera has 0.69 pixels/arcsec (1/1.45). Assume half a pixel for sub-pixel movements, then PHD Guiding's Min. Motion should be set to about 0.35. I guess I can always go a quarter of a pixel and so set about 0.17. In that sense then, the default 0.15 setting is probably alright. 

[fwm891]

I much prefer OAG guiding if possible. If you can use pulse guiding rather than st4 you only then need the usb link not the second st4 cable (less to tangle up)

[Gib007]

I much prefer OAG guiding if possible. If you can use pulse guiding rather than st4 you only then need the usb link not the second st4 cable (less to tangle up)

I was initially resistant on OAG because of my strong liking for a small guidescope system. It seems that for unlimited exposuresl length, OAG is where it's at.

[sloz1664]

I agree OAG guiding is the way forward once you get the spacings right for your imaging combinations.

I also agree that pulse guiding rather than ST4 that removes the awful cable which is prone to failure.

But why stop there. I now control my mount via bluetooth which removes the usb link and another cable.

Good luck on your OAG adventure and hopefully nice round stars.