Guidescope to OAG - experiences?

[Imme]

Hi guys,

I have recently moved to an OAG from a guidescope.....mainly as a weight saving measure. Guiding through the scope was great and if I wasn't on the brink of my mounts weight limit I likely wouldn't have changed.
So, I have gone from a 280mm specific guiding scope to a 925mm guiding (image) scope.
In reality this means I am guiding on a scope approx. 3 times more magnified than previously.
Does this mean I should expect greater errors in my guide graph?
Is it reasonable to expect the greater magnification will result in what 'appears' to be greater errors compared to what I previously saw?

The reason I ask.......using the guidescope I always calibrated fine and I could keep the image/guiding to under 1 pixel on my guidegraph. Now with the OAG it’s pushing out to 2 pixels at times and I am getting a backlash error sometimes during calibration.
The only change to the rig is the OAG and an update to the latest version of PHD.

So I guess the question I have is this. If I am moving to a guiding scope that is 3 x times the length of the previous one is it reasonable to expect my guide graph errors 'could' also be magnified by that same 3 x multiplier?

Stars - yes, on the couple subs I did testing last night they seemed ok

[Chapstick]

Did you change the focal length of the 'guidescope' in PHD2 settings? If not might be best to run through the setup wizard again or make these changes.

[Imme]

Sure did.....all setings are correct.

As I said, stars are looking OK....it's just the guiding error on the graph is now bigger. Guess I was just after verification from others that the change to the longer length guiding scope resulted in a change in guide errors on the graph.....it seems a reasonable assumption to me.

[Chapstick]

Okay cool, thought I would ask. I went from 190mm guide scope to 2800mm OAG and didn't really notice much of a change in my graphs. That is really the only experience I have in regards to this! GL

[Imme]

hmmmm......OK, well it seems maybe I do have another issue.


I did hang a 700gr field flattener off the end of my scope at the same time so maybe that's caused an issue.....I may play with balance again

[RickS]

If your guiding image scale is now 1/3 smaller then, yes, you can expect to see bigger movements on a guide graph that has pixels on the Y axis. But if you were plotting arc seconds on the Y axis then you wouldn't expect to see that.

Cheers,
Rick.

[Chapstick]

Apologies yes - I am graphing arcsec.

[Imme]

Cheers Rick.

I did notice if I switched between pixel and arc seconds then arc seconds were smaller. In the previous setup with the smaller standalone guidescope I was seeing pixels were smaller than arc seconds

[PRejto]

I would suggest that you just bin your guide camera 2x2. This should get you back closer to the resolution you previously guided at with the guide scope. I certainly bin 2x2 with my ONAG or OAG through my TEC180 and I think most do the same using OAG



Peter

[peter_4059]

Hi Jon,

I've just made a similar change to my setup, moving from guiding through a 250mm fl guidescope to an OAG on the SN10. The fl of the SN10 is 1150mm (with the paracor fitted).

Using the guidescope, my guider image scale was 3.09 arc seconds per pixel with the ASI120MM guide camera binning 1x1.
With the OAG on the SN10 I now have an image scale of 1.35 arc seconds per pixel with the ASI120MM camera binned at 2x2.

I haven't had a lot time with the OAG to trial guide settings and the Zfilter guide algorithm has just been updated in the latest version of PHD2 however I'm seeing a similar RMS value in terms of arc seconds and slightly lower peak errors so far with the OAG.

I've included a few screen shots from my PHD logs below - the first three are with the guidescope and the last three are the OAG.

Peter

[Imme]

Correct me if I’m wrong but with the pixel scale being approx 2.5 smaller.....does that mean that with similar figures you’re getting guiding that is 2.5 times better? Or am I just being too simplistic?

[peter_4059]

The comparison of RMS and peak arcseconds is on the same basis - ie that accounts for the binning and focal length differences.