Guiding image scale

[Anthonyexmouth]

When is it too much?

Just thinking of options for when i get my new widefield scope and thinking about swapping my asi290 from my st80 to my ed80 and then use that for guiding when imaging or then using that for lunar imaging now and then. I run a side by side rig.  would 600mmf/l and the 290 be a bit much? 

[Clarkey]

I don't think you can 'over-guide'. However, I don't think you would notice much difference guiding with the ED80 over the ST80 in the final images. I use an ST80 to guide my RC8 at 1600mm FL with a 120 mini so widefield it would be more than capable.

[Anthonyexmouth]

2 minutes ago, Clarkey said:

I don't think you can 'over-guide'. However, I don't think you would notice much difference guiding with the ED80 over the ST80 in the final images. I use an ST80 to guide my RC8 at 1600mm FL with a 120 mini so widefield it would be more than capable.

I'm just wondering about PHD getting a bit flakey because of the scale. TBF, with my pier being solid and the mount running as good as it does I could probably get away without guiding at the f/l i'm going to. 

[Clarkey]

Given PHD will guide with an OAG at 2500mm or more with banana shaped stars, I think the ED80 will be a doddle!!

[Anthonyexmouth]

2 minutes ago, Clarkey said:

Given PHD will guide with an OAG at 2500mm or more with banana shaped stars, I think the ED80 will be a doddle!!

Of course, didn't think about that. 

[newbie alert]

1 hour ago, Anthonyexmouth said:

When is it too much?

Just thinking of options for when i get my new widefield scope and thinking about swapping my asi290 from my st80 to my ed80 and then use that for guiding when imaging or then using that for lunar imaging now and then. I run a side by side rig.  would 600mmf/l and the 290 be a bit much? 

Not at all , I use a 290 and 1280mm ( oag and a SCT)and it's never missed a beat... Never had to look or rotate for a star.. awesome camera

[ollypenrice]

I use an ST80 to guide my TEC140 imaging rig which has a sampling rate of 0.9 arcsecs per pixel. I get a guide RMS of 0.3 arcsecs. That's better than I need but, as said above, you can't over-guide. The ST80 is a great guide scope but be sure to do three things: make sure the cast metal back is tightly held to the main tube with its 3 radial screws. I wouldn't hesitate to put a bit of Araldite between the two parts to be double sure. Make sure the draw tube locking bolt is nice and tight to stop draw tube flexure. Use some kind of extension tube to get focus. One of my guide cameras has a very short backfocus so I needed two Barlow bodies (without lenses) to add the distance. I Araldited these together and also Araldited the complete extension into the draw tube. (This ST80 is going to be a guidescope for life but it's totally reliable.🤣 )

Olly

[Anthonyexmouth]

17 minutes ago, ollypenrice said:

I use an ST80 to guide my TEC140 imaging rig which has a sampling rate of 0.9 arcsecs per pixel. I get a guide RMS of 0.3 arcsecs. That's better than I need but, as said above, you can't over-guide. The ST80 is a great guide scope but be sure to do three things: make sure the cast metal back is tightly held to the main tube with its 3 radial screws. I wouldn't hesitate to put a bit of Araldite between the two parts to be double sure. Make sure the draw tube locking bolt is nice and tight to stop draw tube flexure. Use some kind of extension tube to get focus. One of my guide cameras has a very short backfocus so I needed two Barlow bodies (without lenses) to add the distance. I Araldited these together and also Araldited the complete extension into the draw tube. (This ST80 is going to be a guidescope for life but it's totally reliable.🤣 )

Olly

I'll be taking the st80 off. Been using it for 3 years. The ed80 is the one that may be relegated to guiding duties with a part time job of some lunar imaging. A sharp star 76edph is going to be my new imagining scope on my pier. 

[vlaiv]

If you go too high in sampling rate of your guide system - you'll introduce too much noise.

Upping sampling rate while keeping same aperture - leads to lower SNR for same integration time - so you either need to increase guide exposure or bin.

I'd say that upper bound on guide exposure is determined by your mount. I'd say that you don't need more than x3-4 of ratio between best guide RMS for your mount and guide precision.

Guide precision is set to something like 1/16 - 1/20 of a pixel (let's use 1/16 as conservative figure).

This all means that if you for example have Mesu 200 and expect to guide down to 0.3" RMS regularly - then your guide precision can be 0.1" - no need for more than that. Pixel size in this case can be x16 that - so 1.6"/px. You don't really need to go lower to have precise enough guiding (you can - I'm guiding at 1.0"/px because I use OAG at 1600mm and it gives me ~0.5"/px natively so I bin that by x2).

2.9um at 600mm FL will be also around 1"/px - so you don't really need to do anything. It will be ok as it is - for even very precise mounts. You don't need to bin either.