Image Scale for acceptable guiding

[Startrek]

I’ve read a lot of information on forums etc... which recommend your image scale should be less than 4x between your main imaging system and your guiding system optics to ensure acceptable guiding

I’ve calculated my system below -

Image Scale Calculation

P is Image scale in arc sec/pixel

Formula
P= ( pixel size in microns x 206.3 )/ focal length in mm

Main Imaging Scope
New 8” f5 Newtonian imaging telescope (focal length is 1000mm)
Canon 600D pixel size is 4.3 microns
Therefore image scale would be 4.3 x 206.3 / 1000 = 0.88 arc sec/pixel

Guide Scope
Orion 50mm Guide scope ( focal length 162mm)
ZWO ASI120MM-3 Guide camera pixel size is 3.75 microns
Therefore image scale would be 3.75 x 206.3 / 162 = 4.77 arc sec/pixel

Image ratio imaging camera to guide camera

4.77 / 0.88 = 5.4 x

Guiding Advice
Advice is you should be below 4x image scale from guide scope camera to main imaging camera to guide at an acceptable error

Please let me know if I’ve made a mistake in the calculations or the methodology but my image scale differential is 5.4 x and I can comfortably guide at around 1 arc sec error , a little worse when seeing is ordinary and a little better sometimes

I welcome any comments from experienced IIS members in regard to the above

Thanks

[multiweb]

I guide with an OAG and my guider pixel size is very close to my imaging camera so I guess I guide at the same image scale. I think the more important question is what FL you're imaging at and what your guiding accuracy is in seconds of arc or sub arc sec if you're lucky. Then depending on the seeing conditions at your site you're good enough, or not.

[Startrek]

My guide scope specs say you can use it for focal lengths up to 1500mm
My scope is 1000mm and has an image scale 5.4 times the guide scope image scale
At 1500mm focal length you would have an image scale 7.4 times the guide scope image scale
So the magical < 4x image scale is probably a figure in the “ideal world” I suppose

[RobF]

Marc's advice is gold.

You also need to consider quality of mount, PE curve (rate of change and any irregular parts of the cycle), moment arm of scope (e.g. very long, or prone to picking up wind at your site).

Any flexure in your guide scope arrangement will quickly undermine your calcs. OAG is pretty hard to beat if you work within its limitations.

[ChrisV]

+1 with what the others have said. But I have tried an OAG on my newt and it didn't really improve things compared to using my guidescope - so I stuck with the guidescope as I have no plans to try really long focal length scopes.

Your calculations are correct. I'm using same newt specs as you, but a 200mm guidescope with an asi290 which has 2.9um pixels. So I'm 3x imaging scale on the guidescope. But you're using an EQ6R which I would say is better than my CGEM - which I'm sure would balance out my smaller guidescope imaging scale. So just make sure your guidescope is clamped really well. My guidescope is held by two heavy duty ADM tube rings which are firmly held to the newt tube rings.

The main thing for me is the seeing. On bad nights I'm getting 1.3arcsec rms error in PHD2 - I can see a slight egg shape to the stars on my images. And last night it was 0.8arcsec - all looked good in my images.

So I suppose what does your guiding graph look like and how are your stars coming out in the images? Are you happy with that?

[kens]

The 4x is just a guideline just as the 1500mm is. The 1500mm assumes that you have the same pixel size on both guide and imaging cameras and is made on the basis that autoguiding software can potentially resolve down to about 0.1 pixel movement. Your 50mm guidescope has a focal length of around 160mm which is about 0.1x the imaging focal length. It ignores the differential flexure that often becomes evident at long focal lengths and the mechanical issues of being able to point your mount at that 0.1 pixel accuracy.
The 4x guideline is also based on the autoguider being able to resolve down to 0.1 pixel but given that is an ideal figure it provides a bit of margin for deviation from that. Both guidelines aim for the smallest detectable movement to equate to less than 1 pixel on the imaging camera.
Another guideline is that above 1000mm focal length you should consider using an OAG due to differential flexure in the guide scope.
Astrophotography is very complex due to the large number of variables involved. That's why people come up with guidelines to try and simplify things. But the guidelines are only as good as the assumptions underpinning them.

[Startrek]

Thanks Chris and Ken
I agree at the end of the day you calculate as much you like with optical mechanics and mathematics but good guiding is really dependant on seeing conditions , wind , rock solid equipment on your OTA , stable ground and a bit of luck thrown
I’m usually guiding around 1.0 to 1.3 arc sec error and can take 50 x 5 minute subs with perfect round stars ( with dithering thrown in )
I know I will never get down to 0.5 to 0.8 arc sec error with my big rig but hey I don’t need to , the images are the proof of the pudding

Thanks