Does image scale matter on a guide camera?

[Stonius (Markus)]

So far I've been using the ASI290MM as a guide camera AND a planetary imaging camera, and I've been pretty happy with it.


However, i was thinking of getting another camera just for guiding, so I don't have to break apart my imaging train when imaging different objects (I have separate planetary and deep sky filter wheels).


The thing is ASI290's are not that common at the moment so it got me thinking about the ASI174 mini.


It has the same resolution, but the pixels are bigger - the sensor is twice the size of the ASI290!


So will that double my guiding error? Does it make a difference?


It may also give me more guide stars to choose from, I guess? I'm assuming a bigger sensor would capture a wider field of view?



Cheers


Markus

[PKay (Peter)]

Hi Markus

Last week I started using a ASI120mm mini for guiding.

60mm guide scope.

Very good with guiding accuracy down to fractions of a second.

[Startrek (Martin)]

From my experience I found a little more focal length on your guide scope does improve your guiding marginally
I originally started with a 50mm guide scope with a 162mm focal length ( I use to swap my offset shoe mounted finder scope ) and guide camera ZWOASI120MM-S USB3 pixel size 3.75uM
And now I use a 60mm Orion Guide scope with helical focuser ( 240mm focal length ) central axis mounted on a solid bar off the tube rings with same guide camera. I’m consistently guiding around or just 1 arc sec total rms ( unless there wind or crappy atmospheric conditions)
I image with newts at 900mm and 1000mm focal lengths
So from my experience go with a little longer focal length and aperture
The 60mm guide scope has been a very popular guide scope across the globe and has been rebranded as Orion and other brands but effectively same scope
Cheers

[Sunfish (Ray)]

If you are thinking of going on to an OAG at some point you might want to find out more about that when you buy a guiding camera. Different format cameras have different backfocus and connections which need to pair with the OAG you choose so you can achieve focus without also buying another bunch of adapters or a different guide camera. If you are sticking with a guide scope and shorter focal lengths it will not matter.

I have found that guiding with my larger chip camera on an OAG is easier as more stars are available on auto but harder to see the stars and focus. The OAG is not as good on a newt as on a refractor or SCT due to coma and short back focus but it does work .

[Merlin66 (Ken)]

I’m little surprised that the discussion doesn’t raise the arcsec/ pixel for the guide system vs the arcsec/ pixel of the imaging camera.
PHD is very good and claims it can give effective guiding when the ratio between the two is around 5 to 10 times ie the guide arcsec/ pixel is x10 the imaging scale.

[Sunfish (Ray)]

Maybe you better explain that in detail as other quote much different figures. I think my Cmos cameras are about the same pixel size

https://www.cloudynights.com/topic/6...g-image-scale/

Originally Posted by Merlin66

I’m little surprised that the discussion doesn’t raise the arcsec/ pixel for the guide system vs the arcsec/ pixel of the imaging camera.
PHD is very good and claims it can give effective guiding when the ratio between the two is around 5 to 10 times ie the guide arcsec/ pixel is x10 the imaging scale.

[Merlin66 (Ken)]

OK, maybe a little simplistic.


Assuming (good?) seeing of 2 arcsec, the the FWHM of a star at 1000mm fl would be 9.7 micron. 19.4 micron at 2000mm and 2.4 micron @ 250mm.


The pixel size of a typical "imaging" camera (ASI 1600) is 3.8 micron.
Guide cameras - QHY5Lii 3.75 micron and Lodestar 8.2 micron.


What do you think the outcomes would be? "Mini guider" vs OAG???

[Startrek (Martin)]

Originally Posted by Startrek

From my experience I found a little more focal length on your guide scope does improve your guiding marginally
I originally started with a 50mm guide scope with a 162mm focal length ( I use to swap my offset shoe mounted finder scope ) and guide camera ZWOASI120MM-S USB3 pixel size 3.75uM
And now I use a 60mm Orion Guide scope with helical focuser ( 240mm focal length ) central axis mounted on a solid bar off the tube rings with same guide camera. I’m consistently guiding around or just 1 arc sec total rms ( unless there wind or crappy atmospheric conditions)
I image with newts at 900mm and 1000mm focal lengths
So from my experience go with a little longer focal length and aperture
The 60mm guide scope has been a very popular guide scope across the globe and has been rebranded as Orion and other brands but effectively same scope
Cheers

On my 6” f6 newt ( fl 900mm ) the image scale with my new ZWOASI2600MC is 0.86 arc sec per pixel
My 60mm guide scope ( fl 240mm) with ZWOASI120MM-S image scale is 3.22 arc sec per pixel

So ratio is about 3.7 x

The last 4 or 5 nights in Sydney has been clear for imaging and I’ve been consistently guiding around 0.80 RA and around 0.60 DEC no issues pushing 5 minute subs with round tight stars to edge of field

So the theory about 5x or 10x image scale between main rig and guiding rig may be only a general rule as many factors come into play

Any thoughts or comments

[Merlin66 (Ken)]

I'm used to applying Nyquist/ Shanon sampling which basically says the star image should cover at least 2-3 pixel to ensure adequate signal information.


Some of the guider set-ups don't even achieve a full pixel width on 2 -3 arcsec images.


If the image is smaller than one pixel even as it moves around within the pixel the pixel intensity is the same (??!)


Thinking about your example:
""On my 6” f6 newt ( fl 900mm ) the image scale with my new ZWOASI2600MC is 0.86 arc sec per pixel""
The pixel size is 3.76 micron and the 2 arcsec FWHM is 8.7 micron, this gives a "good" sampling of 2.3.


""My 60mm guide scope ( fl 240mm) with ZWOASI120MM-S image scale is 3.22 arc sec per pixel"
The pixel size is 3.75 micron and the 2 arcsec FWHM is 2.3 micron, this gives a "poor" sampling of 0.6.


This suggests to me that the guide star could move around by >2 arcsec and still be only within one pixel.
What happens to the imaging guiding? It doesn't seem to be <2 arcsec


Does guiding give big round stars or small round stars?

[Startrek (Martin)]

If you mean the stars in the light frame ? Yes tight round stars big and small
Obviously the small stars when zoomed in to 5x or 10x are blocky due to the reduced number of pixels that make up the star
Never had problems with excessive star bloat in 3 years of imaging with any of my newts
Further to the above the stars in my PHD2 star field view are tight too , pin points ( obviously excluding the bright stars )

[Merlin66 (Ken)]

Martin et al,
You mention "" reduced number of pixels that make up the star"

That's the point I'm trying to make....
If your seeing is 2 arcsec then an "ideal" star would be contained in a 3 x 3 pixel matrix


In the meantime the guide star would still not even be covering pixel!
If the guide stars look bigger than 1 pixel then the guiding can't be < 2 arc sec.


(The size of the seeing disk (in micron) vs the guide pixel size I see as an issue. Just my 2c)

[Sunfish (Ray)]

I think if you read the links in the link I sent , Stark explains how a star covering many pixels can guide down to a small fraction of a pixel in accuracy with good seeing.

A larger focal distance for guiding is always going to be better which explains why an OAG is always attractive.

The point is also made by Rista that the ratio of the guide scope and imaging scope rig is not really material which is why the figures shown in PHD 2 do not really tell you how good your result will be . As usual the answer is , it depends.

Just better to have as good a resolution for your guide scope set up as possible. Period.

Mine are equal and the idea of a certain number , say 5x, is the upper limit of what will work not a recommendation. I got a 135 mm lense to work but it was not ideal. An OAG will always be better if you can focus and find a star, but a pain on a newt.

[Merlin66 (Ken)]

Ray,
""a star covering many pixels can guide down to a small fraction of a pixel"


That's the issue I see with smaller finder guiders, short focal length.
With "standard" guide cameras the star image (say, 2 arcsec seeing) will not cover multiple pixels unless it's allowed to drift about - which defeats the purpose.

[Sunfish (Ray)]

I think I understand you, thanks.

I would have to read more Stark to see if I fully understand the sub pixel star image problem.

Originally Posted by Merlin66

Ray,
""a star covering many pixels can guide down to a small fraction of a pixel"


That's the issue I see with smaller finder guiders, short focal length.
With "standard" guide cameras the star image (say, 2 arcsec seeing) will not cover multiple pixels unless it's allowed to drift about - which defeats the purpose.