Pixel scale for guiding

[kalasinman]

Ok, so I found the formula, and tried to figure for my  lenses.

1. Is the formula the same for DSLR lenses as for a telescope?

2. When figuring the FL, when an FX lens is used on a DX camera it operates as a longer FL because of the cropped sensor. Should one use the stated FL or the effective net FL in the formula?

I figured the scale of a potential 5" guide scope and camera as 7.13"

If I use the net effective FL of my 400mm lens as 600mm, then the scale =1.64"

as 7.14/4=1.78 , does this mean guiding would be poor?

I know I'm not getting this. Any help really appreciated-Jack

[lensman57]

Ok, so I found the formula, and tried to figure for my  lenses.

1. Is the formula the same for DSLR lenses as for a telescope?

2. When figuring the FL, when an FX lens is used on a DX camera it operates as a longer FL because of the cropped sensor. Should one use the stated FL or the effective net FL in the formula?

I figured the scale of a potential 5" guide scope and camera as 7.13"

If I use the net effective FL of my 400mm lens as 600mm, then the scale =1.64"

as 7.14/4=1.78 , does this mean guiding would be poor?

I know I'm not getting this. Any help really appreciated-Jack

I don't even worry too much about pixel scale  in long exposure DSO imaging never mind in guiding, just connect the webcam to the guidescope and go for it. Guiding accuracy depends on a long list of factors and pixel scale is way down the list. As far as hobby imaging is concerned this is just an abstract science, once you get into long FL imaging, over 3000mm, then  you may wish to give this some thought but I doubt very much if you need to worry about it for now.

A.G

[Thalestris24]

Ok, so I found the formula, and tried to figure for my  lenses.

1. Is the formula the same for DSLR lenses as for a telescope?

2. When figuring the FL, when an FX lens is used on a DX camera it operates as a longer FL because of the cropped sensor. Should one use the stated FL or the effective net FL in the formula?

I figured the scale of a potential 5" guide scope and camera as 7.13"

If I use the net effective FL of my 400mm lens as 600mm, then the scale =1.64"

as 7.14/4=1.78 , does this mean guiding would be poor?

I know I'm not getting this. Any help really appreciated-Jack

Hi

You can use a FOV calculator like this one:

http://www.skyatnightmagazine.com/field-view-calculator

It's useful to know fov and pixel scale for lots of things, not only guiding. You can manually enter 300mm, say, for a dslr lens fl (ignore the aperture field) and it will do the calculations for your camera (select from dropdown menu). Or you can select a scope from the dropdown menu.

Hth

Louise

[kalasinman]

@A.G. Although your advice is comforting, the reason I posted here was to get an answer to my questions. Nothing wrong with learning?

[kalasinman]

@Loise, thanks.

[kalasinman]

Ok, so I found the formula, and tried to figure for my  lenses.

1. Is the formula the same for DSLR lenses as for a telescope?

2. When figuring the FL, when an FX lens is used on a DX camera it operates as a longer FL because of the cropped sensor. Should one use the stated FL or the effective net FL in the formula?

I figured the scale of a potential 5" guide scope and camera as 7.13"

If I use the net effective FL of my 400mm lens as 600mm, then the scale =1.64"

as 7.14/4=1.78 , does this mean guiding would be poor?

I know I'm not getting this. Any help really appreciated-Jack

Anyone?

[Thalestris24]

Um, there's a guiding help and troubleshooting guide here:

http://stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

There is info there about guide scopes fl vs main scope fl and lots more!

You can ignore the cropped sensor business - just use the fov calculator and enter the actual fl.

[kalasinman]

Thank you again Loise. Ok I know I'm a hard headed person (ask wife), but the more I think about it....if you use an FX lens on a DX camera obviously the camera sensor is what it is, but what it sees decreases because it sees less of the focal plain. The FOV narrows and therefore the arc" per pixel would decrease proportionally for the same reason. It's by a considerable amount too, roughly 50% increase in net FL.

I guess one of my doubts was about this 4:1 or is it 1:4 ratio concerning the pixel scale of the guide camera and the DSLR/lens. I just didn't understand the concept as to how the ratio changing affected the guiding efficiency. So is it that if there is too great a disparity between the scale of the imager and the guider that would make the guiding less accurate?? I guess that this would be a problem if a very small scale for the imager was coupled with a very large scale for the  guider?

Again, I'm just trying to learn principles here.

[Thalestris24]

Hi again

As I said above, ignore the crop factor stuff. The guiding guide on here explains what you need to know better than I can, and I can't see the point in duplicating it... e.g. from the above link:

"So how do you calculate your pixel scale?

pixel scale in arcseconds per pixel = (camera pixel size in µm / scope focal length in mm ) x 206.3

So I image with a Canon EOS 500D with pixels that are 4.7µm square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective focal length of 510mm:

(4.7µm / 510mm) x 206.3 = 1.9 arcseconds per pixel

I guide with a QHY5 with pixels that are 5.2µm square and an Orion ST80 with an effective focal length of 400mm:

(5.6µm / 400mm) x 206.3 = 2.67 arcseconds per pixel

If you don’t feel like doing the maths, use my Imaging Toolbox to do the hard work for you. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.41). That is well within the 4 x rule we established above."

It can't be clearer, can it? Some people here have successfully guided a fairly long fl scope with a finder guider. It's all just guidelines really. As it says in the Pirates' Code... Lol.

Louise

[kalasinman]

"So I image with a Canon EOS 500D with pixels that are 4.7µm square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective focal length of 510mm:

(4.7µm / 510mm) x 206.3 = 1.9 arcseconds per pixel

I guide with a QHY5 with pixels that are 5.2µm square and an Orion ST80 with an effective focal length of 400mm:

(5.6µm / 400mm) x 206.3 = 2.67 arcseconds per pixel

If you don’t feel like doing the maths, use my Imaging Toolbox to do the hard work for you. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.41). That is well within the 4 x rule we established above."  From the link Loise posted.

I've read and re-read this. In fact it's one of the reason for the OP. It seems that he/she plugged in a value of 5.6 for the guider pixel size after above stating it was 5.2 for one thing. By my reading the imager scale is shown being divided into the guider scale. The answer yielded would be that the GUIDING resolution is 1.5 times the IMAGING resolution, not the other way round as stated. This is part of the source of my confusion. These issues kind of throw the whole statement into question for me and I can't figure it out. Sorry if I'm thick...:-)

[Thalestris24]

"So I image with a Canon EOS 500D with pixels that are 4.7µm square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective focal length of 510mm:

(4.7µm / 510mm) x 206.3 = 1.9 arcseconds per pixel

I guide with a QHY5 with pixels that are 5.2µm square and an Orion ST80 with an effective focal length of 400mm:

(5.6µm / 400mm) x 206.3 = 2.67 arcseconds per pixel

If you don’t feel like doing the maths, use my Imaging Toolbox to do the hard work for you. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.41). That is well within the 4 x rule we established above."  From the link Loise posted.

I've read and re-read this. In fact it's one of the reason for the OP. It seems that he/she plugged in a value of 5.6 for the guider pixel size after above stating it was 5.2 for one thing. By my reading the imager scale is shown being divided into the guider scale. The answer yielded would be that the GUIDING resolution is 1.5 times the IMAGING resolution, not the other way round as stated. This is part of the source of my confusion. These issues kind of throw the whole statement into question for me and I can't figure it out. Sorry if I'm thick...:-)

Hi

Yeah, that was obviously a typo... But it doesn't make a lot of difference. Probably more common for imaging resolution to be quite a bit higher (smaller pixel scale value) than guiding resolution and up to 4x higher is a reasonable ratio. A common guide cam is the qhy5l-ii (which I use) with a finder guider. So pixel size is 3.75um, fl about 170mm so pixel scale = 4.55" /pixel. My main scope has fl=750mm, camera = qhy8l with 7.8um pixels so pixel scale = 2.14" /pixel.

So guide / main = 4.55/2.14 = 2.13 - fine . Now if my imaging scope had a fl of 3000mm that would give a pixel scale of .54 and ratio of 4.55/0.54 -> 8.4 - challenging!

Hope that's clearer

[lensman57]

@A.G. Although your advice is comforting, the reason I posted here was to get an answer to my questions. Nothing wrong with learning?

No, nothing wrong with learning. Have a read of the stuff in the link. There is a lot of stuff about Nyquist, , Resolution and Pixel Scale,hope it is of some use.http://starizona.com/acb/ccd/advtheorynyq.aspx

A.G

[kalasinman]

Yes, I finally understand. Thank you both. As I said, the confusion over the juxtaposition of terms and numbers was partly why I was confused. I just couldn't make sense of it as originally stated. 3000mm FL, now that kind of problem I'd love! Thanks A.G. and Loise for your kindness and patience.

[ollypenrice]

In terms of what works for guiding, I think the guiding pixel scale is less important than we might suppose because individual pixels are not what are used. Guiding software calculates the centroid of a multi pixel guide star image. It does not try to resolve that star, it tries to find it's centre. This notional point is the point chosen for guiding. Craig Stark, author of PHD, actually recommends a slightly soft focus for guide stars because this give a more precise centroid position.

Here we see a guide trace from a 400mm guidescope with binned pixels of (effectively) about 18 microns. And in soft focus, as recommended. This is still giving a phenomenally good trace capable of keeping long FL systems comfortably sub pixel.

(That's a Tak EM200. Our Mesu beats it consistently, often running 0.03/0.02 all night.)

So, bottom line; guiding is not just about stellar resolution.

Olly

[Thalestris24]

 @ Olly

[lensman57]

Yes, I finally understand. Thank you both. As I said, the confusion over the juxtaposition of terms and numbers was partly why I was confused. I just couldn't make sense of it as originally stated. 3000mm FL, now that kind of problem I'd love! Thanks A.G. and Loise for your kindness and patience.

It was my pleasure, the short of all that is for most us mortals using short to medium FL scopes and with a popular guide camerasuch as , ASI 120, QHY5 Lii and even the starlight xpress Lodestr even using a 50mm lens as a guidescope will keep the cenetroid of the guide star to within 0.1 of a pixel which is way beyond what is required for normal imaging so atleast you have piece mind.

Regards,

A.G

[Dave_D]

 @ Olly

he's just showing off 

[Thalestris24]

After a long night trying to get some slightly decent subs I lie awake thinking why won't my poor, cheap 'working class' Heq5 guide like that? It's not fair, I tell you!

[Dave_D]

After a long night trying to get some slightly decent subs I lie awake thinking why won't my poor, cheap 'working class' Heq5 guide like that? It's not fair, I tell you!

at least yours has bearings... my poor vixen gp must be from the workhouse then 

[Thalestris24]

Hi Dave

Have you managed to do any decent images on yours? The proof of the pudding, as they say. Sometimes mine is good and well-behaved. I just re-balanced my setup - will try again tonight if the clouds keep away. Yep, bearings are good! However, mine seems to be unstable and won't stay polar aligned even though it's more or less permanently set up. I put it on a pedestal recently - seems ok on that but the mount itself is still problematic. Maybe it's down to me...

Louise

[RikM]

A lot of it is to do with balance and polar alignment. I can a nice flat guide graph with my NEQ6 with a bit of care setting up correctly. I have heard people say that the HEQ5 is potentially smoother running than the NEQ6.

[Thalestris24]

Hi Rik

Yes, indeed. Dunno about comparisons with an Neq6, my heq5 syntrek is my only experience, so nothing to compare it to. I can only assume mine is doing what it should but I've never achieved a flat graph. I have sometimes got a few decent images though - maybe not perfect but ok by me. It gets unhappy if I touch it or change anything - mind of it's own!

Louise

[Dave_D]

Hi Dave

Have you managed to do any decent images on yours? The proof of the pudding, as they say. Sometimes mine is good and well-behaved. I just re-balanced my setup - will try again tonight if the clouds keep away. Yep, bearings are good! However, mine seems to be unstable and won't stay polar aligned even though it's more or less permanently set up. I put it on a pedestal recently - seems ok on that but the mount itself is still problematic. Maybe it's down to me...

Louise

In all fairness, this is a really good mount, just not up to the task of carrying a 10" newt for proper imaging. Getting reasonably good images considering. I really want something like a CEM60 or AZ EQ6-GT to do my optics justice though.

[ollypenrice]

The guide trace in my post can be matched by a good NEQ6. Mine sometimes manage it and carefully tuned ones often do.

Olly