Guide scope Vs OAG

[Rodd]

OK--The name of the thread may be a bit misleading.  I no longer have an OAG because I sent it back.  It may have been able to work--but poor supplier support was getting on my nerves.  So, I am going a different route--a guidescope.  Astro Physics makes a non flexive bracket for the Baader Vario finder (well, as non flexive as they come I am told).  It is called the Astro Physics Vario Guider and is highly regarded.  I intend to use this with my refactors at focal lengths of 318 to 1,000 (probably will top out at 700mm for the most part but may try 1,000mm just to see.  That is the unreduced TOA 130 and at F7.7 its a bit slow.  But reduced by .7x it becomes F5.38 and is pretty decent at 700mm.  318mm is the FSQ 106 with .6x reducer for F3 imaging.  I tried the Astrodon MMOAG but the Tak reducers do not have enough backfocus to permit its use.  I am using the Lodestar 2 s a guide cam, and the ASI 1600mmcool pro for imaging.  My question is:

 

1) The ASI has a pixel size of 3.8um while the Lodestar 2 has a pixel size of 8.2umx 8.4um (something like that--not square).  The pixel scale ratios between guide scope and imaging camera are as follows:  TOA 130 1,000mm ( 6.92  /.78).   FSQ 106 at F3 (318mm) (6.92/2.46)

This seems high and unacceptable.  But I have no experience with guide scopes.  Is the OAG the only way to go?

 

Rodd

 

[drjolo]

I have been using guidescopes since I changed from newton to refractors. Fur guiding I use ASI290MM camera with 2.9um pixel. I have been imaging at different pixel scales from 1.5 to 0.45"/px. And my experiences are:

1. at 1.5"/px scale I was able to guide with 50/165 guidescope, but with 60/240 (aperture/focal) results were better
2. at 1"/px scale 50/165 guidescope could not handle it. 60/240 was minimum
3. at 0.45"/px I used for some time 60/240 with 1.5 barlow and Fringe Killer with decent results, but currently I guide with 80/400 and it works perfectly ok

In my opinion there are two things to consider:

 - guidescopes are achromats. I was able to significantly improve guide behaviour after adding Fringe Killer filter to reduce star size (due to AC) and also after adding 1.5x barlow (though FOV is then smaller)
 - I use currently 8/10 bit CMOS camera with small pixel. 16 bit camera can give bit better accuracy for star centroid calculation (assuming the same pixel size). But CMOS camera is better for guiding due to low read out noise, so with short exposures SNR will be better

I cannot refer directly to Lodestar since I have never used it. I used some time ago Atik Titan and QHY6 for guiding - these both are 16bit CCD cameras. While they behave bit better than ASI120MM, then they both gave up compared to low noise and small pixel ASI290MM. 

And of course when you use separate guidescope some bend and tilt may occur. Even if your guiding works perfectly ok, still at very long exposed frames elongated stars may be visible, or there can be visible displacement in star position between frames in the series. I wrote some time ago a blog entry about guiding http://astrojolo.com/gears/oag-vs-guidescope/

[Rodd]

3 minutes ago, drjolo said:

I have been using guidescopes since I changed from newton to refractors. Fur guiding I use ASI290MM camera with 2.9um pixel. I have been imaging at different pixel scales from 1.5 to 0.45"/px. And my experiences are:

1. at 1.5"/px scale I was able to guide with 50/165 guidescope, but with 60/240 (aperture/focal) results were better
2. at 1"/px scale 50/165 guidescope could not handle it. 60/240 was minimum
3. at 0.45"/px I used for some time 60/240 with 1.5 barlow and Fringe Killer with decent results, but currently I guide with 80/400 and it works perfectly ok

In my opinion there are two things to consider:

 - guidescopes are achromats. I was able to significantly improve guide behaviour after adding Fringe Killer filter to reduce star size (due to AC) and also after adding 1.5x barlow (though FOV is then smaller)
 - I use currently 8/10 bit CMOS camera with small pixel. 16 bit camera can give bit better accuracy for star centroid calculation (assuming the same pixel size). But CMOS camera is better for guiding due to low read out noise, so with short exposures SNR will be better

And of course when you use separate guidescope some bend and tilt may occur. Even if your guiding works perfectly ok, still at very long exposed frames elongated stars may be visible, or there can be visible displacement in star position between frames in the series. I wrote some time ago a blog entry about guiding http://astrojolo.com/gears/oag-vs-guidescope/

Thank--not sure if that means I am in trouble--but I think so.  I can't decipher the 1,2,3 list quickly.  I have to see pixle sacle and have not calculated that for you setup.  

[drjolo]

Ok, so conclusion was missing I think at 7"/px scale with Lodestar and Baader Vario guiding may be not so accurate - basing on my experiences. But it also depends on mount quality - you use AP Mach that is much better than my modded EQ6. 

[Rodd]

Just now, drjolo said:

Ok, so conclusion was missing I think at 7"/px scale with Lodestar and Baader Vario guiding may be not so accurate - basing on my experiences. But it also depends on mount quality - you use AP Mach that is much better than my modded EQ6. 

Yes--the mount will track as told by the guidescope and star position--but if that star position is not accurate--what good is a good mount.  The mount will only go where the guidescope tells it to.  No?

[drjolo]

1 minute ago, Rodd said:

The mount will only go where the guidescope tells it to.  No?

Definitely it should  

[Rodd]

Just now, drjolo said:

Definitely it should  

No--frowning face!  I am saying that if the mount is told to go to the wrong position it will do so and guiding will be bad.  the best mount in the world can't track a star that is displayed in a poor position.  

[drjolo]

That's true. What I meant is that guiding is correcting tracking errors that already happened. Better quality mount with lower PE will have lower tracking errors, guiding will have to work less and overall image quality will be better. But I think for your setup with 0.78 pixel scale you should consider higher resolution guiding scope/camera. Even most optimistic guiding tutorials says that guide to main train pixel ratio should not exceed 10:1 (assuming good guiding scope quality and 16 bit guide camera).

[Rodd]

2 minutes ago, drjolo said:

That's true. What I meant is that guiding is correcting tracking errors that already happened. Better quality mount with lower PE will have lower tracking errors, guiding will have to work less and overall image quality will be better. But I think for your setup with 0.78 pixel scale you should consider higher resolution guiding scope/camera. Even most optimistic guiding tutorials says that guide to main train pixel ratio should not exceed 10:1 (assuming good guiding scope quality and 16 bit guide camera).

That my conclusion--Thanks.

[vlaiv]

1 minute ago, Rodd said:

No--frowning face!  I am saying that if the mount is told to go to the wrong position it will do so and guiding will be bad.  the best mount in the world can't track a star that is displayed in a poor position.  

Yes, that is exactly why you should look at guide error RMS and impact of guide resolution on it rather than relation of imaging and guiding resolutions (usual way people decide if guiding is good enough).

So let's do some numbers. Vario has 250mm FL and Lodestar has 8.2um pixels. This gives ~6.8"/pixel. Then we use some random number between 16 and 20 (inclusive, I say random because you can take 20 and be optimistic or 16 and be realistic - both I've seen quoted as centroid accuracy of guide programs) to get precision with which star position can be measured. Let's be realistic and take 16, that gives you roughly 0.42".

Compare that with expected guide RMS error. If you are looking at 1.5 - 2" RMS error, than I would say you are OK, but if you aim for 0.5" RMS - then you are in trouble, since almost all of it is going to go on "I'm not really sure where the star is" part. I would say (and this is really a wild guess, sort of hunch if you like) that you need at least x5 better measurement precision than your guide RMS error. So for 0.5" guide RMS error I would say you need at least 0.1" centroid accuracy, and for 8.2um pixels that translates into very high FL - north of 1000mm.

If you plan to guide with 250mm FL - get guide camera with really small pixels. Or use something like ST80 for guiding (with 400mm FL, and again small pixels - something like 3.75um - that would give you ~0.12" precision).

But again that depends on your mount and expected guide erro RMS.

[drjolo]

12 minutes ago, vlaiv said:

So let's do some numbers. Vario has 250mm FL and Lodestar has 8.2um pixels. This gives ~6.8"/pixel. Then we use some random number between 16 and 20 (inclusive, I say random because you can take 20 and be optimistic or 16 and be realistic - both I've seen quoted as centroid accuracy of guide programs) to get precision with which star position can be measured. Let's be realistic and take 16, that gives you roughly 0.42". (...)

I may be wrong, but it seems that there can be even more troublesome. For 6.8"/px scale if you have decent quality optics it will result in star FWHM less than 1px. I think that proper star centroid calculation is not quite possible, when 90% or more if its light lands in one pixel only. So that accuracy will be much lower even that realistic number 16. 

[michael.h.f.wilkinson]

In my very limited experience with guiding (only up to about 400mm focal length), I have used either my ASI178MM (2.4 micron pixels) or in one case my ASI178MM (5.85 micron). In all cases the guide scope was an ST80. Both combinations worked well enough, but I have yet to throw any real challenge at them.

[Rodd]

My rms with att 8300 self guiding filter wheel are typically less than .2 pixels when my pixel scale is 1.12 Arsenal/pix (I have Maim report in pixels)

[Rodd]

2 hours ago, drjolo said:

I may be wrong, but it seems that there can be even more troublesome. For 6.8"/px scale if you have decent quality optics it will result in star FWHM less than 1px. I think that proper star centroid calculation is not quite possible, when 90% or more if its light lands in one pixel only. So that accuracy will be much lower even that realistic number 16. 

 

2 hours ago, vlaiv said:

Yes, that is exactly why you should look at guide error RMS and impact of guide resolution on it rather than relation of imaging and guiding resolutions (usual way people decide if guiding is good enough).

So let's do some numbers. Vario has 250mm FL and Lodestar has 8.2um pixels. This gives ~6.8"/pixel. Then we use some random number between 16 and 20 (inclusive, I say random because you can take 20 and be optimistic or 16 and be realistic - both I've seen quoted as centroid accuracy of guide programs) to get precision with which star position can be measured. Let's be realistic and take 16, that gives you roughly 0.42".

Compare that with expected guide RMS error. If you are looking at 1.5 - 2" RMS error, than I would say you are OK, but if you aim for 0.5" RMS - then you are in trouble, since almost all of it is going to go on "I'm not really sure where the star is" part. I would say (and this is really a wild guess, sort of hunch if you like) that you need at least x5 better measurement precision than your guide RMS error. So for 0.5" guide RMS error I would say you need at least 0.1" centroid accuracy, and for 8.2um pixels that translates into very high FL - north of 1000mm.

If you plan to guide with 250mm FL - get guide camera with really small pixels. Or use something like ST80 for guiding (with 400mm FL, and again small pixels - something like 3.75um - that would give you ~0.12" precision).

But again that depends on your mount and expected guide erro RMS.

How about this--I own the Celestron Skyris 236c which has 2.8 um pixels.  According to the info I have, it can be used with maxim DL.  It is a USB camera that should plug into the ASI 1600 like the Lodestar 2 does.  This would afford a guiding pixel scale of 2.31 arcsec/pix--much better than over 6.  That places the TOA at 1000mm at bit over 1/3 and reduced at 700 a bit over 1/2.  Better?

If so--do you think it will work?

Rodd

[Rodd]

Or the QHY5III with 2.4 um pixel and a huge FOV (3,000 plus x 2,000 plus)  At $499 I could return the Lodestar 2 and even have some left over.

Rodd

[vlaiv]

4 minutes ago, Rodd said:

 

How about this--I own the Celestron Skyris 236c which has 2.8 um pixels.  According to the info I have, it can be used with maxim DL.  It is a USB camera that should plug into the ASI 1600 like the Lodestar 2 does.  This would afford a guiding pixel scale of 2.31 arcsec/pix--much better than over 6.  That places the TOA at 1000mm at bit over 1/3 and reduced at 700 a bit over 1/2.  Better?

If so--do you think it will work?

Rodd

I think it will work, most setups work, but the question is how good

According to calculations and pixel scale, that combination - Vario at 250mm FL + 2.8um pixels should be able to get accuracy in range of 0.14" - 0.11". I believe that such accuracy should be good up to 0.5-0.6" guide error RMS. I don't think you will be able to go below that, but I might be totally wrong on this, since it is as I've said, based on guess rather than on any sort of mathematics / reasoning (there is some level of reasoning but not particularly accurate one) / calculations. But indeed it does agree somewhat with what @drjolo said about imaging to guiding resolution ratio of 10:1 (if we take for example that one should aim for guide RMS error being half of imaging resolution and guide resolution being at least 5 times less than guide RMS error - it adds up to at least x10 ratio of guide / image resolutions).

I think that you should certainly give it a go since you already have the gear on hand, and look if guide RMS is what you would normally expect given conditions. Also look at star FWHM sizes in your subs and compare with what you would normally expect (like when using OAG). I did not quite catch that one, what was exactly reason not to use OAG anymore? 

Based on how it behaves, you might later on decide to trade in Lodestar for something with even smaller pixels (like 2.4um) or you might decide that this setup fulfills your needs.

If you do give it a go, it would be really good to hear feedback on it - that would give us a chance to further test / refine this reasoning.

[Rodd]

1 minute ago, vlaiv said:

I think it will work, most setups work, but the question is how good

According to calculations and pixel scale, that combination - Vario at 250mm FL + 2.8um pixels should be able to get accuracy in range of 0.14" - 0.11". I believe that such accuracy should be good up to 0.5-0.6" guide error RMS. I don't think you will be able to go below that, but I might be totally wrong on this, since it is as I've said, based on guess rather than on any sort of mathematics / reasoning (there is some level of reasoning but not particularly accurate one) / calculations. But indeed it does agree somewhat with what @drjolo said about imaging to guiding resolution ratio of 10:1 (if we take for example that one should aim for guide RMS error being half of imaging resolution and guide resolution being at least 5 times less than guide RMS error - it adds up to at least x10 ratio of guide / image resolutions).

I think that you should certainly give it a go since you already have the gear on hand, and look if guide RMS is what you would normally expect given conditions. Also look at star FWHM sizes in your subs and compare with what you would normally expect (like when using OAG). I did not quite catch that one, what was exactly reason not to use OAG anymore? 

Based on how it behaves, you might later on decide to trade in Lodestar for something with even smaller pixels (like 2.4um) or you might decide that this setup fulfills your needs.

If you do give it a go, it would be really good to hear feedback on it - that would give us a chance to further test / refine this reasoning.

The reason I sent the OAG back is because the ZWO OAG was pretty poor--but most importantly the supplier calculated wrong and would not return my calls and I got very frustrated and decided to go with another supplier.  However, the supplier I chose only sells the Astrodon MMAOG s=which is the best, but cannot work with both Tk reducers (not enough back focus).  He recommended the AP vario Guider and I had the Lodestar 2 that I kept instead of sending it back with the OAG.  I did not know its pixels were too big for a guider.  I want to use an OAG--but just spent $700 on the AP vario guider.....I am spinning like a top and starting to come unraveled.

Anyway--you mention rms--I don't want any rms errors.  I view rms errors as a function of the system--not something I chose.  I want tight stars and good guiding--the smallest rms possible.  With the STT-8300self guiding filter wheel my rms errors were always less than .2 pixels, whixh when imaging at 1.2 arcsec/pix is about .25 arcsec.  So, in short, I do not understand how I can determine what rms "I am shooting for".  What ever is good enough I suppose.

[Rodd]

21 minutes ago, vlaiv said:

I think it will work, most setups work, but the question is how good

According to calculations and pixel scale, that combination - Vario at 250mm FL + 2.8um pixels should be able to get accuracy in range of 0.14" - 0.11". I believe that such accuracy should be good up to 0.5-0.6" guide error RMS. I don't think you will be able to go below that, but I might be totally wrong on this, since it is as I've said, based on guess rather than on any sort of mathematics / reasoning (there is some level of reasoning but not particularly accurate one) / calculations. But indeed it does agree somewhat with what @drjolo said about imaging to guiding resolution ratio of 10:1 (if we take for example that one should aim for guide RMS error being half of imaging resolution and guide resolution being at least 5 times less than guide RMS error - it adds up to at least x10 ratio of guide / image resolutions).

I think that you should certainly give it a go since you already have the gear on hand, and look if guide RMS is what you would normally expect given conditions. Also look at star FWHM sizes in your subs and compare with what you would normally expect (like when using OAG). I did not quite catch that one, what was exactly reason not to use OAG anymore? 

Based on how it behaves, you might later on decide to trade in Lodestar for something with even smaller pixels (like 2.4um) or you might decide that this setup fulfills your needs.

If you do give it a go, it would be really good to hear feedback on it - that would give us a chance to further test / refine this reasoning.

Maybe I should try the OAG route again

[vlaiv]

Having such low guide error RMS is superb! I know that premium mounts are capable of such low guide RMS, but I'm used to HEQ5 so I'm over the moon when I get 0.5" or just a tad below total RMS.

Thing with such low guide error RMS is that you need very fine guide resolution to "resolve" it. If you for example have guide resolution of 0.15" and say you have guide rms error of 0.25", than that makes me wonder how precise figure of 0.25" is. It is a bit like trying to get average population height with 1m stick. Some measurements will be 1m, some will be 2m, and on average you might get something like 1.632m, but I'm fairly certain that such figure will be wrong

On the other hand, if your imaging resolution is something like 2"/pixel, then having 0.25" RMS error vs having 0.45" RMS error will have rather small impact on final image quality. Ideally you want the absolutely best guiding - smallest guide error, but at some point you enter domain of diminishing returns. This is what I mean by "setting" expected guide RMS - if you are easily able to go down to some very small number - then great. But if you need to spend a large amount of money / time and effort to go from let's say 0.48" to 0.45" and it has very small impact on resolution that you are working on, then is it worth it at all?

[Rodd]

2 minutes ago, vlaiv said:

Having such low guide error RMS is superb! I know that premium mounts are capable of such low guide RMS, but I'm used to HEQ5 so I'm over the moon when I get 0.5" or just a tad below total RMS.

Thing with such low guide error RMS is that you need very fine guide resolution to "resolve" it. If you for example have guide resolution of 0.15" and say you have guide rms error of 0.25", than that makes me wonder how precise figure of 0.25" is. It is a bit like trying to get average population height with 1m stick. Some measurements will be 1m, some will be 2m, and on average you might get something like 1.632m, but I'm fairly certain that such figure will be wrong

On the other hand, if your imaging resolution is something like 2"/pixel, then having 0.25" RMS error vs having 0.45" RMS error will have rather small impact on final image quality. Ideally you want the absolutely best guiding - smallest guide error, but at some point you enter domain of diminishing returns. This is what I mean by "setting" expected guide RMS - if you are easily able to go down to some very small number - then great. But if you need to spend a large amount of money / time and effort to go from let's say 0.48" to 0.45" and it has very small impact on resolution that you are working on, then is it worth it at all?

Definitely true.  I don't know enough about it. All I know is I start getting annoyed--rightly or wrongly--when I see my rms errors go above .2 pixels using the self guiding filter wheel.  But, now that I thinkmof it--the guide chip in the filter wheel I believe has pixels of 7.4 while the imaging sensor has pixels of 5.4.  So my guiding scale is 1.11 and my guiding scale is 1.52.  So .2 of 1.52 is about .35arcsec.  Most often on a good night bmy rms errors on the DEC axis are .08 to .15 pixels and on the RA axis .13 to .23.  But if they were to jump to .5 or .6--would I be able to see that in an image--I don't know.  I don't know.  Trial and error at $700 a pop is not practical.  I find it hard to understand why it is so hard to find the answer.  the TOA 130 is commonly used, as is the ASI 1600 now.   Why is it so hard to know if a guidescope will work.

To tell the truth--I originally got the Lodestar 2 to work in an OAG with the FSQ at F3 (.6x reducer).  the focal length of the FSQ at F3 is only 318mm.  The guidescope will no doubt work for that system.  But I also want to use the ASI 1600 with the TOA.  Not sure what to do 

[vlaiv]

5 minutes ago, Rodd said:

Definitely true.  I don't know enough about it. All I know is I start getting annoyed--rightly or wrongly--when I see my rms errors go above .2 pixels using the self guiding filter wheel.  But, now that I thinkmof it--the guide chip in the filter wheel I believe has pixels of 7.4 while the imaging sensor has pixels of 5.4.  So my guiding scale is 1.11 and my guiding scale is 1.52.  So .2 of 1.52 is about .35arcsec.  Most often on a good night bmy rms errors on the DEC axis are .08 to .15 pixels and on the RA axis .13 to .23.  But if they were to jump to .5 or .6--would I be able to see that in an image--I don't know.  I don't know.  Trial and error at $700 a pop is not practical.  I find it hard to understand why it is so hard to find the answer.  the TOA 130 is commonly used, as is the ASI 1600 now.   Why is it so hard to know if a guidescope will work.

To tell the truth--I originally got the Lodestar 2 to work in an OAG with the FSQ at F3 (.6x reducer).  the focal length of the FSQ at F3 is only 318mm.  The guidescope will no doubt work for that system.  But I also want to use the ASI 1600 with the TOA.  Not sure what to do 

Yes, so if your guide resolution was 1.52"/pixel, that means that guide software is able to tell difference in star position with roughly 0.08" - that is quite fine guide resolution, suitable to give accurate guide RMS for values in range 0.4" (if my guess is correct, that you need at least 5 times finer guide resolution compared to guide RMS error). I also believe that guiding below 0.4" RMS is adequate for 1.11"/pixel imaging scale (it should not be more than half, but it is of course better if less).

Now if you pair ASI1600 with TOA 130 you will be imaging at 0.78"/pixel - this is really high resolution, and it also makes me wonder if 5" scope is suitable for such high res work.

I need to point something out here. When I say that resolution of 0.78"/pixel is a bit high for 5" scope - that does not mean you can't work on such resolution, it only means that you will get the same amount of detail if you for example worked on coarser resolution, and coarser resolution will get you better SNR for same imaging time. You can always make your final image taken at 0.78"/pixel and present it on lower resolution by binning or resampling (and here I have to say I'm advocating supersampling to other resampling techniques, but no software is doing it as I'm aware).

Anyway your mount should be able to guide/track well for native resolution. With good skies (again something like ~1" seeing or less) you should be able to present your images in native resolution without them looking soft or having swollen stars. But in order to guide with precision you need guide resolution close to what you have with 8300 setup - that being somewhere around 1.3-1.4"/pixel. This should be your starting point when thinking about guide focal length / pixel size.

So for example 3.75um pixels (various cmos sensors have such pixel size) with ST80 will give you 1.93"/pixel, that is a bit less than you need (if going for native 0.78"/pixel). To use Vario 250mm guide scope you would need camera with ~1.5um pixels (I don't think there are any). With OAG and guide camera with 3.75um you are certainly covered as it will give you very fine guide resolution. Even using 8.2um pixels with OAG is going to be almost OK. That would give you 1.69"/pixel.

All of that is based on native resolution of 0.78"/pixel. If you decide to image on slightly less demanding resolution, like above 1"/pixel or resample your images to such resolutions, then things get a bit more relaxed.

You can always guide with sub optimal resolution but your images will be a bit softer / FWHM greater than it could potentially be, which does not need to matter much if you for example present your images resized to smaller size.

 

[Rodd]

8 minutes ago, vlaiv said:

Yes, so if your guide resolution was 1.52"/pixel, that means that guide software is able to tell difference in star position with roughly 0.08" - that is quite fine guide resolution, suitable to give accurate guide RMS for values in range 0.4" (if my guess is correct, that you need at least 5 times finer guide resolution compared to guide RMS error). I also believe that guiding below 0.4" RMS is adequate for 1.11"/pixel imaging scale (it should not be more than half, but it is of course better if less).

Now if you pair ASI1600 with TOA 130 you will be imaging at 0.78"/pixel - this is really high resolution, and it also makes me wonder if 5" scope is suitable for such high res work.

I need to point something out here. When I say that resolution of 0.78"/pixel is a bit high for 5" scope - that does not mean you can't work on such resolution, it only means that you will get the same amount of detail if you for example worked on coarser resolution, and coarser resolution will get you better SNR for same imaging time. You can always make your final image taken at 0.78"/pixel and present it on lower resolution by binning or resampling (and here I have to say I'm advocating supersampling to other resampling techniques, but no software is doing it as I'm aware).

Anyway your mount should be able to guide/track well for native resolution. With good skies (again something like ~1" seeing or less) you should be able to present your images in native resolution without them looking soft or having swollen stars. But in order to guide with precision you need guide resolution close to what you have with 8300 setup - that being somewhere around 1.3-1.4"/pixel. This should be your starting point when thinking about guide focal length / pixel size.

So for example 3.75um pixels (various cmos sensors have such pixel size) with ST80 will give you 1.93"/pixel, that is a bit less than you need (if going for native 0.78"/pixel). To use Vario 250mm guide scope you would need camera with ~1.5um pixels (I don't think there are any). With OAG and guide camera with 3.75um you are certainly covered as it will give you very fine guide resolution. Even using 8.2um pixels with OAG is going to be almost OK. That would give you 1.69"/pixel.

All of that is based on native resolution of 0.78"/pixel. If you decide to image on slightly less demanding resolution, like above 1"/pixel or resample your images to such resolutions, then things get a bit more relaxed.

You can always guide with sub optimal resolution but your images will be a bit softer / FWHM greater than it could potentially be, which does not need to matter much if you for example present your images resized to smaller size.

 

I will most likley image with the TOA 130 with the.7x reducer--which yields a resolution of 1.12 with the ASI 1600.  How about that?   EDIT--I NEVER get 1" skies!  Not even 2"--3" at best.  My sky stinks

[vlaiv]

1 minute ago, Rodd said:

I will most likley image with the TOA 130 with the.7x reducer--which yields a resolution of 1.12 with the ASI 1600.  How about that?

That is perfectly fine resolution, high resolution, but fine resolution for 5" instrument. I image at 0.98"/pixel using ASI1600 and 8" RC. It is native resolution of 0.49"/pixel but I bin x2, precisely because I'm not able to guide suitably (even ~0.5" RMS is borderline). I use OAG and 3.75um guide cam which I also bin x2 to improve SNR, simply because I don't need additional precision. My guide setup with bin x2 gets me guide resolution of 0.97"/pixel which has precision of about 0.05" - and that is more than enough for 0.5" guide RMS error. It would even be fine for 0.25" guide RMS error if my mount was capable of it.

On a side note, maybe you've come across such statements as: "After I switched to OAG - my guide graph got worse, and RMS increased, but my stars are tighter". I have couple of times - and it shows the impact of guide resolution / precision on determining correct RMS value and improving guide performance.

So, if you want the best guiding that your mount can give you - go with OAG and camera that has 3.75um or smaller pixel size. That would give you 1.1"/pixel guide resolution that will be suitable for down to 0.25"-0.3" RMS guide error, and that would be perfect for imaging resolution of ~1.12"/pixel.

But as I've said, why don't you try Vario with Celestron Skyris 236c since you already have that on hand? It gives you guide resolution of 2.31" which has around 0.12-0.14" precision which should be good for RMS guide error down to 0.5"-0.6" (your mount is bound to perform better than this, but it is question how much better, since you won't be able to measure it correctly), and that is still acceptable guiding for 1.12"/pixel resolution. Just try it out, and look for star FWHM. If acceptable, then you have working system, if not completely happy - get a decent OAG or longer FL guide scope (and use it with same Celestron Skyris if you can - 2.8um is going to be good pixel size for it).

[Jkulin]

Rodd, if you are going the Guidescope route can I make a suggestion?

Although I am going OAG on my RC, for my Refractors I will probably continue with a Guidescope until such time as I am comfortable with OAG. I use a couple of these: -

https://www.firstlightoptics.com/guide-cameras/skywatcher-guidescope-mount.html

Absolutely brilliant and rock solid and not expensive, probably about $130 over your side of the pond, I hated flimsy constantly moving guide rings and these have really improved my guiding.

[Rodd]

1 hour ago, vlaiv said:

That is perfectly fine resolution, high resolution, but fine resolution for 5" instrument. I image at 0.98"/pixel using ASI1600 and 8" RC. It is native resolution of 0.49"/pixel but I bin x2, precisely because I'm not able to guide suitably (even ~0.5" RMS is borderline). I use OAG and 3.75um guide cam which I also bin x2 to improve SNR, simply because I don't need additional precision. My guide setup with bin x2 gets me guide resolution of 0.97"/pixel which has precision of about 0.05" - and that is more than enough for 0.5" guide RMS error. It would even be fine for 0.25" guide RMS error if my mount was capable of it.

On a side note, maybe you've come across such statements as: "After I switched to OAG - my guide graph got worse, and RMS increased, but my stars are tighter". I have couple of times - and it shows the impact of guide resolution / precision on determining correct RMS value and improving guide performance.

So, if you want the best guiding that your mount can give you - go with OAG and camera that has 3.75um or smaller pixel size. That would give you 1.1"/pixel guide resolution that will be suitable for down to 0.25"-0.3" RMS guide error, and that would be perfect for imaging resolution of ~1.12"/pixel.

But as I've said, why don't you try Vario with Celestron Skyris 236c since you already have that on hand? It gives you guide resolution of 2.31" which has around 0.12-0.14" precision which should be good for RMS guide error down to 0.5"-0.6" (your mount is bound to perform better than this, but it is question how much better, since you won't be able to measure it correctly), and that is still acceptable guiding for 1.12"/pixel resolution. Just try it out, and look for star FWHM. If acceptable, then you have working system, if not completely happy - get a decent OAG or longer FL guide scope (and use it with same Celestron Skyris if you can - 2.8um is going to be good pixel size for it).

So you use the ASI 1600--great.  I was told that the filter wheel is inaccurate to the point that the filters will not precisely line up so that flats will not be accurate enough.  Is that true?  Is calibrating the ASI 1600 subs difficult?  I am used to the self guiding wheel of the stt-8300, which purports a micron level accuracy of filter placement.  Anything to this?