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Guide camera resolution too low?

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#1 ThatsMyCoffee

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Posted 19 January 2019 - 01:39 AM

Will guiding improve if I increase the resolution of my guide cam?  My main cam is an 1600mcp, but my guide cam is only a 120mm-s.

 

Is it too low a resolution?



#2 overnight

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Posted 19 January 2019 - 01:45 AM

Depends on what you're pairing it with. Are you using OAG or guidescope (if guidescope, what FL)?



#3 happylimpet

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Posted 19 January 2019 - 05:33 AM

Thats the same combo I use with an OAG and it works a treat - certainly no need to change it if thats the case.


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#4 ThatsMyCoffee

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Posted 19 January 2019 - 08:09 AM

Depends on what you're pairing it with. Are you using OAG or guidescope (if guidescope, what FL)?

200mm guidescope.  I've been thinking about switching to an OAG, just not sure it's worth the trouble since my flexture(?) issues have been fixed.

 

What does the math say?  Big improvement?



#5 tjugo

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Posted 19 January 2019 - 09:33 AM

Hi,

 

As already stated it depends. Modern autoguiding programs calculate the centroid of the guiding star to increase resolution. A good algorithm can reliably detect guiding error up to 5 times smaller than the native resolution of the guiding system (Guiding camera pixel size + Guider focal length).

 

Now you just need to do the math to see if your guiding system is sensitive (spacial-wise) enough to properly guide your main imaging rig.

 

Cheers,

 

Jose



#6 Der_Pit

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Posted 19 January 2019 - 09:51 AM

Important is the pixel size, which in your case is almost identical (120 has 3.75μ, 1600 has 3.8μ).  So in your case it just is the ratio of the focal lengths, 200 vs. 750 if I interpret your signature correct.  For my taste it would be (too much) on the large side.  Even more as the absolute size of your guide pixels is 3.75/200*206.265=3.87 arcsec per pixel.  So unless seeing is really bad the guide star will only cover one pixel, and the guide algorithm has not much chance to get guiding to sub-pixel accuracy.

I'd definitely expect improvements if you do either 

  • get a guide scope with longer focal length, or maybe check the option of a barlow
  • get a guide camera with smaller pixels
  • switch to using an OAG


#7 Ken Sturrock

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Posted 19 January 2019 - 11:46 AM

Like others have mentioned, modern centroid guiding algorithms are pretty amazing and can accurately calculate "sub-pixel" centroids. In the case of guiding, you're trying to calculate a centroid based on the pattern of brightness that falls across a grid of pixels. You are not trying to capture minute detail that exists in/on a target. So, I don't think that you need to go to extremes and get real "Nyquisty" for guider sampling, but higher resolution is usually better than lower. At the end of the day, if the guiding routine can produce a decent and reliable centroid then that's all that you really need.*

 

Since you do have a current system: If it seems to track nicely and produces round stars then I wouldn't mess with it...

 

 

 

 

* = This is a little different from a related issue of: My guiding system doesn't produce a great centroid and my mount wanders around a little but my imaging train is low enough resolution that nobody notices in the final pictures.


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#8 WadeH237

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Posted 19 January 2019 - 12:38 PM

Before thinking about guider image scale, I would ask:

 

What is wrong with your guiding right now?  What problem are you trying to solve?

 

Hint:  "I want better guiding" is not a helpful answer.  Are the stars in your image round?  If not, what do your guide graphs show?  Is the mount not responding to guider inputs?  Is it oscillating with each correction?  Does it have occasional excursions that take a long time to settle?

 

Posting a sample guide graph here would make it easier for us to understand how your guider is doing.


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#9 ThatsMyCoffee

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Posted 20 January 2019 - 01:24 PM

What is wrong with your guiding right now?  What problem are you trying to solve?

 

Hint:  "I want better guiding" is not a helpful answer.  Are the stars in your image round?  If not, what do your guide graphs show?  Is the mount not responding to guider inputs?  Is it oscillating with each correction?  Does it have occasional excursions that take a long time to settle?

 

Posting a sample guide graph here would make it easier for us to understand how your guider is doing.

 

VERY appreciative that people are willing to help, but I'm not trying to diagnose a problem.  I just want a better understanding of how things work.  That's what CN is all about right??  My guiding does SUCK, but I'm forced to stick to the theoretical stuff for now.  Unable to even setup my rig for another few weeks.

 

My biggest issue is a cheap mount and I'm going to upgrade it.  By the sound of it, an OAG setup would probably help so maybe I'll do that too.  If I'm still having trouble after all that, I'll come back with a graph.   :)

 

Thanks to all.



#10 WadeH237

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Posted 20 January 2019 - 03:51 PM

VERY appreciative that people are willing to help, but I'm not trying to diagnose a problem.  I just want a better understanding of how things work.  That's what CN is all about right??  My guiding does SUCK, but I'm forced to stick to the theoretical stuff for now.  Unable to even setup my rig for another few weeks.

 

My biggest issue is a cheap mount and I'm going to upgrade it.  By the sound of it, an OAG setup would probably help so maybe I'll do that too.  If I'm still having trouble after all that, I'll come back with a graph.   smile.gif

 

Thanks to all.

It's impossible to make meaningful suggestions to improve your guiding without being able to understand exactly how your guiding is working right now.  That's the reason for my questions.

 

If it were me, I would not seriously consider buying or changing anything until I had an opportunity to analyze the current state of things with some degree of accuracy.  Yes, it does mean that you have to wait until you have suitable skies.  And yes, it does mean that you need to spend time under otherwise usable imaging skies, doing testing and diagnostics.  Such is the way of this hobby.

 

If you want pure speculation as to how others would spend your money, you're all set (Cloudy Nights is really good at that).  If you want a better understanding of how things actually work, get us something to start with and we're happy to help (Cloudy Nights is also good at that).



#11 ThatsMyCoffee

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Posted 21 January 2019 - 08:38 AM

If it were me, I would not seriously consider buying or changing anything until I had an opportunity to analyze the current state of things with some degree of accuracy. 

You're absolutely right and I've done that (in other threads/forums).

 

This was just a simple question about resolution.  Big thanks to those that answered that question.



#12 ChrisMoses

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Posted 21 January 2019 - 10:44 PM

 

My biggest issue is a cheap mount and I'm going to upgrade it.  By the sound of it, an OAG setup would probably help so maybe I'll do that too.  If I'm still having trouble after all that, I'll come back with a graph.   smile.gif

 

Thanks to all.

The Exos 2 isn't a bad mount, but it isn't exactly a premium mount either.  It serves a purpose, but high-quality AP isn't it's forte.

 

I would strongly recommend switching to an OAG when you can. 



#13 cfosterstars

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Posted 22 January 2019 - 03:56 PM

IMHO, the only thing that really matters is the quality of your stars at the longest exposure that you are taking. That being said, the main factor - outside of the quality of your mount - is the guider pixel scale and not the size of the chip or even the size of the pixels on the chip. What you are after is a guider scale that is at least on the order of the seeing that your have during imaging. My rule of thumb is that average seeing is about 2 arc-seconds of variation. You want to have a guider scale of about that order in terms of arcsec/pixel. Do you have to get that small a scale - not necessarily - but it helps. The easiest way to do this is with a longer focal length guide scope and the cheapest way to do that is with a barlow. I use a Stellarvue 80mm guide scope with a 300mm focal lenght and a 2x barlow to get to 600mm focal length. I use an ASI290mm-mini guide camera with a pixel size of 3.8 microns and this give a pixel scale of 1"/pixel. This works well for all seeing conditions. This means that the stars are spread across 3-5 pixels and the algorithm can get a very good read on the centroid. A smaller pixel size or a longer focal length (ie, 3X barlow) would reduce the pixel scale proportionally. I really would likely seen no benefit since the seeing limits everything anyway. I could change to an ASI174 guide camera to get a larger chip size and a wider FOV for more candidate guide star, but with a larger pixel size and a bigger pixel scale - for again probably no benefit other than more guide stars to choose from. 

 

I just got my first OAG for my 10" Meade ACF SCT. I did this not to get a smaller guider scale - which I will get with the same camera. What I get is no flexture - not that I have seen any flexure issues that I can identify. I could also lower the weight on the rig since an OAG weight less than a guide scope and mount. This is at the cost of tricker focusing  for the guide camera with the OAG and a much smaller FOV to chose guide stars. For the FOV issue I am considering the ASI174 for the SCT OAG to get 2X the FOV assuming that the prism will produce enough of a light cone to cover the larger chip.

 

For what it is worth, I have had no real issues with using  my guide scope with my SCT and getting good stars even with 10 minute exposures. Why am I changing then? Well I have a VERY bad case of GAS - gear acquisition syndrome - that is an affliction that is common to astrophotographers and is often found with cases of aperture fever. smile.gif I just cant help myself from tweaking. 


Edited by cfosterstars, 22 January 2019 - 03:57 PM.

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