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Guide scope quality with regards to 1/50th pixel guiding.

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#1 moonrider

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Posted 24 January 2021 - 11:25 PM

Just wondering, how important is optics quality in guiding using relatively small pixels and PHd2 to get sub 1/50th of a pixel in guiding. Granted I'm not talking about APO's  or maybe even doublets. But to really get a very good snr and a very tight center centroid, are excellent optics needed to get there? Is it necessary and or does it improve guiding enough to justify the cost? Also let me preface that in my case, I have Bortle 3 sky's with no light pollution to speak of and generally only image when the seeing is very good. 

 

Just wonderg if I can improve on .12rms guiding, on good nights. I know a mount is also a limiting factor, but I have my mount working very good and I am still using a modified 50mm finderscope with my ZWO120mm and it all works very good. But, can it be better with better optics?

 

p.s. I read Dr. Starks article on 1/50th pixel guiding. This is what got me thinking.



#2 Cfreerksen

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Posted 24 January 2021 - 11:40 PM

If you can guiding to 0.12rms I think you should just quit while your ahead. That is about 5x better than my typical. You must have some very steady skies my friend.

 

Chris


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#3 moonrider

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Posted 24 January 2021 - 11:55 PM

If you can guiding to 0.12rms I think you should just quit while your ahead. That is about 5x better than my typical. You must have some very steady skies my friend.

 

Chris

 

Well, like I said, that is on good nights! But in all seriousness, I just was wondering if it could be better. My stars are nice and round @ 10min subs and it has taken about a year working on the mount given the weather lately.....I should note tho, the rms .12 is with a small(80mm) imaging scope and 50mm guide scope on a CGX. The 9.25 with OAG isn't quite as good 😁😁😁 but still very acceptable.


Edited by moonrider, 24 January 2021 - 11:57 PM.


#4 james7ca

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Posted 25 January 2021 - 04:21 AM

I wouldn't worry about trying to get 1/50th pixel guiding. It's probably not that practical or even possible. I would suggest that something between 1/10th to 1/20th pixel is nearer to a good setup.

 

In terms of using a separate guide scope, you'll probably be okay if your guide scope pixel scale is no more than five times your primary image scale. Plus, you'll likely have more issues with flexure in your guide and primary scope than you will have with your guiding scale (as long as you are within the 5X ratio). However, people can get into problems with guiding scale when they try to pair a short-focus guide scope with a guide camera that has relatively large pixels (that's one of the reasons why the 5X recommendation exists).

 

The only other issue is that you want to keep the spot size of the guide scope's stars LARGER than your guide camera's pixel size. But, this is unlikely to ever become a problem unless you bin your guide camera's pixels. For example, if you were planning on using a 50mm, f/4 guide scope with a guide camera that had a pixel size of 5um you wouldn't want to bin that by 2x2 to produce an effective pixel size of 10um. Reason being that the guide scope's Airy disk (green light) might only be 5um in diameter which means that you are severely undersampled and unlikely to get accurate measurements (the Airy disk could literally wander within the 10um sample pixel without registering much of a change).

 

Contrast that situation with an OAG on a 8" SCT where the Airy disk would be 12.4um, so a 2x2 bin would probably be okay with that same camera.


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#5 t-ara-fan

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Posted 25 January 2021 - 12:51 PM

... improve on .12rms guiding, on good nights. 

0.12 pixels or arc-seconds? 



#6 moonrider

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Posted 25 January 2021 - 01:40 PM

0.12 pixels or arc-seconds? 

 

Arc-seconds.



#7 t-ara-fan

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Posted 25 January 2021 - 01:56 PM

Arc-seconds.

 

 

0.12 pixels or arc-seconds? 

 

 

Arc-seconds.

Your guiding is excellent. That must be a fraction of a pixel on your imaging camera.  Not sure if you need to improve it, or if you would notice. I wouldn't touch it because of Murphy's Law.
 



#8 rgsalinger

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Posted 25 January 2021 - 02:00 PM

Even when using an AP1600AE at 7000 feet in West Texas, the best RMS that I saw was just about .2 arc seconds. With my Paramount I can get down a bit under .3 arc seconds but usually it's more like .4. It's the same with my CEM120EC2. This is at 4000 feet in the desert 60 miles from San Diego. 

 

I've read Craig's article and he has a much better grasp on the math than I do, but I have never seen anything that would lead me to believe that guide stars are so clean that you can actually get much below 1/6 of a pixel. There are tons of other reasons why guiding is what it is and math is math. 

 

As a long time skeptic, I would love to see .12 arc seconds RMS guiding error as a consistent level documented. Also, what equipment is being used to achieve this. I have another in my hat collection available for immediate eating if this turns out to be correct. If documented the OP must tell "us" how this is achieved.

 

Rgrds-Ross 


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#9 moonrider

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Posted 25 January 2021 - 04:20 PM

0.12 pixels or arc-seconds? 

Whoops my mistake, pixels.....



#10 rgsalinger

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Posted 25 January 2021 - 05:16 PM

Thank  God I really didn't want to have to eat a hat this afternoon. 

Rgrds-Ross


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#11 moonrider

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Posted 25 January 2021 - 07:39 PM

Thank  God I really didn't want to have to eat a hat this afternoon. 

Rgrds-Ross

I went and checked my PHd2 logs, I need new glasses.........Bifocal upgrade!



#12 Cfreerksen

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Posted 25 January 2021 - 07:42 PM

Whoops my mistake, pixels.....

And what is the arc-sec rms?

 

Chris



#13 AstroBruce

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Posted 25 January 2021 - 09:46 PM

With a 50mm finder and a ZWO120mm at  .12rms per pixel, I would think about .5 rms in arc-sec.

 

Bruce



#14 rgsalinger

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Posted 25 January 2021 - 10:04 PM

I think that when most people look to see how the guiding is doing they look at the root mean square (RMS) value of the deviations over some period of time rather than some instantaneous measure. I set my window up to 15 minutes so that it spans a number of my usual 5 minutes sub frames. I'm sure that others use different values.

Rgrds-Ross


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