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0.38 RMS With The Longest Guide Star In The World, LOL

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#1 ChiTownXring

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Posted 08 May 2021 - 10:15 PM

So I just connected my filter wheel to my imaging train and needed to re-focus my 120MM in my COAG which I did during the day. Then last night I go to fine focus it and got distracted and hit guide in Phd2 and when I saw the shape of the star I was about to get up to go and get the star in focus because the star looked like the Whale Galaxy but then I saw the RMS and sat back down. I thought this is going to get ugly I'm sure but to my surprise the RMS stayed at 0.38 and even dropped down to as low as 0.24 when things settled down.. I am scratching my head knowing that guiding can be better with an out of focus star but isn't this crazy? I am not complaining just puzzled is all..

 

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#2 rhart426

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Posted 08 May 2021 - 10:43 PM

Focus isn't what causes stars to look like that. The reason the star is so misshapen is because the OAG is pulling light from the very edge of the image circle.  Fortunately, PHD2 doesn't care if it's round or not, it can calculate the centroid either way for accurate corrections.



#3 ChiTownXring

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Posted 08 May 2021 - 10:53 PM

Focus isn't what causes stars to look like that. The reason the star is so misshapen is because the OAG is pulling light from the very edge of the image circle.  Fortunately, PHD2 doesn't care if it's round or not, it can calculate the centroid either way for accurate corrections.

That's what I love about this hobby, I learn something new everyday.. So you are saying that the star got cut off and Phd2 is only picking up half of it due to it being on the very edge of the image circle? If this is indeed the case I am surprised that Phd2 auto selected that star..

 

Thanks for this valuable info..



#4 TOMDEY

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Posted 08 May 2021 - 11:03 PM

Yep --- almost counterintuitively, centroids compute more accurately for otherwise stable impulse-responses that are kinda fat. A lot of reasons for that --- but, thankfully, it works.    Tom


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#5 rgsalinger

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Posted 09 May 2021 - 01:06 AM

When using an OAG good focus is always better than poor focus because you will get more guide stars when in focus and the SNR will be higher.

 

I've never once seen that defocusing improved either my guiding or the resulting images. What I have seen is that poor focus in an OAG causes poor guiding. Never seen the reverse. 

 

The OAG is not pulling light from the "edge" of anything. You are not seeing 1/2 of a star at all. You are seeing a star that's distorted by the scope's optics. I don't know your scope's imaging circle but an OAG is generally outside the best part of any refractor's optics unless you have some kind of flattener attached.

 

Rgrds-Ross 


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#6 F.Meiresonne

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Posted 09 May 2021 - 01:38 AM

Amazing PDH2 is. I am Always complaining about commercial software , not or half working in most cases.and needing updates all the time.....and youeven  have to pay for that..

 

PHD2 never failed on me, works great , guides great and it comes for free. This on both my mounts on ST4.

 

I once had 1 problem , PHD2 loosing connection...but that was me...not connecting a cable good enough...



#7 Tom62e

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Posted 09 May 2021 - 01:55 AM

The only issue I have ever had with PHD2 is it dropping the connection to my ASI120MM-S but since it only happens with this camera, I'll assume it's an issue with the camera and not PHD2.  PHD2 never drops my Lodestar or my Orion SSAG, just the ZWO camera.



#8 michael8554

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Posted 09 May 2021 - 06:51 AM

I've never once seen that defocusing improved either my guiding or the resulting images. What I have seen is that poor focus in an OAG causes poor guiding. Never seen the reverse. 

 

Good to know Ross, I was developing a thought that a more rounded, slightly defocused whale, would guide better - shan't try that then !

 

 

HFD of 5 is as good as I can ever get with an OAG at 1400mm FL



#9 Tom62e

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Posted 09 May 2021 - 11:47 AM

Good to know Ross, I was developing a thought that a more rounded, slightly defocused whale, would guide better - shan't try that then !

 

 

HFD of 5 is as good as I can ever get with an OAG at 1400mm FL

I 2nd that.  I experienced better results in PHD2 with pin-tack focus. 

 

Same with SharpCap but maybe even more so.  I started another thread about how I couldn't "solve" for Polar Alignment in SharpCap (despite being focused enough that I could clearly see and count 40 or more stars on my screen).  Once I used the SharpCap Bahtinov Mask tool to fine-focus - it solved immediately. 



#10 rgsalinger

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Posted 09 May 2021 - 01:07 PM

Dropping connections, assuming that they are USB is chronic and due to the cabling topology, the quality of the cables and the quality of the USB chipset in the computer. My systems are all configured to use really short USB cables and "fancy" hubs. That has made them very reliable for years now. If you aren't using a quality hub to reduce the cabling to your PC, give it a try. 

Rgrds-Ross


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#11 rhart426

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Posted 09 May 2021 - 03:36 PM

This literally happened to me last night; guide camera was throwing USB connection errors, had to make a late-night Walmart run for a temporary replacement.  I've had nothing but grief with the ZWO-supplied flat USB cords.  Replacing all of my cabling with higher-quality stuff is my next priority, along with acquiring a spare of each.




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