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What PHD2 numbers are good?

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#1 rjhat3

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Posted 11 August 2020 - 01:07 AM

Just curious what the RMS numbers are for acceptable vs good vs excellent guiding. Trying to fine tune PHD2 and my mount. Anyone have any guidelines?

I have made mine dialed in as best I can get it, and I’m wondering how it compares to someone who actually knows what they are doing. 🤣 

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#2 rkinnett

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Posted 11 August 2020 - 02:19 AM

"Acceptable" error depends on your imaging scale.  You generally want your guiding error to be about the same as your imaging scale or better.  If you're imaging with a 1000mm focal length Newt then that pointing error is more than acceptable.  If you're imaging with a larger SCT, your stars will appear slightly smeared.  Application aside, 0.8" RMS is decent by most people's standards.

 

Try a longer exposure time.  At 0.5s, you're chasing atmospheric fluctuations.  2-5 sec is generally recommended, depending on seeing.

 

You'll get more useful replies to your posts if you provide more information about the equipment you're using.


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#3 PirateMike

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Posted 11 August 2020 - 02:31 AM

What Ryan said.

 

FYI... Image Scale =  (Photo Site / Focal Length) * 206.265

 

Miguel   8-)

 

.


Edited by PirateMike, 11 August 2020 - 02:33 AM.


#4 Huangdi

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Posted 11 August 2020 - 07:06 AM

I got the best results when I saw guiding at 2/3rd of my imaging scale. For my wide field setup, that's 1.7"/px.when guiding at around 1.2", my stars are about as tight as they can get. They're still round at 1.7" guiding, just a little bigger (but very fine details are smeared).

I would consider 0.6 good for a mount without encoders and 0.3-4 for one with encoders

Small edit: this is obviously wind&seeing-dependant. My local observatory has mounts that usually consistently guide at 0.4, but in a night with bad seeing and wind they easily go above 1"

Edited by Huangdi, 11 August 2020 - 07:35 AM.


#5 terry59

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Posted 11 August 2020 - 07:15 AM

Are you happy with the FWHM and eccentricity? If so you are good


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#6 TelescopeGreg

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Posted 11 August 2020 - 12:00 PM

It will also help if you reduce the vertical scale on the History graph.  You're losing a lot of detail that could help reveal how things are going.  I run mine +/- 4", vs yours at +/- 12".


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#7 kisstek

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Posted 11 August 2020 - 12:09 PM

I'd consider 0.17 pixels to be pretty dang good!



#8 Huangdi

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Posted 11 August 2020 - 12:17 PM

It will also help if you reduce the vertical scale on the History graph. You're losing a lot of detail that could help reveal how things are going. I run mine +/- 4", vs yours at +/- 12".


But then it's not pretty anymore :(
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#9 F.Meiresonne

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Posted 11 August 2020 - 12:20 PM

What Ryan said.

 

FYI... Image Scale =  (Photo Site / Focal Length) * 206.265

 

Miguel   8-)

 

.

variable "Photo site" means what? 


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#10 OldManSky

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Posted 11 August 2020 - 12:38 PM

variable "Photo site" means what? 

Size of the pixels (photo sites) on your imaging camera :)

 

I try to stay under 1/2 the imaging scale in terms of total RMS error in PHD2.  At shorter focal lengths with 1.5" or greater image scale, that's easy.

With my 103 at 0.8" or so image scale, it's tougher...but my mount can usually pull it off.


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#11 F.Meiresonne

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Posted 11 August 2020 - 01:32 PM

(3.72/274)*206.265 = 2.8 

 

Is this correct? for my 61 mm EDPH scope at 274 mm FL, whichs means i should stay under 1.4 RMS?

3.72 is the pixelsize of the canon 800D camera sensor..



#12 Stelios

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Posted 11 August 2020 - 04:05 PM

(3.72/274)*206.265 = 2.8 

 

Is this correct? for my 61 mm EDPH scope at 274 mm FL, whichs means i should stay under 1.4 RMS?

3.72 is the pixelsize of the canon 800D camera sensor..

What it means is that once you are under 1.4" RMS, improving your guiding won't make much if any difference to your images. 

 

It also means you should dither and drizzle :)


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#13 Stelios

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Posted 11 August 2020 - 04:07 PM

Just curious what the RMS numbers are for acceptable vs good vs excellent guiding. Trying to fine tune PHD2 and my mount. Anyone have any guidelines?

I have made mine dialed in as best I can get it, and I’m wondering how it compares to someone who actually knows what they are doing.  

In addition to what others have said, you should change the displayed graph so you can see what's happening. Change +/- to at most 4".


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#14 rjhat3

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Posted 11 August 2020 - 05:55 PM

Thanks for the info! I am imaging with a 1000mm f.3.9 Newtonian Astrograph (10”). Guiding scope is a 50 mm Orion at 162mm. My guide cam has a pixel size of 3.75. My imaging cam has a pixel size of 3.7.
So, if I do the math for my imaging train, I come up with an image scale of .763. If I guide at 2/3rds, that means I need to get it down to .51. Is that correct?

Edited by rjhat3, 11 August 2020 - 06:09 PM.

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#15 PirateMike

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Posted 11 August 2020 - 09:57 PM

variable "Photo site" means what? 

The size of the photo site (pixel) can be found in the specs of the camera and should be in the scale of "um".

 

The focal length is the telescopes focal length in mm.

 

For example...

 

My camera has pixels sized as 3.8um

My scope has a focal length of 740mm

 

so...

3.8 / 740 = 0.00513

then...

0.00513 * 206.265 = 1.05919

 

If you use a reducer then you need to use the reduced focal length in the equation.

 

A general rule is that you would like your image scale to be between 1.0 and 2.0, but that's a

just a general suggestion.

 

I hope that explains it. smile.gif

 

 

Miguel   8-)

 

.


Edited by PirateMike, 11 August 2020 - 09:58 PM.

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#16 PirateMike

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Posted 11 August 2020 - 10:08 PM

Thanks for the info! I am imaging with a 1000mm f.3.9 Newtonian Astrograph (10”). Guiding scope is a 50 mm Orion at 162mm. My guide cam has a pixel size of 3.75. My imaging cam has a pixel size of 3.7.
So, if I do the math for my imaging train, I come up with an image scale of .763. If I guide at 2/3rds, that means I need to get it down to .51. Is that correct?

That is correct by the assumption that you need 2/3rds of the image scale.

 

Perfectly good photos have been taken at 1 times the image scale. One thing not mentioned is exposure time. The shorter the better as long as you are getting signal from important (for you) areas of the scene.

 

The thing that counts the most is if you are satisfied with your final images. 2/3rds or lower is wonderful but is it achievable with your equipment, maybe, maybe not. Do you need that level of guiding accuracy, maybe, maybe not. In any case you have to work with what you have or buy a better mount.

 

Just enjoy what you have and do the best you can do.

 

 

Miguel   8-)

 

 

.


Edited by PirateMike, 11 August 2020 - 10:11 PM.

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#17 F.Meiresonne

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Posted 12 August 2020 - 12:08 AM

Good info here!


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#18 PirateMike

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Posted 12 August 2020 - 06:41 AM

I forgot to mention that in the example I gave above, the image scale of 1.05919 is the number of arcseconds of sky that is projected onto a single pixel,that is what image scale is. If my tracking is below that then the center of that star will not move off that pixel and will not be recorded onto a neighboring pixel.

 

Then again that does not include the seeing. My skies are usually about 1.8 arcseconds for seeing, so my guiding appears well below the seeing. 

 

What this all means in the end is hard to say. My images come out just fine for my taste, so I'm a happy camper. If I had the tracking of the Hubble Telescope I would be a much happier camper.

 

In the end the tracking RMS is just an indication of your tracking accuracy and it includes the effect of the seeing. it should be used as a number comparison in future imaging sessions.

 

 

Miguel   8-)

 

.


Edited by PirateMike, 12 August 2020 - 06:56 AM.

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#19 PirateMike

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Posted 12 August 2020 - 07:03 AM

If you want to see what my example setup is capable of (using a reducer, so image scale is actually 1.45 and an RMS of probably about 0.65) then check out the link.

 

This image was taken with 20 minutes of exposure. You can go to the full scale image to see the data in more detail.

 

Enjoy.

 

https://www.astrobin.../full/384810/0/

 

 

Miguel   8-)

 

.


Edited by PirateMike, 12 August 2020 - 07:05 AM.


#20 rjhat3

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Posted 12 August 2020 - 11:33 AM

I forgot to mention that in the example I gave above, the image scale of 1.05919 is the number of arcseconds of sky that is projected onto a single pixel,that is what image scale is. If my tracking is below that then the center of that star will not move off that pixel and will not be recorded onto a neighboring pixel.

Then again that does not include the seeing. My skies are usually about 1.8 arcseconds for seeing, so my guiding appears well below the seeing.

What this all means in the end is hard to say. My images come out just fine for my taste, so I'm a happy camper. If I had the tracking of the Hubble Telescope I would be a much happier camper.

In the end the tracking RMS is just an indication of your tracking accuracy and it includes the effect of the seeing. it should be used as a number comparison in future imaging sessions.



Miguel 8-)

.


Maybe a dumb question, but how do you accurately gauge current seeing conditions in arcseconds?
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#21 PirateMike

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Posted 12 August 2020 - 07:26 PM

Maybe a dumb question, but how do you accurately gauge current seeing conditions in arcseconds?

No such thing as a dumb question, just dumb answers. waytogo.gif

 

I use the FWHM number in the "Star Profile" window of PHD2. after following the instructions described in it's user manual.

 

Basically the manual says... use the smallest star you can find and focus it as accurately as possible.

 

 

Miguel   8-)

 

.



#22 OldManSky

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Posted 12 August 2020 - 08:02 PM

Maybe a dumb question, but how do you accurately gauge current seeing conditions in arcseconds?

Since my mount is very, very well polar-aligned and on a permanent pier, to gauge seeing I start up PHD2, turn off corrections, and watch the guide graph.  That way it's showing star motion only in DEC, since I have essentially no dec drift due to polar misalignment.  I use short exposures, and let it go for a minute or so.  The graph will show seeing "bounce" in arcseconds on the DEC graph.

My seeing ranges between 1.2" and 2.0" on most nights.  Every now and then it'll jump up to 4" or so (when unusual weather conditions prevail).  On nights like that, I shut down :)


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#23 nimitz69

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Posted 12 August 2020 - 08:21 PM

Just curious what the RMS numbers are for acceptable vs good vs excellent guiding. Trying to fine tune PHD2 and my mount. Anyone have any guidelines?
I have made mine dialed in as best I can get it, and I’m wondering how it compares to someone who actually knows what they are doing.


When your total RMS is less than your seeing limit there is no more to worry about wrt guiding. Most places in US don’t see conditions below 2 arcsecs that often


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