Image scale

[cesco]

Hi all,

my guiding scope is a refractor, 80mm aperture / 900mm focal length, equipped with an Atik Titan videocam, 7.4u square pixels, yelding an image scale 1.7 arcsec/px. I use PHD2 for autoguiding, 2x binning, and 0.45 pix min move (the threshold beyond which a correction is triggered) both in RA and Dec. Imaging scope is a Meade 7" ED refractor 1600mm focal,  equipped with an Atik 428EX mono with 4.54 u  square pixels, and an image scale of 0,6 arcsec/px, 2x binned. How many pixels will images in the imaging scope move when the guide star moves 0,45 pix away? How can I evaluate whether a 0,45 pix movement of the guide star is an excessive image movement in the imaging scope?

Thank you!

Francesco

[RafalT73]

Look at ccdware page where they explain how to calculate some guiding settings, if that’s the point…

https://www.ccdware.com/resources/autoguidercalcv4.html

[The Lazy Astronomer]

If I'm reading that right, then you're binning your guide cam x2 for an image scale of ~3.4"/px, and also binning x2 for your imaging cam for a scale of ~1.2"/px. A 0.45px movement in the guider = 1.53", which equates to a movement of ~1.3px in the imaging camera. 

[cesco]

...and so, it is assumed that an 1.53" movement in the guider is also read as an 1.53" movement in the imaging camera....right? That corresponds to an amplification of the guider movement, in pixel, as seen in the imager, by a factor 1.7/0.6, which is the ratio of the guiding scope scale to the  imaging scope's. This factor must also take into account different binning of the guiding and imaging camera, while remaining unaffected  by equal binning.....

[michael8554]

1 minute ago, cesco said:

This factor must also take into account different binning of the guiding and imaging camera, while remaining unaffected  by equal binning.....

The guide and imaging cameras don't have different Binning, they both have the same Binning, X2 ?

The factor takes into account any amount of Binning, if you include that in the calculation of the respective Image Scales.

Michael

[cesco]

Yes, binning is the same.

But it would be better that the guider be unbinned. This would yield a factor of 1.44 from guider to imager scale, and a min move of 0,45px in the guiding camera would be reflected as only 0,65px move in the imaging camera.... if I have understood the calculation. This, in turn, requires that a brighter guide star should be preferred to a dimmer one compensated for dimness with 2x binning.

[The Lazy Astronomer]

5 hours ago, cesco said:

...and so, it is assumed that an 1.53" movement in the guider is also read as an 1.53" movement in the imaging camera....right?

Yes

5 hours ago, cesco said:

This factor must also take into account different binning of the guiding and imaging camera, while remaining unaffected  by equal binning.....

For simplicity, calculate your image scales to include any binning.

4 hours ago, cesco said:

Yes, binning is the same.

But it would be better that the guider be unbinned. This would yield a factor of 1.44 from guider to imager scale, and a min move of 0,45px in the guiding camera would be reflected as only 0,65px move in the imaging camera.... if I have understood the calculation. This, in turn, requires that a brighter guide star should be preferred to a dimmer one compensated for dimness with 2x binning.

I wouldn't worry too much about this, just aim to be achieving guiding performance (in arcseconds) of less than half your main camera imaging scale. If you're not able to achieve that, then consider changing binning or phd2 settings (use the guiding assistant too).

[cesco]

Are RMS values (RA, Dec and total), reported in PHD2 log viewer, an appropriate measure of guiding performance? Total RMS is 1.75" (0.52 px), and the image scale of the imaging scope is 1.17"/px 😬...... On the other hand, the error in polar alignment is around 1.5 arcmin, as measured by PHD2 Guide Assistant. Polar alignment was performed with PoleMaster. 

[The Lazy Astronomer]

1 hour ago, cesco said:

Are RMS values (RA, Dec and total), reported in PHD2 log viewer, an appropriate measure of guiding performance? Total RMS is 1.75" (0.52 px), and the image scale of the imaging scope is 1.17"/px 😬...... On the other hand, the error in polar alignment is around 1.5 arcmin, as measured by PHD2 Guide Assistant. Polar alignment was performed with PoleMaster. 

Yes - total RMS is a good measure, but ideally the RA and DEC RMS should be the same as each other too.

Your polar alignment is fine, phd2 can usually deal with anything under 5 arcminutes pretty well. Bad polar alignment will usually show up as lots of guide corrections in the DEC axis.

What mount do you have? For the size of your scope, l would have thought you must have a fairly decent mount, so 1.75" RMS seems a little high. 

[cesco]

My mount is Losmandy Gemini II, dating back to late 1990's, manual... I stick to star hopping and graduated circles to locate DSOs... There is some backlash in RA and Dec motor worms I can fix by hand, periodically, not so bad. Total RMS value I gave above was from PHD2 guide logs, which was continuously updated during guiding also in response to (and affected by) seeing, I guess. More appropriate values should be those measured by the guide assistant tool, that is 0.82 " (0.24px) total; 0.67" (0.20px) RA and 0.47" (0.14px) Dec, much better than the former. I attached some pictures of the guide assistant output, and PHD2 logs.

Thank you for useful discussion,

Cesco

PHD2_GuideLog_2022-02-10_190833.txt PHD2_DebugLog_2022-02-10_190833.txt

[michael8554]

Hi Francesco

According to the GuideLog the Minimum Move setting is 1.5arcsecs on RA and Dec.

So PHD2 will not start correcting until guide errors exceed that figure.

So no chance of sub-1arcsec guiding :-<

Imaging at 1600mm FL with a 900mm FL guidescope is asking for Differential Flexture.

I suggest you get an Off Axis Guider (OAG).

Michael

[Stuart1971]

With DEC backlash it’s good to have polar alignment misaligned anything up to 10marc mins, this helps as it then drifts in one direction so you only need to correct on the DEC axis in one direction, this makes any DEC backlash moot, this is a recommendation from PHD2 developers themselves…mine is 4 arc mins off in PA, and I guide in south direction only, as I have 3220ms of DEC backlash…

Like me you also have large guide camera pixels, but you BIN it….why ?? that gives a massive almost 15 micron pixel, so with min move set at 0.45 it has to move 7 microns across your pixel before it registers, this then means the imaging camera has already seen the move, and will show in images, you want the guide cam to see the error first and correct, so don’t bin the camera, and reduce min move to about 0.10.

Edited February 21, 2022 by Stuart1971

[cesco]

Binning the guide camera is an attempt at increasing the guide star brightness in order to avoid “star lost” messages, while binning the imaging camera helps increase the brightness of the target DSO. My site has turned from rural dark to urban-rural transition, in the last years. I then use the “multi-short-shots” method in order to increase the S/N ratio, and binning the imaging camera helps getting a bit brighter 45-s long images. I take several (80-100) frames, which should yield a 9-10 S/N ratio, after combining.

I agree on avoiding binning the guide camera….I realize that makes the camera wander a lot. I should try to stabilize the guide star another way…

I attach my last picture: NGC2683, 85 45-s frames, about 65 min for each color channel.

Thank you.

Cesco

[AstroMuni]

On 16/02/2022 at 16:25, cesco said:

How can I evaluate whether a 0,45 pix movement of the guide star is an excessive image movement in the imaging scope?

Check out this video, which might answer some of your questions ......