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Re: PEC training
Arun Hegde
RMS stands for Root Mean Square. It is a measure of, on average, how large the deviations are from the baseline. An RMS value of zero means a perfectly flat graph.
In the context of autoguiding, we are concerned with how far the centroid of the guide star deviates from the point where it should be (or we want it to be). These deviations are measured in two directions which are at right angles to each other - Right Ascension and Declination (RA and DEC), Now think of your star as a circle with two diameter lines at right angles to each other. If the deviation in the direction of RA is greater that in DEC, the diameter of the circle is greater in the RA direction than in DEC. The circle is "stretched", it is no longer a circle, but an oval or ellipse, longer in the RA direction than DEC.? If the RA and DEC RMS are close to each other, then the circle remains a circle. Now you can also think about the case where the deviations in RA and DEC are both close to each other, but both very large. That's still a circle, but a rather bloated one. Your images will have round stars, but large ones. Fine features of your image will be smeared. The ideal case is when both deviations are much smaller than one pixel in your image. At that point, all the deviations are "contained" within one pixel, and your guiding no longer limits the size and roundness of your stars, nor feature resolution. The finer your image scale, the more demanding your guiding becomes, and the more you'll be able to see faults in your guiding. Many people consider that a value of lower than 0.8 " RMS to be a good number to aim for. Because at point, under most conditions, deviations caused by things like atmospheric seeing become more critical. With proper polar alignment, balancing, appropriate clutch tightening, etc., you should be able to guide consistently under this value with good mounts. That has certainly been the case for me (with some work) with the GM811G and a small (80mm) refractor. |
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Re: PEC training
¿ªÔÆÌåÓýHi I am interested in this discussion. Can someone explain the statements that the closer RA and Dec are in RMS the more round the star. What is the underlying explanation. ? Thanks. ? Aubrey ? Sent from for Windows 10 ? From: Arun Hegde
Sent: Friday, December 11, 2020 12:20 PM To: [email protected] Subject: Re: [Losmandy_users_io] PEC training ? "Eccentricity (star roundness) really depends on how close RA and DEC RMS are, not necessarily how high is your RMS ? low RMS = small stars RA and DEC RMS close in value = round stars"
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Re: PEC training
Arun Hegde
"Eccentricity (star roundness) really depends on how close RA and DEC RMS are, not necessarily how high is your RMS
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low RMS = small stars
RA and DEC RMS close in value = round stars"
True - but I'd think that if both were less than your image scale, whether one is greater than the other would not matter; you'd get round stars because your pixels would be too large to resolve the difference. And if both were larger than your image scale and close to each other, you'd have round stars but they'd be larger than they could be from optical effects alone, plus you'd lose feature resolution. So a desirable end point is to have both as well as your total RMS smaller than your image scale. On Thu, Dec 10, 2020 at 10:42 PM, Brian Valente wrote:
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Re: ASI120mini question
As others have suggested, take the guide camera out of the guider (but first use a parafocal ring on it so you don't loose focus)? and insert an eyepiece. Place a bright obvious star in the center of the main scope and then adjust the guider so that the same star is centered in the guide scope. Put the guider back in and re-check focus.
There is another reason (esp. if you don't have a permanent setup) to get this as good as you reasonably can: Field Rotation, which if present causes the images of stars to appear as little arcs center on (and proportional to the distance from) the guide star. Field rotation depends on four things: 1. The precision of the polar alignment. 2. The any difference in the angular alignment of the imaging and guiding scopes ( how well they are co-boresighted, to use the artillery term) - what your question was about. 3. The duration of the exposure. The longer the exposure the worse (if present) field rotation will be. 4. The magnitude and the orientation of the polar alignment error relative to the RA and DEC of the imaging target. You can not do anything about the 4th unless you just give up and move to another object in a different part of the sky and pray you are lucky. The third you can compensate for by taking more shorter exposures and then using a stacking tool that does a two point stacking alignment. The first two (polar alignment and co-boresight accuracy) you can do something about. Having the two scopes co-boresighted has two benefits: First, it makes it easier to diagnose (if necessary) polar alignment problems because the arcs of the stars will all be orientated around the center of the image (where the guider was tracking), making it obvious what is going on. Second, a large amount of boresight error can magnify even a tiny amount of polar alignment error. With regard to 2, the problem (if present) is not less for shorter focal length imaging instruments. This is because the size of the arcs in angular measure (degrees, minutes of arc, whatever)? for a given exposure duration is linear in the angular distance between the star images and the guide star. So while cutting the focal length of the imaging scope in half will reduce the plate scale (Image size in inches per degree, for example) the width of the field, in degrees will proportionally increase (for the same chip, obviously)? so the two effects cancel each other out. Likewise cropping won't do anything in general, nor will going to a shorter focal length (wider field) instrument. For a general discussion see: For a detailed discussion, see Michael Covington's book "Astrophotography for the Amateur", pages 276 to 282 where he goes through all the mathematics of polar alignment error, DEC drift, and field rotation. On page 281 he gives some nice illustrations and a summary (B.3.3 How Much Field Rotation is Tolerable). The reading is not required, just a general understanding. The Calgary (Canada) section of the Royal Astronomical Society has a detailed qualitative discussion of Field Rotation, including the extreme case of ALT/AX mountings: https://calgary.rasc.ca/field_rotation.htm The bottom line is that the better the polar alignment the less all this matters. And for off-axis guiders improving polar alignment or shortening exposures (if feasible) is the only cure. And of course what really matters is how satisfied you are with your final images. Clear skies and Happy Holidays. Mark Christensen PS: Can you do the alignment during daytime? Of course, just make sure the alignment target is far enough away. How far? Far enough so the parallax between the two scopes (guider and imaging) doesn't matter much. If your 60mm guider is the type I am familiar with its focal length is between 240 and 300mm.? Let's call it 300mm. Most guide cameras have small chips in them so let us suppose you want to get within 1mm of the center. That means the parallax error should be reduced to less than 1/300 or so (about 0.2 degrees). If the spacing between the center lines of the guider and the imaging scope is something 6 inches then the alignment target should be at least 300 * 6 inches or 150 feet and 300 feet would be better. Further is you wish to achieve more accurate co-boresighting. The same discussion applies to finder scopes. |
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Re: ASI120mini question
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If the centers are too far off you introduce cone error and over the course of a night on one target, the guide scope will stay on a star, but your image frames will start to travel around it in a small circle.? you won't get cone error from misalignment of your guidescope/main ota, since you are really using the OTA for goto and related functions. You can still of course have cone error from the main telescope. it's always something right :) Tony - my suggestion is for the moment maybe don't worry about that too much. If you're getting your system together and working on focus, etc. try to get that stuff working first, and then move on to more precise alignment.? Yes it's important and you should try to get it close, but i personally wouldn't sweat it too much for now. On Thu, Dec 10, 2020 at 11:29 PM John Kmetz <jjkmetz54@...> wrote: Tony, --
Brian? Brian Valente portfolio |
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Re: ASI120mini question
If you want to line both scopes up, point your main scope at a bright star and center it up in your camera. Take the auto guider camera out of your guide scope and stick an eyepiece in the guide scope. It doesn¡¯t have to be in focus. You are just looking for the same bright star your main scope is pointing at. If you can¡¯t see it, adjust your guide scope until you can see it. Put your guide scope camera back in the guide scope and the bright star should be visible now. Fine tune the guide scope until the star is centered. If you have a green laser it is even easier. Just remove the guide camera and point your green laser in the guide scope. You will be able to see the green laser come out the other end of the guide scope and see where it is pointing and adjust it.
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On Dec 10, 2020, at 11:21 AM, Tony Gabriele via groups.io <tgabriele73@...> wrote: |
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Re: PEC training
On 12/11/2020 3:07 AM, Paul Goelz wrote:
On 12/10/2020 10:55 PM, Brian Valente wrote:Guys,Any of the things you mentioned could contribute to that. a bad PEC could certainly create havoc tooOK, I have uploaded the log from this session. After a night of reflection I realized that since the orientation of my oval stars was not coincident with RA or DEC, it is likely it was not caused by a guiding issue. So this morning I had a look at my subs and found the following: 1. My subs are not perfectly in focus. 2. My collimation (this is an 8" SCT) is just a touch off. I may have a small amount of sensor tilt as well.... not sure. But coupled with the fact that I was not perfectly in focus, that caused the aggregate star image during the 10 minute subs to be oval in an 11:00 / 5:00 orientation. That matches the oval in the final stack. 3. The ten 10 minute subs do show creep along the RA axis (12:00 / 6:00) from one sub to the next that could be tracking issues or flexure or mirror flop..... or some combination of all three. So I still have questions about the tracking / PE and am interested in what you see in my log. But I am not sure how responsible any RA issue are for my oval stars. Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: multi-star PHD2!
I was out again tonight and I've come to the conclusion that multistar is a big improvement to guiding. I was getting solid sustained 0.6 rms range of guiding tonight. Previously I was getting around 1.0 rms.? Testing both on the same night would have been better, but I wanted to take pictures and didn't want disrupt things, so I kept multi on.?
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Re: PEC training
On 12/10/2020 10:55 PM, Brian Valente wrote:
Any of the things you mentioned could contribute to that. a bad PEC could certainly create havoc tooOK, I have uploaded the log from this session. You should see a couple short guiding runs as I played around with it before starting an imaging run. Then there should be a long 100 minute guiding run while I imaged. During that 100 minute imaging run I did a PEC train, watched it for a bit, then turned PEC off and left it off. Some time later (still during that 100 minute run) I changed RA aggressiveness from 70 to 90, then to 100 and then back to 90 (as I recall). Except for the RA aggressiveness, all other settings are per the last guiding assistant run. Note that DEC backlash started around 1000mS but drifted upwards from there to a little north of 2000mS as I recall. DEC guiding looked near perfect to me. RA looked pretty good as well aside from the occasional vaguely periodic swings +/- 2" or thereabouts. I'll be interested to hear what you find. Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: ASI120mini question
Tony,
Like everyone has said, the imaging and guide scopes do not have to be perfectly aligned, but they should be pretty close. I use the crosshairs in both PHD2 and Sequence Generator to get as close to center as I can on a bright star without over doing it. If the centers are too far off you introduce cone error and over the course of a night on one target, the guide scope will stay on a star, but your image frames will start to travel around it in a small circle. Then when you go to stack your light frames you will see they will not overlay as well, and you will have wasted margins you will need to crop off, reducing the total good image area you might have had. Just as important is good polar alignment. Even if you exactly center both scopes, the imaged frames will drift off target in one direction (declination), and then the other way after the meridian flip, when PA is bad. Then again you will end up cropping off bad margin areas.? If you are imaging a target that has a lot of empty space to the edge, these factors are not as important. But if you have something large that just fits onto the sensor, then you could be losing part of your intended target. But with good centering and good PA, the light frames will pretty closely overlaid and you are getting the most from your given sensor size. And we all like bigger sensors :). Best regards, John |
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Re: PEC training
On 12/10/2020 10:55 PM, Brian Valente wrote:
Any of the things you mentioned could contribute to that. a bad PEC could certainly create havoc tooIts bedtime for Bonzo here.... I'll upload tomorrow and see what you can see. Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: PEC training
Any of the things you mentioned could contribute to that. a bad PEC could certainly create havoc too best way is to upload your guidelogs and we can do some analysis On Thu, Dec 10, 2020 at 7:51 PM Paul Goelz <pgoelz@...> wrote: On 12/10/2020 10:41 PM, Brian Valente wrote: --
Brian? Brian Valente portfolio |
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Re: PEC training
On 12/10/2020 10:41 PM, Brian Valente wrote:
Eccentricity (star roundness) really depends on how close RA and DEC RMS are, not necessarily how high is your RMSI should also add that although seeing seemed OK, transparency was awful due to high humidity and (I think) thin high cirrus. Bands of high cirrus passing through could have I guess caused asymmetrical guide star movement.... even with multi star guiding enabled? Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: PEC training
On 12/10/2020 10:41 PM, Brian Valente wrote:
Eccentricity (star roundness) really depends on how close RA and DEC RMS are, not necessarily how high is your RMSYes, I agree. The oval stars tonight were because the P-P RA error at about 2-3" was noticeably higher than the DEC (which was VERY smooth and <1"). Usually they are roughly equivalent and the stars are round. I would not have even mentioned it except that there was a noticeable periodic component to the RA guide graph that I was assuming is PE. Maybe not? Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: PEC training
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?if guiding is working correctly, should periodic error beyond fractional arcseconds even be visible?? As an experiment, I increased the RA aggressiveness from 70% to 90% and things got better.? >>
?tonight was the only time I have actually had oval stars.??? Eccentricity (star roundness) really depends on how close RA and DEC RMS are, not necessarily how high is your RMS low RMS = small stars RA and DEC RMS close in value = round stars On Thu, Dec 10, 2020 at 7:36 PM Paul Goelz <pgoelz@...> wrote: On 12/10/2020 10:25 PM, Brian Valente wrote: --
Brian? Brian Valente portfolio |
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Re: PEC training
On 12/10/2020 10:29 PM, Brian Valente wrote:
PS i should have also mentioned you can deactive/ delete PEC from inside the Gemini Telescope.net applicationYe, I am aware of those checkboxes and functions. While I can see how to turn PEC on and off, I don't see a way to ERASE it. Again, not an issue if it only stores one curve. Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: PEC training
On 12/10/2020 10:25 PM, Brian Valente wrote:
the gemini II built-in pec is really more designed for visual - one cycle, no multiple passes, no averaging.Thanks Brian, that makes sense. Too bad the G2 does not average multiple sessions though. I can certainly try PemPro on a trial basis but I'm afraid that I find the purchase cost of $149 excessive. On a side note.... if guiding is working correctly, should periodic error beyond fractional arcseconds even be visible? As an experiment, I increased the RA aggressiveness from 70% to 90% and things got better. Again, it is not excessive at all and tonight was the only time I have actually had oval stars. Mostly it does not seem to be affecting my images. Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
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Re: PEC training
PS i should have also mentioned you can deactive/ delete PEC from inside the Gemini Telescope.net application the PEC checkbox to enable/disable is right there on the soft hand controller The ability to autoload pec on startup or clear PEC is under Setup menu->PEC settings On Thu, Dec 10, 2020 at 7:26 PM Brian Valente via <bvalente=[email protected]> wrote:
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Brian? Brian Valente portfolio |
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Re: PEC training
Hi Paul >>> I have done several PEC training sessions (training with PHD2 / pulse guiding) but if anything, my (assumed) PE gets worse.??For high quality astro imaging, you need a high quality PEC. this approach is not high quality enough. you really need to be using PEMPro for PEC? the gemini II built-in pec is really more designed for visual - one cycle, no multiple passes, no averaging.? you also have a lag there between observed movement and reaction so you will always be a tiny bit behind i really recommend you save yourself grief and give PEMPro a shot - they have something like a 45 or 60 day trial period B On Thu, Dec 10, 2020 at 7:21 PM Paul Goelz <pgoelz@...> wrote: First of all, I'm aware this is a dumb and basic question.? But for the --
Brian? Brian Valente portfolio |
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PEC training
First of all, I'm aware this is a dumb and basic question. But for the life of me I can't find a definitive answer online.
Now that I am using multi star guiding which reduces the effects of transient seeing effects, I can see what appears to be a couple arcseconds peak of what certainly looks like periodic error. I have done several PEC training sessions (training with PHD2 / pulse guiding) but if anything, my (assumed) PE gets worse. So I have a couple questions. 1. If you train more than once, does the G2 store / average more than one PEC curve, or does each new training session overwrite / replace any previous sessions? 2. Is there a way to erase the PEC curve? Only necessary I guess if it averages multiple sessions. TIA, Paul -- Paul Goelz Rochester Hills, MI USA pgoelz@... www.pgoelz.com |
Jamie Amendolagine
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