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I know that guiding is a moment-by-moment process, everything from polar alignment, quality of mount, quality of guide camera and of course sky seeing conditions and wind all impact guiding; the latter two particularly are vulnerable to continuous change. I take great pains to get PA and focus the best I can and I always calibrate on a star on the same side of the meridian as the object I am going to image, near the celestial equator. My guiding often starts off around .25" to .45" total error (combined RA +DEC). But very often, the guiding doesn't stay that way, a few seconds or a few minutes later the numbers either creep up or jump up to numbers above 1 arc-second. I noticed last night that there was a halo around the moon. I assume that the ice crystals that cause this effect might be impacting autoguiding, yes? Even when it's not particularly windy, at least at ground level, this error-creep often happens. Because I was imaging last night with an 8"SCT, guiding with an OAG, I found out that even the slightest error above .7" or .8" can cause the stars to not appear perfectly round, even though my collimation checked out very well. Any thoughts? Jerry |
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Not sure if you've looked into this before but it is a great reference to get started with the asi settings. East Wind Astrophotography: How To Adjust ASIAir Guide Aggression |
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Jeff Horn: Yeah, I've read that numerous times. It has helped me to understand better both the ASIAIR Plus and guiding in general. |
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| I looked for your setup but couldn't find it. What mount, what guide camera. Are you using multistar guiding? What is guide exposure length? Is mount set to sidereal? |
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Jeff Horn: AM5 harmonic drive mount, ASIAIR Plus multi-star guiding with the ASI220mm mini guide camera, Edge HD 8" with .7 reducer and OAG. |
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Maybe is something awry with your setup...but maybe not. So many of these kind of things that I've experienced end up being related to seeing conditions. Some nights I get round stars on my 9.25 even while guiding is 0.9-1.2. Other nights, stars are awful. Some nights I get oval stars while guiding 0.5-0.8. Other nights, they are perfect. The fluctuation in dimensions seem to be independent of anything I do. Right when I think I need to collimate and check for tilt, the next night I'll get perfect stars. It might be variations in set up, but, in the end, I think it is the effect of weather being amplified by the scope's tremendous magnification. Of course, if RA and DEC guiding numbers are wildly different (i. e, RA 1.24, DEC 0.6) then stars will tend to be oval. More often than not, my stars are most asymmetrical on nights the Jet Stream is above my house. Ovals are also sometimes caused by the turbulence and friction between overlapping fronts...causing a bubbling layer of changing density way above despite calm winds at ground level. I notice that RA/DEC can be great one night and awful the next despite a solid setup and the same aggression forces. So I started tracking more data affecting seeing (humidity, dew point/fog formation, high haze & ice crystals, moon phase, and the jet stream). There is a correlation between all of these and worse guiding. So now I relax a little and tell myself tomorrow might be better. It usually works out that way. |
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Dave Rust: Well said Dave. Astrophotography deals with much that is uncertain and out of our control, which is a paradox as the objects we're imaging have been there for millions or billions of years and will be there in millions or billions of years from now. Go figure! |
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Jerry Gerber: You will find there is a very high correlation between seeing and your guide error. Some essential reading here on optimising autoguider settings for strain wave mounts. https://bbs.astronomy-imaging-camera.com/d/15989-getting-the-best-performance-from-my-am5 |
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Chris Beere:Jerry Gerber: I'm pretty sure I have the max RA and max DEC numbers set to the best options, somewhere between 150ms -250ms. the setting I am still not sure of is the number of calibration steps. I have a Skywatcher Evoguide 50mm guide scope (242mm focal length) that I use with my refractors, and a OAG that I use with my Edge HD 8". My understanding is that the lower the focal length of the guide scope the higher the calibration step should be. but I am still not sure what would be the best number for my setup. Is there a math formula that could give me some idea as to what to set the calibration step to, based on the FL of my guide scope? Thanks, Jerry |
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| I have had similar issues too. I suspect it’s seeing conditions but results vary a lot from sub to sub sometimes. When I image with a 9.25” I typically use bin2 to improve my image scale and s/n, and sometimes do that also with my Newtonian when the conditions are iffy. With pixinsight you can do this after the fact so you can just compare results. It may be possible to improve things by fine tuning your mount. My cem40 had a lot of play in RA and I was able to adjust worm gear which helped some. |
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Hey Jerry, yes there is a formula for the calibration pulse. This is a copy pasta from a thread in which Chen (autoguide guru over on the ZWO forum) laid it all bare. "That is true, when all else is equal. Remember that the "Calibration steps" is in terms of the duration (milliseconds) of a calibration pulse at the Guide Rate. So, the same Calibration steps duration will move the mount by twice as much (in terms of arc seconds in the sky) if you use a Guide rate of 1x sidereal instead of 0.5x sidereal rate, for example. 1x sidereal rate moves at 15 arc-seconds per 1000 milliseconds of time. At 0.5x guide rate, the mount will move by 37.5 arc-seconds in 5000 milliseconds. You want each calibration pulse to move a star by about 2 pixels on your sensor. PHD2 will try to move the star by 25 pixels during calibration; so the 2 pixels per calibration pulse will finish calibration in 12 or 13 steps. You can use this tool to determine the pixel scale (arc-seconds per pixel) for your guide scope: https://astronomy.tools/calculators/field_of_view/ Select Imaging Mode, and select any Messier object (M1 will do; the tool won't produce a result unless you have chosen an object first). Enter the focal length of your guide scope and the guide camera that you use. Then look at the result for the "Resolution". It will be in arc-seconds per pixel. From what I mentioned earlier, you want to scope to move about 2 pixels per calibration pulse. The plate scale will convert that to arc-seconds per calibration pulse. For example, if the plate scale is 6" per pixel, and you want 2 pixels, then you want to move 12 arc-seconds per pulse. If you use 0.5x sidereal rate as the guide rate, this 12 arc-seconds corresponds to 12/(15*0.5) = 1.6 seconds. So,for this example, you will want to use a Calibration Pulse of 1600 milliseconds. Get your own guide pixel scale and plug all the numbers in for your particular case. I suspect 5000 milliseconds is way too long even for a small guide scope. ASIAIR has all the available data (guide scope focal length, pixel size of guide camera, and guide rate of mount) to determine the Calibration pulse duration. I don't know why ZWO won't just use those parameters to determine the Calibration pulse automatically, given that their silly mantra is "Simple as 1,2,3." Chen" |
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Thanks Chris! My resolution using the Evoguide 50 ED guidescope and the ASI220mm mini camera is 3.41 arc-seconds per pixel. I get that. What I don't understand is what to do with that information. My mount guide rate is .5x sidereal time, it cannot be changed using the AM5 mount and ASIAIR Plus. With my 8" SCT, using the same camera with the OAG, my resolution drops down to .58" per pixel, same guide rate. So if you know the math behind these numbers please tell me so I can enter the correct calibration step size. Best, Jerry |
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Ratio between Scope and Guiding scope in arcsec/px should be 1:3, max 1:5. Check it by astronomy.tools. Ratios are mentioned in Cloudy nights forum............ Your ratio is 1:6 that could cause problems........ CS Frank |
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Jerry Gerber: ((2×3.41)÷(15×0.5))×1000 = 909.33333333333333333333333333333 Set the calibration pulse to 903 milliseconds An ASI178MM would be a good investment to pair with the Evoguide as it has much smaller 2.4µm pixels and would give you a higher resolution guiding image scale of 2.05" per pixel That would be a closer match to your actual imaging scale giving you a 1:4 ratio. |
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Chris Beere:Jerry Gerber: I also have an ASI290mm mini which has an arc-second per pixel size of 2.47". I am getting good results with either camera with my refractors, but with the Edge SCT I haven't tried the ASI290 yet. I think seeing conditions are really going to be the largest determining factor in regard to sub quality, as guiding seems deeply dependent upon sky conditions. No matter how good the equipment and how much skill is involved, sky conditions are out of our control will always be a major influence, except of course for NASA, who gets to put their scopes far above the atmosphere! |
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Let me see if I understand this: R = resolution of sensor in arc-seconds per pixel S = number of steps GR = guide rate ms = 1000th of a second 2R / (S x GR) x 1000 = calibration step length in milliseconds in the case of the ASI290mm mini and the Evoguide 50ED guide scope, the resolution is .42" per pixel 2 x .42 / (15 x .5) x 1000 or .84 / 7.5 x 1000 or a calibration step size of 112 My guide rate is fixed at .5 as I am using the AM5 with ASIAIR Plus, and, so far, the ASIAIR Plus doesn't have a guide rate setting for this mount (yet). What I don't understand is the 15 (number of steps). Why is this the best number? |
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Jerry Gerber: 1x sidereal rate moves at 15 arc-seconds per 1000 milliseconds of time |
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Chris Beere:Jerry Gerber: Deleted |
