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Hi All! Since I am relatively new to Astro-Photography and looking at some of the wonderful shots here on AB I asked myself what good guiding really is. After conquering the initial setup steps in PHD I thought that my guiding was fairly good, but my pictures show significantly larger stars than other really good pictures. Of course the exposure time plays a role, still, when taking 60 seconds exposures I expected more pinpoint stars (being fully aware that stars with mag less than 8 will be bigger since overexposed). So I started to blame my guiding, which might be wrong, maybe it is the resolution of the camera. To eliminate the guiding from this chain of thought it would be nice to hear from you what guiding accuracy you have. Mine is around an average of 1-2 arc seconds over a span of 5 minutes (PHD gives you a nice statistic), is that too much? Is the goal to be below 1 arc second? Would be great if you can pass some of your results so that I can compare. |
8.04
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I don't think guiding has anything to do with the size of stars, only the shape of them after long exposure. I think the pinpoint stars come down to the optics and type of filters used. I have no idea what my guiding accuracy is, I just look to see if the stars are round. Carole |
11.07
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Can you tell us about your equipment? Focal length, aperture, camera with pixel size, and mount. I can tell you what I get from a guiding error standpoint but may not be relevant to your situation. Also, do you know what your seeing is in arc seconds? Kevin |
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Uwe, I saw that you were using basically the same optics as me. I'm getting between 0.5-0.8" RMS over subs. (windy nights in the 1.40's) I have it mounted on a EQ-6R Pro Also from some of your images, the stars are round in one corner, but oblong in another. Maybe the sensor isn't parallel with the imaging train. (ex. M1 the stars look round in the bottom right but oblong mostly elsewhere), also the same for several others. Focusing could also be a culprit. I recommend a motorized focuser for that wonderful refractor you have. It really does it justice getting pinpoints with this apo. If you go the motorized focuser route, kstars/ekos works very well to use an autofocus routine and monitor HFR (Half flux radius) of a select star in the frame. That program works very well controlling autoguiding/autofocus/platesolving/planetarium/camera sequence control. (example of it all working in a stream i did not long ago https://www.twitch.tv/videos/239216619) |
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| I ran the numbers and at f/7 your imaging resolution is 0.71 arc seconds per pixel, because that camera has tiny 2.4 micron pixels. With a 0.8x reducer, it is at 0.88 arc seconds per pixel. That is going to be a tough image scale to use with the AVX... unless you were to use the camera on a smaller APO like a WO GT71 or SV70 = 1.47" per pixel. |
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Thank you all so much for those answers, gives me a perspective! The oblong stars at the edges is definitely a problem, Alan, you might be wright that the sensor is not exactly parallel with the train, or even the telescope itself, I have to check this out again. The telescope arrived with the focuser not mounted correctly, maybe I did not do a 100% job to get it back in place. Robert, your numbers are a good explanation, thank you for calculating this out for me! One of my other suspects is the light pollution that I am taking the pictures in, Bortle scale 6 to 7. And i noticed when I try to reduce the back ground noise in the pictures by cutting of the light at the low end it increased the star sizes (wonder why?). In a couple of weeks I will do a short vacation in a less light polluted area, that at least should give me an answer to this. But as always, the challenge is part of the whole, without it everybody would make great picture and then no one would care as much as we do right now. Again, thanks to all who answered, very much appreciated! |
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In my experience guiding has a lot to do with star FWHM, but of course also with star shape. Every time I've upgraded my mount FWHM has gone down. Working with guiding so the guider sends so few corrections to the mount will further improve FWHM |
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Hi Uwe. There are all sorts of reasons that stars may be larger than you expect. It could be guiding, it could be focus, or it could be how you have stretched the stacks. Keeping star sizes to a reasonable level is one of the key challenges in processing. Round stars do not necessarily mean good guiding since a series of random errors could result in rounding. (That’s not my opinion, incidentally, but that of Olly Penrice - a highly respected imager and processor). I’d certainly re-iterate the advice on focussing. If you cannot stretch to autofocus, make sure you use a Bahtinov mask (or similar) and make sure you check focus often. This was a key mistake I made when starting out. ... Focussing was a bit of a nuisance, and I figured if I got focus at the start of the night, how much could it change? The answer is ... quite a lot - especially, in the first few hours. At the beginning of the evening, it is not unusual for my autofocuser to kick in after every 20 minute sub (due to temperature drop). I’d check at least every hour if I were you. Note that, in general, poor focus will tend to make any sensor tilt even more obvious. PHD’s Total RMS is useful if you have configured your equipment correctly. Work out what your imaging resolution is - you can do this using the Astronomy.tools website. If your Total RMS, as shown in PHD is around half your imaging resolution you should be OK. Don’t get too anxious about guiding - if it is set up correctly, then PHD really does live up to its name. But the quality of guiding will change markedly from one night to the next. Some nights look OK, but the atmosphere just will not allow decent numbers. Watch how poor the guiding tends to be as your equipment is still cooling, and note how it often settles down as the night progresses. The cardinal error is to have too much ‘intervention’ - you don’t want to ‘chase the seeing’. Don’t be tempted to cut down exposure lengths in an attempt to get ‘better’ guiding. I use 3-4 second exposures with my EQ6 and 6 secs with my Mesu. Good luck. |
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| ebbene io vengo da molto lontano x la fotografia astronomica, era ancora agli albori della foto analogica, allora la breve esposizione su una galassia era di sessanta minuti, guida fuori asse oculare illuminato 12,5 mm., certo ero molto lontano dai risultati che si ottengono oggi con il digitale, ma riuscivo ad avere molte immagini con stelle molto sferiche, certo il loro volume era abbastanza degenerato ma questo oggi posso dire che la causa prima, era la lunga esposizione, oltre la qualità delle emulsioni, oggi guido ancora le mie foto con lo stesso sistema, con un tempo di esposizione molto inferiore tra i quindici e i 22 minuti in media ma questo perché non ho ancora imparato a usare le guide automatiche e il sistema remoto, ma di una cosa sono certo, che le belle immagini si ottengono in ogni caso solo se ai un buon cielo, ossia primo nemico la turbolenza atmosferica che non solo ingigantisce le stelle ma toglie molto dettaglio al soggetto, sempre secondo il mio punto di vista ritengo che se si vuole determinare più stelle è necessario aumentare il tempo di esposizione, per andare più in profondità, anche ahimè qui è necessario un cielo molto scuro cieli sereni a tutti c,colombo |
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Thank you all again for more information about guiding, they all do make sense. I am a bit surprised about the focusing though, I will have to try that out. I do use a Bahtinov mask and my initial focus is certainly spot on. I do not think that I experienced that much change, when I switched from one object to the next one in one evening I did not see a major difference, but maybe I was lucky those times. A "good" thing here in Florida is that the temperatures in Spring and Fall are not changing much in the evening, maybe that helped. For sure my guiding "error" (RMS) is more than half of my resolution, I will try some 2x2 binning, I am curious what that will produce, I am trying it right now with M3. Carlo, I used google translate since I do not speak Italian, I believe I understood the content that was translated, was in some sentences funny though :-), google translate is not perfect yet. But I definitely understood that you are for quite some time into Astro-Photography, and I certainly agree that darker skies would the best. I will try to do my best to get the advises into reality! Thanks again!! Uwe |
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| I am also fairly new to astrophotography. A lot of great points mentioned here about guiding but I wanted to zero in to the point of pinpoint stars. There is a group of us here in St. Louis that have been buying new equipment over the last 2 years and comparing. The one thing that made a huge difference on pinpoint stars and sharpness was using a mono camera. I still use a bayered color camera myself and have moved from a Nikon DSLR to a Zwo asi 071 PRO bayered color camera. I have barely used it because of clouds but I like what it's adding as far as color and sensitivity. BUT my stars are no better than they were with the DSLR. Two of my buddies went from Canon DSLRs to a ZWO 1600mm with filters (astrodon and Baader filters). Immediately the stars became sharp and pinpoint. No other changes in their equipment. I'll let someone else explain the physics but I believe the fact that with mono, each and every pixel is used for luminance and RGB using filters. This creates a more precise star. As opposed to bayered where 50 percent of the pixels (green) barely get used. There is also interpolation of pixel values in stacking because of the bayer matrix. If you look at the mono pics in astrobin and compare to color camera pics, you will see a pattern. Even knowing this, I chose to stick with a one shot color camera for now. BUT mono is king. |
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I agree with you Mark, the camera makes a big difference, as well as the filters. And maybe the mono cameras gain from the fact that you always have to use filters if you want to have an RGB image. Whenever I take images with my Hα or OIII filters the star sizes are significantly smaller. I tend now to use the OIII image as the "master" image, and add then others on top of it. And when you do that you see that the CLS-CCD images have significantly bigger halos around the stars. I am close to the point to generate just Hα, OIII and SII images in the future, they seem to result into some really nice ones. Thanks for the feedback Mark! |
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Very good points made by some seasoned astrophotographers. Another thing to consider is seeing. I've had nights of hazy skies rolling in and the seeing going from average to very poor and the stars will be round but bigger. I am currently dealing with this, so if focus and collimation is good then consider the seeing. I have learned now that when I'm shooting monochrome that when i have night of rgb to shoot them cycling through each filter one at a time over night rather than complete one filter at a time, that way it all averages out in stacking. However I don't think it's a good fix for bad seeing. I didn't get to read all the comments but make sure you do a focus check every now and then due to temperature change and the possibility that gravity at certain angles can slightly change focus. Hope you get it all worked out! |
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| ora forse in tutte queste discussioni, mi pare che si perde di vista il motivo per cui si fa foto astronomica, perché non tutti hanno possibilità economiche che consentono ricche attrezzature, oppure devono fare molti km. per trovare un cielo discreto, allora si fa quello che si può, anche perché poi è la meteorologia ad avere l'ultima parola, tutto questo per dirvi che il risultato di una foto astronomica va prima di tutto, inserito come ricerca di soggetti poco noti o molto complicati da riprendere visto la loro latitudine. personalmente focheggio attraverso la mia canon 60 d a e ho notato che se vuoi fare una buona messa fuoco la fai se il cielo di quella notte e trasparente e tranquillo, questo lo posso valutare meglio quando mi metto a guidare con la mia guida fuori asse e oculare con reticolo illuminato illuminato. la stella guida diventa un puntino luminoso molto piccolo, poi seguito un collega che mi ha fatto vedere la stessa cosa attraverso il sistema remoto, si sono d'accordo è più facile poi la guida non impegna l'astrofilo, ma la stessa serata rivelò per ben tre volte un errore evidente di guida e si scopri, grazie al fatto che io ho imparato la guida manuale intervento con pulsantiera che io posso guidare stelle anche un po voluminose, mentre nel caso della guida automatica è stato necessario cercare una stella di piccole dimensioni perché cessasse l'errore, aggiungo non ho una grande attrezzatura, ma sono molto contento quando riesco a fotografare oggetti poco conosciuti, sono molto estasiato per le immagini che molti di voi realizzano, ma rimango dell'idea che per me contano i soggetti poco conosciuti. cieli sereni c.colombo |
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Thank you all again for your views on this subject! After more analyzing I would like to summarize: 1. Seeing conditions are certainly number 1 on the list of problems, we cannot help this, just hoping for better weather conditions. Dark areas certainly help too! 2. Camera (mono/color) and filters. 3. Good focus over a period of time. Refocusing more often. 4. Guiding. So living with my camera for now and the weather that Florida has in the summer I am still fighting my guiding issues (THIS I can work on :-)). After some more tests in the evenings I can clearly see a pattern in my guiding: Good declination performance (I polar align with SharpCap and believe that my alignment is within 5", hence pretty good!) but sine waved right ascension, which makes the stars oblong (and I can see this in all my images when I zoom in). I played with the parameters in PHD2, and indeed, some of the settings cause more and bigger waves, but the sine pattern never disappears whatever setting I use. I read a bunch of articles about this, and beside the standard answers (polar alignment, load too heavy, etc.) which do not apply to my setup I haven't seen anything concrete when it comes to this. For me the sine waves are even worse when I switch the guiding from 'On Camera' to ASCOM, which EVERYBODY recommends to be much better. I will continue trying to find the source of this 'misbehavior', but I am not confident that I will find it (maybe with a new mount?). So further question for all: does/did anybody else experience this sine wave pattern in RA? Did you ever solve this problem? And if yes, please let me know what you did :-)! Uwe |
1.20
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Hello Uwe, I highly recommend that you head over to the PHD2 Google group. You can post your questions along with your logs from PHD2 guiding sessions. You are sure to get some detailed analysis along with some excellent advice on how to improve your guiding performance. https://groups.google.com/forum/#!forum/open-phd-guiding Clear skies! John |
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| Thanks John, will do! Need to get some typical log files the next time I am out :-). |
