How good is good guiding?

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

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)

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!

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.

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

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!

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

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