Gerhard Posted April 1, 2015 Share Posted April 1, 2015 hi all,I was just reading up on optical aberrations, and it appeared to me that maybe my pics taken with my Vixen suffer from coma...It looks to me like the stars are all malformed, and maybe this is coma, very common to newtonians...Coma correctors can be a solution, as I gather, but backfocus issues might ruin this, if there is not enough room to focus with a corrector on.Does anybody know if my Vixen R150S can handle a coma corrector with a DSLR attached? or will I not be able to focus anymore, and so would have spent 160 euros in vain?I haven't found info on the backfocus of my scope...Any info would be great!!! Gerhard.ps: also anything you might have to say about my pics would be appreciated! :-) Link to comment Share on other sites More sharing options...
Dude_with_the_tube Posted April 1, 2015 Share Posted April 1, 2015 i am not an imager so dont take my post as granted,but looking at the pictures it looks more to me that it is a tracking/exposure issue then it is coma.Coma only appears on edges of the image,where both your images show prolonged stars across the image what can only mean:Tracking or your exposures are too long.Proper imagers will chime in and get you sorted with proper advise. Link to comment Share on other sites More sharing options...
alan potts Posted April 1, 2015 Share Posted April 1, 2015 I can only agree with advice given this is not coma, nice pictures though, get sorted with either tracking or whatever and maybe next time the images will really take off. Link to comment Share on other sites More sharing options...
Pompey Monkey Posted April 2, 2015 Share Posted April 2, 2015 I think that there are both coma and tracking artefacts visible in the m42 image as the elongation is in different in different parts of the field.It is hard to tell in the m51 picture as it is cropped.I have no idea if there is enough back (or is it front?) focus on the vixen to get a corrector in there. Are you in a club? If not, it is a good time to join so that you can discuss and try out each others equipment before splurging out hard earned cash! Link to comment Share on other sites More sharing options...
Gerhard Posted April 2, 2015 Author Share Posted April 2, 2015 thanks for the responses!If the problem is tracking, then, being as I am using the mount only (no laptop or guide scope), the only thing to do better would be the alignment with the polar, and afterwards the 3 star alignment on the gopro? Should I do alignment on more stars? I am member of a club, and as a matter of fact my scope was property of that club, I bought it from them, when they were renewing.The president already told me that a coma corrector would not do me much good, but the photos to me seemed to have coma in them...I wanted to have a second (third, fourth... ) opinion...I attach two other non cropped photos:Thanks again for your trouble! Link to comment Share on other sites More sharing options...
Gerhard Posted April 8, 2015 Author Share Posted April 8, 2015 I went out observing again this evening, but only for a "test drive".Just to see if I could get better results by doing all the setup more carefully...Some aspects are "improvable":- I haven't been able to check yet if my polar scope is properly aligned with the mount (the guy I got it from said it should be ok)- I don't yet have a dynamometer to perfect the balancing of the mount- I'm getting more confident about the collimation, but there is always the "danger" of a tensioned primary, I hear...- 3-star alignment: the first star usually is way off the mark, when the mount positions itself... is that a sign of bad polar alignment?- I use a Bahtinov to focus on a nearby bright star, then move to the subject itselfanyway, this evening I tried, just out of curiosity, a 5 minute exposure. There is slight tracing, but I'm guessing that's not the only problem:another one is at 48 secs, and looks not bad, but zooming in, all the stars are deformed again: I get the same triangular stars also at 33 and 18 secs...anyone have any ideas what could cause this? or is it "just" a combination of all of the above mentioned aspects?Thanks for any info!Gerhard. Link to comment Share on other sites More sharing options...
toxic Posted April 8, 2015 Share Posted April 8, 2015 it looks like you are a little out of focus and maybe just a little bit of coma and i think your PA maybe out.Polar Align then focus or visa verse then have a look at what you get when that is sorted before spending any money. Link to comment Share on other sites More sharing options...
wxsatuser Posted April 9, 2015 Share Posted April 9, 2015 Think there is coma, scope and mount issues here.Your pushing the mount hard if that 5minute exposure was unguided.Check you collimation, polarscope reticule is aligned to the mount, your polar alignment is spot on.Don't perfectly balance your mount.If imaging east of the meridian make it counterbalance heavy, west of the meridian scope heavy. Link to comment Share on other sites More sharing options...
Gerhard Posted April 9, 2015 Author Share Posted April 9, 2015 The 5 minute exposure was only to exagerate the anomalies, so as to be able (hopefully) to put a finger on the problem... I know on a non-autoguided mount 30 - 60 secs is the max attainable...But, as you guys point out, and as I already feared, it's probably a combination of things...I really need to check the polar scope alignment, and perfect my PA...Currently I'm doing my PA in this way: - align approximately using the finderscope, so as afterwards to find the polaris in the polar scope- check my polarfinder app to see in which position the polaris is at the moment- position the mount (turn on RA axis) so that the reticule shows the polaris position as indicated by the app- adjust alt and az to position polaris in the polaris circle of the reticuleI find using the polarfinder app much quicker than figuring out the correct position using the indications as for example by astro-baby: http://www.astro-baby.com/HEQ5/HEQ5-1.htmAny idea why the 3-star alignment is off by so much on the first star? PA cause?Interesting what you say about balancing... I may try that out!thanks again!!(I keep reminding myself that I have the scope now for only two months and should expect a long learning curve, but my expectations keep exceeding my patience.... ) Link to comment Share on other sites More sharing options...
wxsatuser Posted April 9, 2015 Share Posted April 9, 2015 I don't think the star alignment is affected by polar alignment.On my setup, a permanent one, if I move the mount manually away from the home postionon next start up I need to star align and the first star is normally out quite a bit. Link to comment Share on other sites More sharing options...
Gerhard Posted April 9, 2015 Author Share Posted April 9, 2015 Maybe a stupid question, but why do I have double traces on the stars in the M51 pic (the 5 minute exposure)?is this still a tracking / alignment problem, or is this something else? Link to comment Share on other sites More sharing options...
jsmoraes Posted April 10, 2015 Share Posted April 10, 2015 At least my coma corrector from GSO has a little behavior of barlow: 1.14x . Therefore it is welcome for DSLRs.If I have a bad alignment primary/secondary mirrors, the coma is strongly present ! More than normal. Check it. Link to comment Share on other sites More sharing options...
Thalestris24 Posted April 10, 2015 Share Posted April 10, 2015 HiYou may have collimation out also...Louise Link to comment Share on other sites More sharing options...
Gerhard Posted April 10, 2015 Author Share Posted April 10, 2015 HiYou may have collimation out also...Louise tomorrow evening I'll be observing with other astronomers, from my club. I will definitely break their b.... to help me with all of the above issues, or at least indicate if I'm doing it in the right way...The last pics I posted were collimated quite good, IMHO... using a laser collimator... and taking my time... Link to comment Share on other sites More sharing options...
Gerhard Posted April 12, 2015 Author Share Posted April 12, 2015 The results of yester-eve:M3:M81/M82:M101:M104:Enlarging, the same anomalies result for all 4... roughly triangular stars...I learned a few things yesterday:- do not forget the Bahtinov mask on the tube, after you're done focussing...- do not forget the lid on the tube after doing darks, and turning to another subject for lights....- apparently light coming from the viewer of the DSLR can spoil the image, so it should be closed while photographing....- enabling mirror lockup in the DSLR should also help avoid anomalies, but the remote control did not work well with that option, so I could not test this...I tried my new dynamometer, but it's one of those fisherman's tools, so it goes up to 50 lbs, so is of no use... Balanced by touch / feeling, as usual... But had a bit more care to try and feel if movement was the same in either direction when pushing / pulling...I tried harder to get the collimation right, trying to get it so as to fix the mirrors in place with the knobs when collimated, instead of just collimating and leaving the mirrors "hovering" without "fixing" them in place (afraid to spoil the just attained collimation)Focussing as usual with the Bahtinov, got it about spot on, IMHO... and checked again to see if it had changed, but it didn't (the Vixen holds well, even with the DSLR on).So, in the end: the anomalies seem caused by coma & tracking (all subs are 33 secs, with darks and bias)...The guy who sold me the scope (president of our club) told me to hold off buying a coma corrector, though, because when I will start guiding (which I will, sooner or later) matters will be much better, and usually one can eliminate coma anomalies by cropping on the subject in the centre... Also, it may happen that later on, for photography, I will be getting an apo, so a coma corrector would have been wasted money....And, as said before, I am only just starting, so should continue with the learning curve, and keep practicing all the different setup aspects to perfection...PS: caught my first glimpse of Saturn in the scope yesterday!! Awesome!!!! The rings were bright as hell! Link to comment Share on other sites More sharing options...
Dave In Vermont Posted April 12, 2015 Share Posted April 12, 2015 Whatever the initial cause, this latest proves it wasn't coma-related. Most likely tracking, as was pointed out previously. I'm thinking this may be caused previously by your telescope suffering from hypochondria. This rather disturbing malady results from reading about the issues a scope's optics have been known to be vulnerable to. It is rarely fatal. Even untreated it usually fixes itself. But these latest images show the telescope seems to be in near full remission. I suggest not allowing the telescope near any websites or books for a few days.On that note:http://en.wikipedia.org/wiki/HypochondriasisClear & Dark Skies,Dave (going to hide now...) Link to comment Share on other sites More sharing options...
Thalestris24 Posted April 12, 2015 Share Posted April 12, 2015 Um, I'd say there's definitely coma there - you can see it in the corners. Plus egginess which can be potentially caused by several problems. With good PA you should be able to get at least 60s subs with round stars in the centre of the fov. Leaving the mirror adjustment screws untightened is a recipe for collimation errors - as I found out myself recently!Louise Link to comment Share on other sites More sharing options...
Gerhard Posted April 13, 2015 Author Share Posted April 13, 2015 I'm thinking this may be caused previously by your telescope suffering from hypochondria. This rather disturbing malady results from reading about the issues a scope's optics have been known to be vulnerable to. It is rarely fatal. Even untreated it usually fixes itself. But these latest images show the telescope seems to be in near full remission. I suggest not allowing the telescope near any websites or books for a few days.IOW: keep calm & keep on tracking! :-DI had another try at M3, using some "archive" flats:Anyway, have to keep practicing all techniques to perfection. But I am happy with the progress I seem to be making.The pres of my club told me he already has a guidescope to sell me... :-DBut to make that step, I need lots more stuff: ccd for the guide, laptop, guiding software, and connection to my DSLR... Plus annex folding table, and sundry... So, will have to wait a bit.... I don't want to get ahead of myself... Link to comment Share on other sites More sharing options...
Gerhard Posted April 13, 2015 Author Share Posted April 13, 2015 ah, and same prez also suggested he might be kind and construct a flatbox for my tube! :-) I HAVE to keep him as a friend! pity I'm not much of a DIY guy myself... I also wouldn't have a place to set up shop.... Link to comment Share on other sites More sharing options...
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