Trouble with Off Axis Guiding? Let's fix that! ZWO ASI2600MC Duo too :)
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- Опубліковано 12 вер 2023
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In my last video about off axis guiding and off axis guider setup, many pointed out common issues with such a setup, I address them in this video :)
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Great discussion Cuiv! Two points:
(1) Another solution for the issue of guiding when re-focusing after changing filters: Use "filter offsets" and also temperature-compensating focus changes. Once these two automated processes are dialed-in, one only has to focus using a luminance filter, then guiding can continue uninterrupted until a new target is chosen. The good news is that NINA & some other imaging software (and software from some focuser manufacturers) can handle "filter offsets" and - with a temperature sensor added to the system - temperature-compensating focus changes.
(2) The issue of finding a guide star with longer focal length systems (especially in fields with few brighter stars) brings us full-circle in our search for equipment for astrophotography. It used to be that a very finely-machined, ultra-precise & often heavy mount with minimal Backlash & Period Error was a necessity. Now, 'harmonic mounts' are becoming all the rage, because of their amazing portability, and (if carefully-designed) virtually zero backlash. The problem, however, is that 'harmonic mounts' inherently have a large 'periodic error', plus the rate at which the mount 'loses stars' can be very rapid/sudden. This means that most 'harmonic mounts' require very frequent guiding corrections, therefore very short guide exposures (e.g. 0.5 second for the AM5), which, in-turn, exacerbates the challenge of finding suitably bright stars for guiding with the off-axis guider (especially with longer focal length systems)! Yikes! lol
Why is this a problem? Well, the only way to mitigate this issue is to use longer guide camera exposures, and/or rotate the guiding camera...but in the former, that means the mount needs to have smaller periodic error & no sudden changes in tracking, which 'harmonic mounts' cannot deliver. In the latter, it often means purchasing a rotator, so that the process can be automated & be highly reproducible (for imaging on successive nights, and also for reliable flat frames) So, if one wishes to keep the system highly portable, there are several solutions, including (a) purchasing a rotator for the system, (b) keeping the same longer focal length, but use a much 'faster' system (therefore more aperture), (c) employing imaging & guide cameras with much higher sensitivity, (d) using a harmonic mount that has encoders on the RA axis, or (e) giving up & imaging at a shorter focal length, With every solution except (e), it will require purchasing new, more expensive gear....which is why, IMHO, astrophotography represents 'a black hole for wallets & bank accounts'! lol
Yep, aware of filter offsets (was actively involved in NINA development at the time and added the default AF filter that relies on that :) ), it worked great when doing filter looping, like L->R->G->B->L... in sequence
And agreed on harmonic drive mounts, although my AM5 seems to perform very well with 1s exposures, although a bit better with 0.5s exposures :)
Perfect summary of the deepening black hole of astrophoto lol
Thanks, Cuiv. This helps. You are correct about the long focal lengths scopes. Mine is 2800mm and there are times when there are no stars in the view.
Yeah 2800mm is an extremely long FL, kudos for imaging on such a long FL!
Thanks for in depth discussion on oag. Plenty of valuable points were taken into account. However, for this telescope operating @520 mm with comma comma corrector (simultaneously working as a reducer also) absolutely no need of oag. Using the same setup, i have taken 5min exposure shots without any problem. The employed mount was am3, which does not offer heavy pay load capacity load capacity than am5, utilized by you.
Definitely needed an OAG because of the terrible default focuser on that scope causing differential flexure :)
@@CuivTheLazyGeek sorry, I haven’t provided complete details of the setup. I use asi120 mini scope for guiding. It will be exciting if you give a detailed comparison of the oag with traditional guiding scope mechanism, so that we will be able to know, how much significant impact oag has over the traditional. And it will also generate more sense and validity of using oag on small focal length telescopes. Thank you
Lots of great information Cuiv! Once again I wish I knew some of this stuff when I was shooting long focal lengths! OAG is really the only way to go in that case! Thanks Cuiv and clear skies!
Absolutely agreed Dave! It will be interesting to also see the Duo type cameras releases
I also have the Celestron OAG with the ASI174MM mini on my 8" EdgeHD, and you're correct. It's like a match made in heaven! I always have stars with or without the 0.7x reducer.
Yeah it's such a beautiful combo!
Thank you very much. A lot of valuable information for me as I'm currently setting up Celestron OAG on an edgehd this week. Clear skies 👍
You'll love that OAG, it's amazing!
Just got an oag to try a couple weeks ago and I'm still putting things together. Now you bring me another bucket of issues to work through on the ever steeper learning curve... Climbing... Hanging by my fingernails, looking and reaching for the rope that's dangling just out of reach! (don't look down...) I... Will... Get... There...... 🥵
Your fingernails are strong, you'll get there!
Don't forget to do a calibrate in PHD2 when changing the angle of the guide camera so that PHD2 can correlate what pulse moves in what direction. ;)
Wonderful information. 😎 Thank you.
Glad it was helpful!
Excellent in-depth presentation of OAG issues. Regarding prism size and the ability to get more stars, yes, the Celestron OAG has a 12x12mm prism. However, weight and backfocus requirements are really too constraining to be a good option for many use cases. Also, it requires a larger camera and altogether the price is eyewatering: about $750 for celestron+ASI174MM! Not to mention that a 12mm high prism can be difficult to manage. A great alternative is the TOAG: it has a small prism but i) has 2 screws to move the prism and camera laterally, and ii) the adjustment of prism depth can easily be done by moving the whole camera port+prism tube (in addition to moving the prism tube relatively to the camera port). Basically, with a small prism and a small sensor, the TOAG can reach more stars than an ASI174MM on a 12x12mm prism. The TOAG has limited options for connections on the camera side, it won't work well with 2" filter drawers, but it is very thin and very light. An excellent option (only option if 55mm backfocus limit) for Canon EOS EF users with dedicated drop-in filters. Also very cheap: TOAG+ASI120MM mini is about $250.
Oh that TOAG looks great! lateral camera movement sounds awesome, but does it mean the slit is long and thus letting light in? Excellent option indeed!
@@CuivTheLazyGeek the slit is just over 4.6mmx7.8mm, good for guide cameras up to the ASI220MM. The camera doesn't move relatively to the slit. The whole bloc, from the prism to the camera, including the slit, moves laterally or vertically together.
You confirmed my intuition about choosing between ZWO & Celestron OAG. My current plan is to buy an edgehd. Celestron OAG seems to be the best choice. Thank you.
For the EdgeHD, which has a lot of backfocus, the Celestron OAG is a no brainer n
Thanks Cuiv!
My pleasure ;)
Hi Cuiv, thanks for the extra info. PHD2 is very forgiving of poor focus, last winter my 50mm guidscope drifted out of focus as the temperature droped and it kept working in fact it even worked with a light coating of dew on the lens. I have just bought an OAG but needless to say there are no clear nights forcast anytime soon so can't try at the moment.
Your OAG came with clouds, as it is wont to do, I hope you get clear skies soon!
Perfectly focused stars especially when undersampled will take up only one pixel (mostly on guide scopes) and your guiding software might confuse it for a hot pixel. Also, if you have a good quality telescope and a solid mounting system for your guide scope, don't bother with an of axis guider. People make it sound like it is the only way to image. On my f2.8 newtonian gave up on the OAG, kept having shadow of the prism on my zwo asi533. Works perfectly fine with guide scope
Also, your OAG should be in front of filter wheel
Hi Cuiv, how do you think it does perform on an RC, since i'll soon get a 6" rc i am planning to get one as you do but im worried that i won't be able to track well enough because of the long focal lenght
Another issue with OAG during Galaxy season is after a meridian flip on an EQ amount, you have a different star field due to the flip, there may or not be a star available to guide on after the flip. This can be a pain when doing automated imaging without a motorized rotator.
True, especially with large focal lengths.
Thx Cuiv for nice explanation, but i'm ditching OAG. Can't get it to work (prism good, dist good and no go).
That's a shame, although I know how hard it can be, hope a standard guide scope will work well for you!
Hey Cuiv!
I got a question:
Since I am using the OAG, my star shape issues disappeared, so all is good in this regard.
However, since I now have to use a filter drawer (so I can use it with DNB filter), I have literally 0mm space for a rotator. (16.5mm OAG + 21mm filter drawer + 17.5mm from the camera = 55mm exactly).
My only way of rotating is by rotating the coma corrector inside the focuser, which is very suboptimal, since this will cause the collimation to vanish. I tested that with the OCAL, no matter how careful I turn and lock it again, probably simply due to the way the CC is built, I lose collimation.
Also, even though it is not a big issue, I couldn't turn the camera in a way so that the prism of the OAG is parallel like it is for you, so I have quite some big vignetting in the guidecam.
How did you solve the rotating issue?
Interested on the collimation. Maybe I'm not demanding enough of my equipment, but the CC rotation doesn't seem to affect collimation. Maybe some astigmatism in your CC?
@@CuivTheLazyGeek Hm that's a good question. I don't think so as my cc has very good quality and I don't see any optical issue with it.
It is probably just the way it lies inside the focuser.
I read in the OCAL instruction that this is normal, simply because nothing is built 100% perfectly, so that you should collimate on a specific rotation and don't change it if possible.
I guess if I could just turn my camera it wouldn't be an issue, though
Bonjour. I was hoping to see in this video a chapter on the ASI2600MC Duo (as it shows on the title), but no luck. Dommage car tes vidéos sont très utiles...
I was wondering if all the weight on the focuser can cause flexure problems in the image train. We have the same "no good" focuser on the Quattro 150P and I'm afraid this can lead to poor FWHM and Eccentricity... could you evaluate this? and, as always, keep up the good work!!!
I got the OAG to deal with the flexure! And now it's gone :)
Well, the flexure is still there, but the OAG corrects for it
Cuiv, do you still use your hyperstar setup? You were so happy with it a few years ago, but you seem to be back to your old rig in all your newer videos. Has the magic faded?
That rig in the latest videos is my new rig, not an old rig at all :) everything in it is pretty much new (newt, camera, mount, even guide cam and EAF). So it's been stealing the spotlight for sure.
Could you tell me please if ZWO 2600 DUO work well with Newtonian Quarto 6"?
I don't see why it wouldn't - but I haven't tested since I don't own a Duo!
@@CuivTheLazyGeek Thank you
Will this work on skywatcher 250 Quattro?
No reason why it wouldn't, you just need to make sure to have a large prism and sensor to always have stars available in in the OAG FOV!
if you don't focus also the guide camera after a filter change and autofocus has happened, you will run into a risk that the guide stars that are now defocused, will not be detected any longer by PHD and it will stop guiding on "star lost". It happened to me many times.
Yes, I should have mentioned in the video that I assume the filters are reasonably parfocal - if you're using 3mm thick filters and 1mm thick filters, you will have issues. It's weird though, for me PHD2 keeps trying and trying and trying until it finds the star again, never stopping. And even if it then locks on the wrong star and pulls the target off center, NINA will catch that and recenter
PHD2 does indeed recognize misshapen stars, but guiding quality drops.
What about ASAir? Does it recognize misshapen stars during guiding?
What happened with your SCT Hyperstar system and IOptron mount? Sold?
ASIAIR apparently uses the PHD2 code under the hood, so it should be the same! I still have the Hyperstar, but I'm getting such good results with the Newt it hasn't seen much use recently...
@@CuivTheLazyGeek Great to hear about Newtonian scope. I thought a lot to get Hyperstar or Takahashi Epsilon. Got Epsilon, it catapulted sensitive to “new level”. It doesn’t much resolution of Edge 9.25, but perfect for many large and dim targets.
💯💯💯
My experience is that the best way to deal with « comet shape stars » is to fix your backfocus distance…. But sometimes it’s not perfect… depending on quality of the corrector…
Christophe
True - if the issue is backfocus, then this helps, but it would also then be affecting your main imaging camera (assuming the sensor isn't too small), so it should be fairly easy to notice!
すごい！ありがとうございます。
Is the ASI2600MC Duo even mentioned in this?
Us astrophotographers are suckers for punishment, thanks for sharing.
Hey what's a hobby without a bit of masochism? A pleasurable hobby? Not for me thank you lol
Thanks Cuiv, for the information. I plan to move to an OAG in the future. Meridian flip can also be a problem with OAGs. James Lamb has a video Using Stellarium "Markers" to Setup an Off-Axis Guider to help plan an imaging session. Link: ua-cam.com/video/L7xY-efQ_Tg/v-deo.html
James always hits the mark, thanks for the link! What's the issue with the Meridian flip and OAGs??
He uses Stellarium Markers to make sure there are guide stars available in the OAG before and after the flip. The planning can be done during the day and there is a little math involved. @@CuivTheLazyGeek.
@@bobbryant4410 I see - losing the guide star after the flip is the issue
Dude, do you really think that there are people who does not know what is OAG are watching your videos? ... and get some helical focuser ... by the way I am using OAG on my 2700mm SkyMax with 2600 camera - works perfect in 90% of the cases when I have one-two stars :)
Yep there are quite a few actually!
Dude? 😂 Of course there are beginners out there (like me) who appreciates Cuiv explaining things like these.
@@TheTabugman If you are a beginner do not mess with OAG, until you have at least 3-4 years experience with beginner friendly setups because the OAG is probably the biggest pain in the ass in this hobby.. And maan - OAG without a helical focuser - please - this is a pure masochism :)))
@@lachezarkrastev7123 I dunno... I've gotten to grips with everything pretty quickly so far, albeit with a lot of research. And yes, a helical focuser seems like a life saver.
ZWO ASI2600MC Duo? Did I miss something in this video? What was that in the title …. click bait?
The Duo is also discussed along with solutions
I used OAG years ago, and I wasnt fond of them. Ive been using guide scopes for years now. However, the products I wa using are nothing like they have now. I may look into for longer focal length imaging. It would makes things much lighter also.
Agree - usually there wasn't a reason for them except for long focal lengths, but it's a bit easier these days
Hi Cuiv! I’ve been told that you are French… maybe we could work together sometimes? How can I contact you?
Christophe
He's in Tokyo
@@samwarfelphotos i know!
Salut Christophe - tu peux m'envoyer un mail a cuivlazygeek at gmail!
@@CuivTheLazyGeek capture écran faite. Je t’écris demain!