Hi all,
Im about to try guiding while I've got these clear nights coming up but before my wife puts a me on the missing persons list just a question.
At the moment im getting pretty good subs at 1 minute, and even 2 minutes but i tried some 3 minute last night and got some rotation.
My question, is 10x3 minute the same data as 30x1 minutes?
Im pretty sure that the 3 minute subs will be better but with my 450d am I pushing the sensor to its limits anyway so even with the guiding im not going to get better images in the long run.
I might get longer subs but the signal to noise will be less because of no cooling etc.
I'm not getting slack but at the moment im not using my laptop for imaging only processing and guiding means a lot deeper learning curve and more to go wrong.
Cheers
Andy
My question, is 10x3 minute the same data as 30x1 minutes?
A question that gets asked quite often. Unfortunately, the answer is a little complicated. If you capture enough data in every pixel so that the shot noise from the signal overwhelms the read noise of the camera in a 1 minute sub then 30x1 mins is going to give you a result that's as good as 10x3 mins.
Whether a 1 minute sub is sky limited will depend on your aperture, focal length, the amount of central obstruction (if there is a secondary mirror), the amount of vignetting, the sky brightness, and the QE and read noise of the camera. It's possible to do some measurements and figure out how long your subs need to be but you need some basic maths chops and you need to do some work to measure your camera parameters.
Andy,
I’m certainly no expert but I’ve read Allan Halls book on Long Exposure Astrophotography and he talks about “read noise” every time you open the shutter and “shot noise” which relates to exposure time
He recommends longer exposures or a mixture of both to large numbers of short exposures. The increased read noise cancels out any advantage you may get from shooting shorter exposures
I guess you just have to experiment yourself as your capture settings vary from object to object and seeing conditions change from night to night
I guess there’s no real fixed set of rules, just recommendations and trial and error
Cheers
I used to image with a 450D, a lot of imaging actually. I even installed a cold finger behind the sensor, so I know that camera pretty well. In its favour, the 450D is not a hot camera, as it doesn't have multiple processors and lots of sub routines generating heat. I would stick with short subs for the time being. It's summer, and hot nights are pretty common, you can't escape heat build up. If you were out at a dark site in the middle of winter, then much longer subs would be fine.
The comments on read noise, etc are relevant, so find the sweet spot for your camera; but if you were to switch over to an ultra low noise CMOS camera then the equivalency between many short subs and one long one becomes pretty true, especially if your using a cooled camera.
I think its is good to start with no guiding to get to understand your mount and get it working as best it can...and you seem to be there.
As to exposure length you may not realise even though I say over and over literally all my photos have been no longer than 2 minutes and that was for only one photo ...the prawn nebula from recollection...but everything else has been 30 seconds with a few at 35 seconds.
I have had to do this because of guiding problems which I still have not fixed...maybe next time...however it surprises me how well short exposure works.
I take heaps ...the recent Orion comes from a bank of about five hours but I stack now only the very very best...so out of five I may het three...
At first I used short exposure because I would not get good polar alignment but now I sortta am getting a preference for short ecposures...I just think shorter mrans a better chance of it being real sharp.
Anyways until I get auto guiding working perfectly I have to say short is great ☺.
Alex
Thanks Rick, Martin, and Glen.
Thanks for for your advice.
Im pretty lucky here as its a dark site and normally has cool nights so camera noise will be down to start with.
Well ive just installed PHD2 and now getting my firewire card working so i can connect (for the first time) my Imaging Source DBK 21AF618 camera to use as a guide camera.
Ive never tested this camera yet so more learning curve.
Rick, I had a quick look at the link but i need to get my head around it when ive got some more time so back to testing the camera.
I installed eqmod and stellarium the other day and tested ok with the mount in the lounge so hopefully it all comes together.
Thanks for your advice.
Cheers
Andy
Thanks too Alex (just read your reply) been so busy lately with work i keep forgetting to reply to peoples input and even look at other members images.
I agree about short exposures without guiding and im pushing it a bit with 2 mins.
Can't wait to get everthig working and give guiding a go.
Cheers
Andy
I've tried 10,15 and 20 min exposures, all guided. 20 min with a dslr has quite a lot of noise in the shot which cut the detail down. I've found for me is between 5 and 10 min depending on what I'm shooting.
I use a 700D which can run rather hot depending on ambient temps.
A solution to guiding problems can lie in perfect alignment of your mount, which also requires good balance, precise tolerances in the worms/axis, excellent base stability (pier, tripod, whatever).
Drift alignment tuning after initial polar alignment setup goes along way towards carefree subs.
I believe that PHD2 has become a crutch, much like folks not being able to drive anywhere without GPS in their cars.
If you have a polemaster would you bother with drift aligning?
Many do not have Polemaster, and some see it as an un-neccesary expense. Sharpcap PA is probably more useful for many folks.
That said, old fashion drift alignment maybe a dying art, and retaining the skill is useful. Polar alignment might be spot on at the time done, but particularly if your gear is on a tripod, a bit of use can see the alignment shift as weight transfer on the legs causes compression settling. Drift can be useful for refinement.
I would see a Polemaster as a very useful aid to quick setup, I will probably buy one down the track a bit, in the meantime I am drift aligning via PHD2.
A question for Andy back on an earlier post, on the long subs are you getting field rotation or more random movement of the object? Guiding will help with the random movements related to wind or mechanical issues in the mount, but if it is field rotation then that relates back to the polar alignment. You would end up with a nice crisp guide star and rotation impacting everything else in the image if you try to fix rotation with guiding.
I've used sharpcap PA and it's pretty spot on takes about 10-15 mins depending how accurate I want it, but no good if you use OAG.
For the moment I have to dismantle my set up when I'm done as I have fairly harsh conditions here.
With poor PA, guiding will help keep everything sharp but have slightly more adjustments. Great PA overall will have everything sweet.
I've used sharpcap PA and it's pretty spot on takes about 10-15 mins depending how accurate I want it, but no good if you use OAG.
For the moment I have to dismantle my set up when I'm done as I have fairly harsh conditions here.
With poor PA, guiding will help keep everything sharp but have slightly more adjustments. Great PA overall will have everything sweet.
You don't need a guidescope to use Sharpcap PA, in fact you shouldn't because rarely is a guidescope pointed in the exact place as the imaging scope. You use your imaging camera through Sharpcap PA, simple. Use OAG for guiding if you wish, but not required for PA. Now this works fine for all the modern cameras supported by Sharpcap, not sure about dinosaur DSLRs.
With poor PA, guiding will help keep everything sharp but have slightly more adjustments. Great PA overall will have everything sweet.
I have to disagree to a point, depending on the focal length you are imaging at and how big the PA error is, poor PA will result in elongated stars and loss of detail. It does not really show much but my M42 image was a case in point, at two minutes there is some field rotation showing in the subs and the further from the guide star, the more visible as effectively the guide star becomes the point about which the image is moving. I did not have time to refine the PA with drift alignment and I knew it but I was only trying to test the mount and see if guiding was happier than before so I grabbed some subs. If I had time to grab some four minute subs it would have been more obvious.
Every setup issue is going to cost some detail or result in misshapen stars, the difference between the PA being off the mark and something like wind pushing the rig around is that guiding can't fully compensate for PA errors, if it is off far enough that the target drifts across the frame sub to sub it will keep it pretty well centered, but at the price of revealing the field rotation.
You don't need a guidescope to use Sharpcap PA, in fact you shouldn't because rarely is a guidescope pointed in the exact place as the imaging scope. You use your imaging camera through Sharpcap PA, simple. Use OAG for guiding if you wish, but not required for PA. Now this works fine for all the modern cameras supported by Sharpcap, not sure about dinosaur DSLRs.
I use a dinosaur dslr for imaging and so far sharpcap doesn't support them.
Good old days driving with half a kilo of UBD on the steering wheel.
I preferred Gregory's.
Go on give guiding a go. I went nearly two years without it, but I was live stacking/imaging with max 30sec subs. Guiding was surprisingly not to hard to get started - harder to make it work really well. But it taught me a lot.
Thanks karl, Paul, Glen, Marc and Chris.
Been flat out at work so I still havent had a go guiding yet.
I have been having a go with drift alignment but the last time i was imaging i just did a visual with my spotter scope and then a quick look with my guide scope and it looked pretty close.
I took a pic of my pole alignment and im out a bit but still pretty close.
I reversed the image and put a red dot where th SCP is but it should be in the centre of the frame.
Also took a 3 minute sub of the Tarantula just for a test and theres a bit of rotation.
Thanks for all the input and next time I'll try the Darv thing.
Cheers
Andy
