This is my attempt at good old M42. I've met some challenges with my Astro modded 350d in combination with CLS filter but I'm pretty happy with this result and it's by far the best M42 in my collection.
42 x 90sec @ ISO 800
38 x 60sec @ ISO 800
62 x 30sec @ ISO 800
No Darks, Bias or Flats ( Causing me processing difficulties ) ( I think )
Canon 350d modified
Astronomik CLS filter
Celestron C5
HEQ5 pro unguided
Constructive comments here we go. When checking tracking accuracy use
the fainter stars, as brighter ones can blot out tracking error and remain circular when fainter ones are clearly elongated. Having said that, your stars are elongated about 11.30pm to 5.30am; I'm guessing that your subs are nearly twice as long as your focal length will allow whilst retaining round stars when unguided, assuming same PA accurac as attained with this image.
Much more importantly, When magnified, the stars at left are clearly out of focus, showing the black shadow of the secondary at their centres, but the
stars at right don't show the black shadows suggesting some camera tilt.
You'll never get images that will satisfy you until your stars are round.
The colour is very muted for my taste, but I'm guessing the CLS filter is at
least partially responsible for that. I gave up using mine, didn't like the
colour cast it introduced. Hope this helped.
raymo
Thanks all for your input. It seems the main issues are with colour and star elongation which doesn't surprise me in the least. I've made quite a few changes to my imaging setup recently and as we all know, that presents many challenges.
We"ll start with the colour. With this image I concentrated mainly on the extra detail in the image ie. The dust region. I quite like the depth that it's added to my image. In the perfect world, I would have put the AM camera with the CLS filter and the entire universe would be blasted perfectly across my sensor but that was never going to be a reality. So ( and I believe I said this in my first ever image ) it's baby steps. My subs have more blue in them than in the final image and, knowing that the CLS gives the entire image a blue hue, I think that when the stacking is done, the blue is reduced. I'm going to try a restack without aligning the channels and see what happens.
Next comes the elongation. This is an easy one. With all the recent changes, I hadn't rebalanced my mount, it was quite out. On the west of the meridian it didn't cause any issues but on the east side it exaggerated the periodic error. I'd never seen this mount do that but all of my other images were taken on the west side. The reason why the stars are out on one side and not the other is because of an uneven crop. I'm hoping all of this bit will be taken care of in my next image.
When I raced (50 years ago☺) starts were critical...simple but easy to get wrong. They were "bike in nuetral" and hand in the air or on helmet.
So what could go wrong...three areas..you could take you eye off the starter for a split second in the excitement, you could fumble because you did not keep your hand wide open as you grabbed the clutch or you would let the engine revs creep to maximum making pushing the bike into gear hard.
So I would get to the line and say over and over to the exclusuon of everything, hand open, watch the tip of the flag, blip the motor...a three part mental list of the raw basics others forgot when under pressure...I always went thru that first corner no less than 4th (finished further back most times however☺)...my point we forget the simple things that one must do or at least check each time every time...recetly not flatning the grass under the paver one leg sat on brought me undone later as the mount moved slightly taking away the perfect PA I started out the night with...I have a mental list (which I am writing down) that I will look at in the future when I sit back after set up for a rest...its all about the little things☺ So easy to overlook balance for example....
Alex
To me a great image but can I ask a question of you (professional) imagers please
How does RJ or anyone work out how many images to take at what exposure
Thank you Jeremy,
It's far from perfect hence the critiques but it's a step up in my journey.
The choice of number of subs in my experience is really dependant on a lot of factors not least of which is the object itself. The main reason for multiple subs is to increase the signal to noise ratio in the image. Digital photos suffer from noise with the length of subs, temperate of the sensor and ISO. If you have a feint object, to capture enough signal ( photons ) you need to either take longer subs or increase the ISO. This makes the image noisier so to compensate for this you take more subs to increase the signal verses the noise. The more subs you take, the better this ratio but it doesn't equate linearly. If 20 subs is twice as good as 10 subs then another 10 doesn't make the ratio three times as good , it would take 40 subs . As you can see, it gets to the point where 100 more subs makes only a small difference ( all things being equal ) but it is up to you where you stop. Obviously 100 subs at even 90 seconds takes a long time to capture and if you've already got 300, you've got to weigh up weather it's worth it.
As far as the varying lengths on his particular target, that is in an attempt to not blow out the core. M42 has a high dynamic range ( really bright bits and really feint bits ). If you take all long subs, you'd capture lots of feint areas but the core just get washed out to pure white ( highlight clipped ). Taking the shorter subs and combining them balances this out.
+1 with Ryan. It's guesswork for me Jeremy. Lesser time on brighter targets. Something dimmer I just keep going until I can see the fine detail I want without too much noise. So I try and get a few hours on target per night when it is in an optimal position. And do 1 - 3 nights. I try for 4-6hours on these. Now I'm using SGPro it's kind of automated so I can let the scope run all night and get a few hours on 2-3targets
That's my aim. But given the weather lately I've been using a fast rokinon lens where I need a lot less.
With older DSLRs and unguided AP the options are limited but dithering will make a world of difference. Calibration frames help as well.
Bore da... UKAstronomer
The length of unguided exposures is limited to a large extent by the equipment and polar alignment. ISO and aperture are probably the most useful variables in this case. Importantly, exposing to get signal over read noise - dark noise is not so limiting with shorter exposures, particularly if the shots are dithered generously. You can almost get away with bias and flats, which makes life a whole lot easier.
How does one know what ISO setting, length of exposure and how many frames to take using a DSLR
Simple answer - time and experience
You need to make the most of your time outside when skies are clear and experiment with your camera on the same and different objects
After a while you will get to know your set up and camera but it takes time and plenty of it
I’ve watched a lot of YouTube videos, read books , asked a lot of questions on various forums , spoken to astronomers but the simple answer is time and experience, no way around it
YouTube clips, books , forums and talking to people give you a starting point but it’s up to you after that I’m afraid , plenty of time outside will answer your questions
This is my 3rd year at astronomy both visual and AP at nearly 60 years of age and I’m only on the first rung of a big ladder . That’s the exciting fun part of this hobby - heaps of learning , setting goals, making progress and achieving a result whether it be good or bad
Clear Skies
You can use rules of thumb or refer to the various tables advice etc., published on-line by some very accomplished APs.
From experience - unguided - Canon 1000D / 450D - ISO800 3 minutes is a good place to start or ~2 mins at ISO1600, then adjust for your sky conditions. It wont be the same for everyone - just a guide.
But please do DITHER with older DSLRs. DITHERING will cover so many unavoidable flaws. It's very easy to do even manually with a hand controller. APT and BEOS have the capability built in.
Did I mention DITHERING??? Dithering... dithering... di..th..er..in..g...
My rules of thumbs....
Take as many as possible as you wont use them all ... so if you have 200 select your magic number and select the best however many...
If you need say only 50 and take only 50 what do you do when you discover trails from planes or sats...
If you have time on you hands try ISo and exposure times outside what you planned...
And this is a game where I think even mistakes are beneficial as you do learn from them.
Good luck ...just keep shooting until the Sun comes up☺.
Alex
