ribuck
sage
Reged: 11/01/05
Posts: 329
Loc: Newcastle, England
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Hi all,
I'm looking to buy a skyscan EQ6 Pro which I believe will track very well for about 2 mins, before needing to be autoguided.
I was wondering if it's possible to get some decent images from 100-120 second exposure lenghts, as i cant afford to buy all the extra kit required for guiding right now.
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Dragon Man
Carpal Tunnel
Reged: 03/07/06
Posts: 1534
Loc: Snake Valley, Australia
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I have an EQ6 with Skyscan and I find that depending on varying circumstances, guiding is essential. I have imaged for 5 seconds and had drift, whereas I have imaged for 5 minutes and had perfectly round stars! A slight breeze, periodic error, backlash etc etc all effect an image of any length. But guiding is not expensive if you choose to hand-guide. I have just posted my hand-guided images using a Toucam in the thread 'Modified Toucam - Images' http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/1500215/page/0/view/collapsed/sb/5/o/all/fpart/1
My first attempts at hand-guiding were a bit messy but I have now got the hang of it.
My hand guiding set-up consists of a Toucam (unmodified) and a 100mm refractor. On my laptop, I place the curser on a star and use the Skyscan hand controller to keep it in place. Simple as that. The pics I posted consist of stacked 3 & 5 minute images.
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Ken James - Snake Valley, Australia - 12" Dob | ED80 | 120mm Achro | 6" Achro | Modded Toucam Pro II | Canon 350d | Samsung SCC-A2333 Video Camera | Mallincam Xtreme video Camera | My CN Gallery | My site: Ken James Astrophotography |
President-SVAA
Broadcaster on NSN as 'SNAKE VALLEY AUSTRALIA'
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Dragon Man
Carpal Tunnel
Reged: 03/07/06
Posts: 1534
Loc: Snake Valley, Australia
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This is how simple hand-guiding can be.
The pic shows an ED80 as the main imaging scope, using a Modified Toucam for long exposures.
The guidescope is the 100mm cheapy home made refractor on the left which has an ordinary Toucam in it. This toucam image is displayed on the lappy where I use the Skyscan hand controller to keep a star still.
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Ken James - Snake Valley, Australia - 12" Dob | ED80 | 120mm Achro | 6" Achro | Modded Toucam Pro II | Canon 350d | Samsung SCC-A2333 Video Camera | Mallincam Xtreme video Camera | My CN Gallery | My site: Ken James Astrophotography |
President-SVAA
Broadcaster on NSN as 'SNAKE VALLEY AUSTRALIA'
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
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LOL!!! Looks like a great imaging setup though!
Here's my simple version. No cables, no laptop, about as basic as you can get!
And here is a LINK to my webpage describing Guidescpes and Manual Guiding.
Clear Skies to you! (I have none!)
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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Dragon Man
Carpal Tunnel
Reged: 03/07/06
Posts: 1534
Loc: Snake Valley, Australia
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Yep, it's a complicated unit 
I only recieved the guidescope that day at Astro camp and wacked it on to use it. Fortunately the next day I took off the silver 'visual' scope and mounted the guidescope properly on top of the ED80. Much neater, more practical, and better balanced
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Ken James - Snake Valley, Australia - 12" Dob | ED80 | 120mm Achro | 6" Achro | Modded Toucam Pro II | Canon 350d | Samsung SCC-A2333 Video Camera | Mallincam Xtreme video Camera | My CN Gallery | My site: Ken James Astrophotography |
President-SVAA
Broadcaster on NSN as 'SNAKE VALLEY AUSTRALIA'
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jgraham
Postmaster
Reged: 12/02/04
Posts: 11575
Loc: Dayton, Ohio
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I do all of my imaging unguided using exposure up to about 30 seconds and I'm very pleased with the results. I've done some work using exposures as long as 60 seconds, but to be honest I don't gain much as I beging to run into some serious sky glow problems. In my case I've found I'm better off averaging more 30 second exposures to quite the noise down rather than using fewer 60 sec exposures to get the signal up. Sooo, I'd consider trying short unguided exposures first, then adding guiding later if you find you need it.
-John
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-John
The best advice on imaging I've ever been given... don't forget to look! 
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
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John, that works to a point. In some cases you will need more data, and averaging doesn't give you more data, just improves the SN ratio.
Can you elaborate about "adding" instead of averaging? What are some of the problems with adding stacks?
I have been fortunate to be at a dark site where LP isn't a problem, but I can see your point about imaging in a polluted sky. This is one of those areas that digital imaging has a huge advantage over film, as with film it would impossible to catch anything at all!
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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jgraham
Postmaster
Reged: 12/02/04
Posts: 11575
Loc: Dayton, Ohio
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The overall outcome of adding and averaging is the same; the noise level tends to decline with the square root of the number of images added or averaged. The difference is computational; computers can only count so high. If you average frames the values converge to an average value, but if you add them you eventually end up with numbers so big your computer can't handle them. As you get close to this point you'll also start running into truncation and round-off error. Soooo, it generally makes more sense to average (or median combine, or some other flavor of averaging).
It's amazing to watch an image build on the screen live. I never keep this image, it's only to keep an eye on how things are going, but it's sooo neat to watch. The first image is full of static, a few stars, the bright core of a galaxy, the rest is all salt & pepper. The second image immediately starts to clear things up, the stars are sharper, more detail in the galaxy. By the time you get to the forth frame the galaxy looks like a galaxy, arms, dust lanes. By the 16th frame things are looking pretty good, the noise is way down and you can start to see fine detail in the arms, the outer arms, and the hint of halo even further out. By the 25th fram you're just about done, you can play with the black & white points and gamma to explore your target, and it's here you often start to see the faint fuzzies hiding in the background. From here on out it's just a matter of how quiet you want the image to get to give you the best chance of pulling out as much as you can above the sky glow. You'll often loose the faintest fuzzies and outermost halo to processing the pretty pictures for posting (that's the price of imaging under light polluted skies), but they're there in the source images I keep in my records.
I must say I get the same sense of wonder watching these images form in real time as I do out at the telescope when I track an object down visually. In fact, in the summer I often do both; the images make great star maps and I know exactly what the target looks like without any processing at all.
Neat stuff.
-John
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-John
The best advice on imaging I've ever been given... don't forget to look!
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Andrew Welsh
Carpal Tunnel
Reged: 03/28/06
Posts: 2571
Loc: Rochester, NY
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The way the math for S/N ratio works out is it's better to do fewer long exposures than many short ones. Due to random photon noise and the efficiency of the sensor, you'll get more of the faint stuff with longer exposures.
This S/N statement is true for "large" numbers of subs, more than 10-20. (i.e. it applies to 20 x 5 min is better than 40 x 2.5 min, but not to 1 x 10 min vs 2 x 5 min)
The downside to long expsosures is you run into several problems:
- more cosmic ray strikes
- increased odds of plane/satellite/ meteor in the frame
- light pollution saturating the image
- increased chance of mechanical issues- tracking, flexure, etc.
- you lose more data and time if something does go wrong, and feel worse throwing out a long sub
It's best to find a happy medium, but if you're pushing for really faint stuff, you're best off doing the longest exposure possible and the most subs possible.
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LX200 8" classic, f/10, Meade eq. wedge, .63x FF/FR
Canon 40D (LifePixel clear glass mod) and 5DMkII, unmodified
Canon EF 200/2L IS, 400/5.6L, 100/2.8 Macro, 50/1.4, 85/1.8, 35L, 24L, 17-40L and Peleng 8mm fisheye
Orion Apex 102mm (4") Mak-Cass
Pimped out with accessories and bling
My DSLR Astrophotography Webpage and photo bucket with full equipment list
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
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So John, using your logic.. from a dark site:
240 exposures at 15 seconds per exposure has the same overall outcome as 60 exposures of 1 minute each?
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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ribuck
sage
Reged: 11/01/05
Posts: 329
Loc: Newcastle, England
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Wow I have opened a can of worms here. I always thought it better to do 10x 2 minute exposures than 40x 30sec , as you pick up more detail in the longer exposure. Am i wrong ?
Anyway the real quesiton which we are straying from, is not which is better but, would i be able to grab some decent photos of the brighter pobjects using 2min exposures. (i.e. 10x 2min)
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Strgazr27
Reged: 10/04/04
Posts: 7104
Loc: StonyHill Observatory
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I'm curious as to why the subject of Focal length hasn't been brought up? An EQ-6 when properly polar aligned and balanced should have no problems going 2 minutes unguided as long as you keep the focal length under about 600mm. An 80mm F/6 should get you at least 5-7 useable subs for every ten you shoot. Granted every one may not be useable but certainly not undoable. The biggest factors to your success are going to be a critical drift alignment of the mount and a careful balance.
The only thing I could think of preventing this is if the 6 is more than a year old. Many of the older EQ-6's had some real ugly PE curves.
CS's
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Bobby
http://www.strgazr27.zenfolio.com
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Qkslvr
Pooh-Bah
Reged: 06/23/06
Posts: 1316
Loc: NE Ohio, US
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You can either add 2 sub's together or average them.
The statistics of the captured light is the same for 2 30 sec subs as it is for 1 60 sec sub. But you'll have read (Bias) error on each sub you take, and while subtracting a Bias frame in your processing will remove most of it, there will probably be some left behind. So 2 30 Sec subs added will probably have a little more read noise than a single 60 sec, But if the noise from light pollution is much larger than read noise, it might be hard to see.
If you average 2 30 sec subs, it will have less than 1/2 the LP noise a single 60 sec exposure will have, But it will also have captured half as many photons (think exposure level).
ClownFish, If you added 4 each of your 15 sec exposures into 60 1 minute subs, then averaged those 60, they'd be about the same. The combined 4@15 would have a little more read error, But the actual amount might still be quite small.
And you might get a higher % of good subframes at 15 sec than you would at 60.
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Mike
Onyx 80ED/N8/CG-5/40D
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Strgazr27
Reged: 10/04/04
Posts: 7104
Loc: StonyHill Observatory
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What happens if the object your imaging is of a magnitude that it doesn't show in 15 or 30 sec subs? I don't care how many subs you stack at that point it's not going to compare to a stack of 5 minute subs.
A lot depends on sky conditions, camera and scope as well as object. You cannot forget about sky conditions either. Take the same scope under mag 6 skies and shoot an object than move it to mag 3.5 skies and those 30 second subs are pretty much gonna tank.
If this was true there would be no need for the mounts and cameras we have today. We could all just use a cheap mount and stack 30 second exposures Lol. My bank account sure would be happier 
CS's
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Bobby
http://www.strgazr27.zenfolio.com
Edited by Strgazr27 (03/23/07 01:28 PM)
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ribuck
sage
Reged: 11/01/05
Posts: 329
Loc: Newcastle, England
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ok, well if anyones up for an experiment, lets get someone to do 10x2 min exposures and 4x5 min exposures of the same subject using the same kit and site and see what we end up with. Anyone game ?
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Chuck Anstey
scholastic sledgehammer
Reged: 01/17/05
Posts: 960
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Quote:
Wow I have opened a can of worms here. I always thought it better to do 10x 2 minute exposures than 40x 30sec , as you pick up more detail in the longer exposure. Am i wrong ?
More information is needed to know if you are correct. You need to know the read noise of the camera and LP noise (or signal with noise = sqrt(signal)) for the exposures. If I was to guess without more information I would go with 10x2min. You are correct that fainter signals need more exposure but longer exposures have more noise. This assumes long exposure DSO targets and not bright solar system targets where target noise is much higher than read noise or LP noise.
The goal of stacking is to drive noise down to an absolute low level but not to the point where there is little gain by reducing the noise any lower. I have found in my research that noise between 6-7.5 ADU RMS for a 16-bit camera is low enough. Theoretically it should be the same for an 8-bit camera also but I don't have one to prove it. If it takes 40 sub-frames to reduce noise to that level from a 30 second shot then that is appropriate. However that would be alot of noise in 30 seconds. What I would normally expect is that noise has been driven too low in 40 frames and those 20 minutes would have been better used as 20 1-minute or 14 1.5 minute shots (yes its 21 minutes).
Darker sites have the advantage of less noise for a given exposure length so it takes fewer sub-frames (less total time) OR longer exposures for the same number of sub-frames (more detail) OR some where in between depending on your goal.
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Qkslvr
Pooh-Bah
Reged: 06/23/06
Posts: 1316
Loc: NE Ohio, US
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Quote:
What happens if the object your imaging is of a magnitude that it doesn't show in 15 or 30 sec subs? I don't care how many subs you stack at that point it's not going to compare to a stack of 5 minute subs.
A lot depends on sky conditions, camera and scope as well as object. You cannot forget about sky conditions either. Take the same scope under mag 6 skies and shoot an object than move it to mag 3.5 skies and those 30 second subs are pretty much gonna tank.
If this was true there would be no need for the mounts and cameras we have today. We could all just use a cheap mount and stack 30 second exposures Lol. My bank account sure would be happier
CS's
If 15 sec doesn't capture but 1 or 2 photons, and your read error is equal or greater than that, you're right.
On the other hand if you capture more photons/sub than read error (5:1, 10:1) You can sum some of your subs.
If each sub captures 50+/-5 photons, and has +/-2 adu's read error, 10 sub's will have 500+/-50 in signal and +/-20 adu read error for a total of 500+/-70 vs a single sub with 500+/-52 A difference of 18 adu of error.
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Mike
Onyx 80ED/N8/CG-5/40D
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Chuck Anstey
scholastic sledgehammer
Reged: 01/17/05
Posts: 960
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Quote:
ok, well if anyones up for an experiment, lets get someone to do 10x2 min exposures and 4x5 min exposures of the same subject using the same kit and site and see what we end up with. Anyone game ?
Been there, done that, and wrote a large research paper on it. For a comparison of different exposure lengths for the same total time see this thread here.
For the research paper in all the gory details see here.
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Strgazr27
Reged: 10/04/04
Posts: 7104
Loc: StonyHill Observatory
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Q,
Lets say 2 hours of exposure time is what we want for an object. Your shooting 15 second subs and I'm shooting 5 minute subs. Same mount and camera under identical skies.
Your stacking 480 subs and I'm stacking 24. Your going to find that with that many chances for variables to pop up, your noise statistics may not follow your rules exactly. On paper what you state is probably very accurate but we don't shoot on paper.
I would much rather stack 24 images than 480.
I may be spoiled as 5 minutes unguided at anything under 800mm F/l is possible with my NJP. I don't do that as I do throw out some subs. I use 10 second guide star exposures and with my skies, right around the 6 minute mark seems to work the best.
Bottom line is if you can shoot 5 minute subs it's easier and based on what I have seen, results in a smoother, deeper image.
JMO's
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Bobby
http://www.strgazr27.zenfolio.com
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imhotep
Vendor - Optical Supports
Reged: 02/14/07
Posts: 1718
Loc: Floriduh
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I've been reading this thread with intense interest. May I ask I side question?
How does one go about using factors like LP to estimate a good exposure lenght for your subs? Does everybody just have to experiment until they figure it out, or is there a more quantitative way to determine how long your exposures should be?
Edited by imhotep (03/23/07 02:36 PM)
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