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Sorry, another question (camera and mount)

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#1 DRK73

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Posted 30 September 2020 - 01:06 AM

One thing I'm still not getting (as I get back into the hobby) is that, when I left, I was expecting to be needing to get 15+ minute exposures with the cameras I was using. I didn't have the SBIG years ago (and am only borrowing it now), I was using an Atik One 6mp camera, but started out with an Atik 383l mono and along the way tried out a few other KAF-8300 based cameras and one or two other CCDs. 

I'm strongly leaning towards a OSC camera for my first camera purchase - or an upgrade to my mount. The mount is, of course, the AVX in my sig and it's been *okay* at short focal lengths with either the SBIG or my Nikon as long as I only image to the east. I cannot reliable get the longer subs I need for the ST-8300 or anything to the west. 

 

So upgrade the mount, right? 

 

But, I'm seeing (and trying to wrap my mind around) people doing the "lots and lots of shorter exposures" thing with the CMOS cams, and I'm wondering if I went that route then I could hold off upgrading the mount for the time being? Collecting a few dozen 3-5 minute subs should be pretty doable I think? Especially because (for the moment) I'm only imaging at 300-420mm focal length? 

 

*note* I'm trying to get a better understanding of how to determine correct exposure length with a CCD/CMOS given optics, sky conditions, sensor, etc. Some of the discussions I've read through here on CN are incredibly helpful, but I'm at information overload right now...



#2 the Elf

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Posted 30 September 2020 - 01:59 AM

Older CCD cameras typically have large pixels with a giant full well depth of 30k to 60k and they have a high read noise of some 5 to 10 electrons. Over all this is a good amount of signal compared to the read noise. It of course requires long exposure time to get enough photons in and it needs a long focal length to resolve fine detail with the large pixels. With a long focal length the scope gets slow and you need an even longer exposure time. That is where the 15 minutes come from.

Modern CCD cameras have a low read noise in the 1 to 2 electrons range, some modern Sony sensors are below 1 for higher gain. The pixels are way smaller and the full well depth is lower, say, 15k e-. The well is filled in a short time and the small pixels allow for a shorter focal length to achieve the same resolution (arcsec/pixel). As the read noise is low the SNR is also fine.

The short exposures help poor mounts but otoh you have to process several 100 frames. That needs a computer with plenty of disk space and lots of cores.

 

Independent of all that for most of us fiddling with gain/ISO settings and exposure time leads to almost invisible improvements while driving to a dark site makes a real difference. The sky background is by far the most problematic thing of all. Buying a car is a better investment in image quality than buying a better camera for most of us.

 

I am an AVX victim as well, sold it and upgraded to the EQ6-R. Taking into account what I payed for an illuminated guide scope and adjustment the price difference is not that big but the step in quality is huge. Get one. You won't regret unless it is to heavy for you to handle.


Edited by the Elf, 30 September 2020 - 02:00 AM.

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#3 sg6

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Posted 30 September 2020 - 03:41 AM

It really is the change of technology. Although there does seem to be a competition for who can get the longest single usable exposure going on. From assorted talks and data presented there is a point that after which the return for a longer exposure lengths has diminishing returns. I need to get that graphical representation.

 

So it will come down to longer is better for an individual exposure but is it worth it over 2 stacked shorter ones? All that will be a mix of camera, scope, operator. Another often ignore factor is a longer exposure has a greater chance of it going wrong (wind shake/vibration)

If you were collecting 2 exposures of 5 minutes and at T = 3 minutes something causes a vibration you are left with 1 5 minute exposure, if you took 5 exposures of 2 minutes and at T = 3 minutes it vibrates you still have 4 stackable exposures

Have been some interesting ideas put forward over time. One I may "try" went against common belief, but astronomers seem somewhat conservative.

 

If not guiding then the shorter approach is I suppose the only one. I use a small EQ5 unguided, so I am limited to shorter exposures. And I have no concerns about that. I do image (when/if I do any) on the "budget" side.

 

As it stands I would say take what you have and try shorter length exposures. Try 150 second ones (assumes guiding present), stack and see what the result is like.

 

You will have to determine the best length for yourself. Too many strange claims of exposure length around.


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#4 Phishin_phool

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Posted 30 September 2020 - 05:33 AM

I recommend watching this

 

https://youtu.be/3RH93UvP358



#5 the Elf

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Posted 30 September 2020 - 06:35 AM

Was about posting the same link. Here is the missing part about gain:

https://www.youtube....h?v=ub1HjvlCJ5Y



#6 the Elf

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Posted 30 September 2020 - 06:51 AM

When imaging unguided a few test shots are the best way to find out the best time. For example on my EQ6-R I can image 1min subs unguided at 420mm focal length. More details here:

http://www.elf-of-lo...t_unguided.html

In the first step check for the longest time with stars oblong just tolerable. Next check how high you can go in the ISO without saturating too many stars or the object. For example a 2 min sub at ISO 400 and a 30sec sub at ISO 1600 have the same saturation level. With the higher ISO you can keep the read noise down. Provided this is relevant. If you image under Bortle 8 or more it isn't.

When imaging NB guided by an OAG I use 15 or 20 min subs, not longer. First loosing a single frame hurts and second I want 20+ subs from one object and filter for a good pixel rejection during stacking and for enough variation from dither. With only 10 subs dither does no longer work well. So when I have a 5 hour window of darkness and clear skies 15 minutes is the maximum, 10 minutes might be better to get more positions in dither. With a faster scope/lens I even go down to 5 or 7 min in NB.

If the object is bright you have to use short subs. For example the Orion Nebula. 2 minutes at ISO 400 is the longest possible exposure for me or the core blows out using my f/6.5 refractor broadband. With only two hours of data under Bortle 4 skies Orion makes a decent image.

The general rule is this: pick objects that fit your rig. Large enough, bright enough and high enough over the horizon. There is no point trying the Cat Eye Nebula with a kit lens and a tracker.



#7 TelescopeGreg

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Posted 30 September 2020 - 12:37 PM

I'm strongly leaning towards a OSC camera for my first camera purchase - or an upgrade to my mount. The mount is, of course, the AVX in my sig and it's been *okay* at short focal lengths with either the SBIG or my Nikon as long as I only image to the east. I cannot reliable get the longer subs I need for the ST-8300 or anything to the west. 

 

So upgrade the mount, right? 

Before tossing the AVX (though that might still be the right long term solution), I'd check the counterweight balance.  With the rather stiff bearings in the mount, it's hard to know quite where the balance point is.  That your East vs West tracking is different tells me that your balance is off.  Try moving the weights a half inch in either direction from where they are now, and see how that changes the tracking in both hemispheres.


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#8 DRK73

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Posted 30 September 2020 - 01:11 PM

Before tossing the AVX (though that might still be the right long term solution), I'd check the counterweight balance.  With the rather stiff bearings in the mount, it's hard to know quite where the balance point is.  That your East vs West tracking is different tells me that your balance is off.  Try moving the weights a half inch in either direction from where they are now, and see how that changes the tracking in both hemispheres.

 

Yeah, I've done that :( 

 

I had one of these years ago and remember that there ended up being certain "no-go" areas of the sky: anything towards the south or anything west. 



#9 TelescopeGreg

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Posted 30 September 2020 - 02:00 PM

Yeah, I've done that frown.gif

 

I had one of these years ago and remember that there ended up being certain "no-go" areas of the sky: anything towards the south or anything west. 

Interesting.  The traditional "no go" zone for the AVX is directly overhead, as the slight balance-offset trick doesn't work there on the Dec axis.

 

South (in the Northern hemisphere) is always harder, because the movement is larger, but I can't figure why west vs east would matter if the balance is right.  Is it RA or Dec that has the problem?  Unless you have a bum mount, I wonder if you're still not properly balanced.



#10 DRK73

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Posted 30 September 2020 - 10:27 PM

Interesting.  The traditional "no go" zone for the AVX is directly overhead, as the slight balance-offset trick doesn't work there on the Dec axis.

 

South (in the Northern hemisphere) is always harder, because the movement is larger, but I can't figure why west vs east would matter if the balance is right.  Is it RA or Dec that has the problem?  Unless you have a bum mount, I wonder if you're still not properly balanced.

 

I'm not saying I can't make mistakes. Honestly, it's been several years since I was going out imaging with any regularity. My last imaging run a week or so ago I was doing IC1396 and did okay up until it was just about directly overhead. I did my meridian flip, recalibrated my guiding - but got no usable subs in about an hour or so before I called it quits. I did try moving the counterweight - but at the time I was doing this the target was, while technically "west", was pretty near overhead. Stars were streaked left-right only, which in my image I think corresponds to east-west (because image orientation was nearly 180deg). 



#11 TelescopeGreg

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Posted 30 September 2020 - 11:23 PM

Well, to be honest, my strategy is to avoid median flips...  Depending on the target and length of the OTA, the AVX can go quite a ways past the meridian crossing.  I let the mount go as far as it can, before running into something, and by then it's probably time to pack it in anyway.  The one time I attempted a flip, it didn't end well.  I forgot to manually slew the camera out a bit before pushing the "go" button, and it ran the camera into a tripod leg.  Nothing damaged, but it killed the alignment for the night (this was before I was doing plate solving). 

 

I currently have the mount balanced pretty close to neutral in RA, so that neither side is favored when I pick a target.  For a while, I had trouble getting PHD2 to calibrate without first losing its star, and the fix for that turned out to be about 3/4" inward movement of the counterweights.


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#12 DRK73

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Posted 01 October 2020 - 07:30 AM

I'll keep at it, then. It just looks a little odd when the counter weight bar starts getting higher than the telescope!



#13 the Elf

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Posted 01 October 2020 - 08:49 AM

My long hard and frustrating AVX journey can be summarized like this:

- open the mount, adjust the worms, replace the grease

- stay away from the zenith

- give it a strong east balance. After the flip move the counter weight about an inch / 2.5cm in to be east balanced again

- use the shortest guide images you possibly can, below one second. This is chasing the seeing but it it is also correcting the poor tracking

- Use short subs like 3min

- Be prepared to sort out 20-50% of your subs for a decent image. That doubles your imaging time.

- Use a light scope

- Get an extra counterweight and rather use tow near the mount than one at the end of the bar.

 

This was the best result I could get:

http://www.elf-of-lo...l_2018_1080.jpg

 

With a lazy don't care about anything setup of the EQ6-R I got this:

http://www.elf-of-lo...1_2020_1080.jpg

This is a larger scope so the comparison is not totally fair. The point is getting this one was a Sunday walk. The AVX image was a struggle.

 

Life is too short to bother with poor stuff.


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#14 TelescopeGreg

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Posted 01 October 2020 - 12:11 PM

My long hard and frustrating AVX journey can be summarized like this:

- open the mount, adjust the worms, replace the grease

- stay away from the zenith

- give it a strong east balance. After the flip move the counter weight about an inch / 2.5cm in to be east balanced again

- use the shortest guide images you possibly can, below one second. This is chasing the seeing but it it is also correcting the poor tracking

- Use short subs like 3min

- Be prepared to sort out 20-50% of your subs for a decent image. That doubles your imaging time.

- Use a light scope

- Get an extra counterweight and rather use tow near the mount than one at the end of the bar.

 

This was the best result I could get:

http://www.elf-of-lo...l_2018_1080.jpg

 

With a lazy don't care about anything setup of the EQ6-R I got this:

http://www.elf-of-lo...1_2020_1080.jpg

This is a larger scope so the comparison is not totally fair. The point is getting this one was a Sunday walk. The AVX image was a struggle.

 

Life is too short to bother with poor stuff.

All agreed.

 

But I believe the OP is very well within the weight parameter.  The puzzle is why he's having such different results between east and west, and the only thing I can think of is balance, or the lack thereof.  Interesting that you had better results with a strong easterly bias; I found that it needed to be a bit more subtle, or PHD2 wouldn't calibrate.  Perhaps the OP's is more similar to yours than mine.

 

I think the bottom line is that every AVX has a personality, and one needs to come to understand the particular quirks of the copy they have.  I've come to an agreement with mine, and it's behaving itself pretty well, considering the 26lbs and 910mm focal length of telescope that it's carrying.  Can I do better with a better mount?  Absolutely.  But are there new things I can do within the parameters of what I have?  Yep.  I'll probably wait until the pain of continuing to use the mount becomes higher than the pain of moving to a different mount architecture.  That would require a new physical and operating structure, touching almost everything from the trolley on up.


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#15 DRK73

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Posted 01 October 2020 - 07:14 PM

I'm going to give the setup a few more chances - Saturday looks promising.

 

We'll see, though. If the struggles I'm having are mostly because I need longer exposures because of the camera I'm borrowing but the mount is limiting my exposure-length...but if I bought my own camera that didn't need such long exposures then I may be able to get away with not upgrading the mount. Really tough predicament. I know I will almost certainly need to upgrade the mount at some point as I'm sure I'll want to go a longer focal length at some point...On the other hand...I can get a higher resolution camera that'll get me to a tighter image scale without having to go big on optics...but I'll need a mount that'll maintain an error well under the image scale...

 

But - if I got a camera hoping that short exposures will save me but I still can't get the mount under control....

 

What to do, what to do....



#16 TelescopeGreg

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Posted 01 October 2020 - 11:52 PM

Yeah, what to do?  We all face that same question.

 

I was all set to upgrade my mount late last year, then happened to look at the Stellarvue "Certified Pre-Owned" website just before Christmas and saw my "dream scope" listed there.  There went the Mount budget...  But, oddly enough, that new scope's weight distribution was enough different from the prior one, that the AVX started behaving itself a whole lot better.  Not something I would have predicted, and certainly not why I got the scope.  But, I realized something.  Ice skaters spin more easily with their arms and legs tucked in, and that applies to telescopes and mounts, too.

 

The mount still isn't performing as well as I would like, but it's doing well enough that it was no longer the limiting factor in my imaging.  Nikons and Nebulae just don't get along, and I was tired of fighting its lack of Ha sensitivity.  So next upgrade, as funds became available, was the camera.  Next after that might be the mount, or perhaps a reducer for the scope, to improve its speed a bit, and for some of the larger targets.  That will depend on how things go.

 

The thing is, I've had that mount since May of 2017, and over time we've come to an understanding.  It's taught me a lot about how mounts and guiders work, what makes them happy, and what causes their flaws to become visible.  In return, I try to keep it within its happy zone.  But building that understanding took time and careful observation.  When it did something unexpected (to me, the novice, nearly everything was unexpected), I would stop and try things to see what caused it, ask questions, and work to make it better.  That AVX is carrying way more than it's "supposed" to be able to carry for DSO AP.  Maybe I have a prize mount; I don't think so.  Rather, I've found a good match of equipment configuration and settings that seems to work well with it.

 

Many have suggested the "life's too short" argument for putting it out of my misery and replacing it.  There's a lot of wisdom (and experience) in that.  On the other hand, when I spend money to make things "just work", I won't learn as much about how and why they do.  In the end, it's a personal decision based on one's desires, skills, budget, and time. 

 

In your shoes, I would keep working on the mount's setup.  It is working, and it has some quirks.  You're not "dead in the water", so there is imaging that can take place while progress is being made.  But, yeah, what to do?



#17 the Elf

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Posted 02 October 2020 - 01:04 AM

 

On the other hand...I can get a higher resolution camera that'll get me to a tighter image scale without having to go big on optics

Yes and no. Have a look at this simple example by Robin Glover, just watch the gray circle story, 20 sec or so:

https://youtu.be/3RH93UvP358?t=1510

 

As you can see noise eats up small structures first. If you go for a higher resolution one way or the other (more focal length or smaller pixels) and if your tracking is up to the task and if your seeing is good enough the higher resolution only shows up in the image if you get the noise low enough. First a higher resolution needs better tracking no matter how you get it and the tracking of the AVX is limited. Second to get the noise down by a factor of 2 you need 4 times the exposure time. That is why you will soon look for a faster scope which is large and heavy. Last but not least the resolution is limited by the scope diameter. That is the diffraction limit. Scopes with obstruction are worse than refractors because there are two perimeters where diffraction occurs plus the spider in case of newts and RCs. Out of experience with my RC6 I'd say 1 arcsec per pixel is fine. Going lower does not gain much. I'm at 0.73 now with the RC8 at 1100mm and the Canon 800D / T7i. There are few nights when seeing is good enough for the scale. Unless you live on a high mountain or in a desert the best seeing you have is about 2 arcsec. Shannon tells us to sample using twice the spatial frequency, the design of a bayer pattern if you go OSC suggests to go for 3 times. That is 2/3 of an arcsecond per pixel. To collect enough signal you want your system to be faster than f/7. A high QE camera using Sony's backside illuminated sensors may help.

Bottom line: as long as you enjoy imaging there is no need to change anything. When you find yourself frustrated because the target you want to image does not come out fine it is time to think about an upgrade. A hobby is meant to be enjoyable not a pain.


Edited by the Elf, 02 October 2020 - 01:06 AM.



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