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Tracking...Guiding...terminology help please. :)

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#1 Intheswamp

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Posted 17 December 2013 - 10:46 AM

I was going through a thread pertaining to imaging with an iOptron Skytracker when an experienced user remarked that being "unguided" that the max exposure would be about 60sec for M31 the Amdromeda Galaxy. I thought the Skytracker *was* guiding the camera. I understand that tracking planetary objects is a bit different but are guiding and tracking two different things when referring to following stars and DSOs...???

I'm really a rank newbie here and trying to understand it all. I've got some basic, humble equipment and I'm just getting started tinkering with doing some night sky photography. I'm trying to build a barndoor tracker for my DSLR so the computer guidance and "GoTo" systems are waaayyy beyond me. Most sites state that for a fairly well built curved rod barndoor tracker that 10+ minute exposures are possible (even stating 1-hour exposures but I'm just hoping for those 30-60 second shots :)). So...guidance and tracking, what's the difference between them when referring to stars and DSO's?

Sorry for rattling on, just trying to figure some things out...thanks for any feedback!

Ed

#2 jbalsam

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Posted 17 December 2013 - 10:51 AM

"tracking" refers to the mount's RA axis being motorized to turn at the same angular velocity as the earth
"guiding" refers to a feedback mechanism that measures the actual position of a star and corrects for any errors in the rate of the RA axis movement (this is generally done by using a camera separate from the main imaging camera to take short exposures of the sky so star positions can be measured).

In tracking, there's no feedback. So if the motion of the RA axis is not perfect (i.e. if it has periodic errors, as all of them do), those errors will cause stars to be deformed in long exposures. Short exposures might avoid the intermittent errors (hence the "less than 60s" recommendation).

EDIT: The iOptron sky-tracker appears to be just a tracking device for digital cameras. It doesn't appear to have any built-in guiding.

#3 nodalpoint

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Posted 17 December 2013 - 11:20 AM

Most sites state that for a fairly well built curved rod barndoor tracker that 10+ minute exposures are possible


With a wide lens longer exposures will be possible. The wider the lens the less movement you'll see in the stars. So for things like Milky Way shots the barn door or Scotch mount will work well. But start putting on lenses a little longer and you'll start to see star trails if you don't have very good tracking.

Search on amazon for some older (90's!) astrophoto books and you can find good instructions.

#4 Intheswamp

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Posted 17 December 2013 - 05:10 PM

Thanks for the clarification, jbalsam, that helps me understand the difference. I know what terms to use now with my barndoor tracker.

nodalpoint, thanks for the reinforcement regarding the star trails. I've managed to get some 18-20 second shots with little egg shaping at 18mm. I've got a 17-55 zoom now and will try it...not much wider but a little.

In regards to using a longer lens... Will using the barndoor tracker help in keeping shorter exposures close together for stacking? In other words, help lessen the impact of star locations drifting so far apart that software like DSS trashes images because the stars have shifted too far? I can't recall what this image drift is called...for some reason I have the word "curtain" stuck in my mind. :question:

Thanks for the help!
Ed

#5 jbalsam

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Posted 17 December 2013 - 06:20 PM

Any tracking device will help, compared to nothing.

I have an Orion EQ-1 table-top tracking mount that you can have if you want it. I bought it from Company 7 when I was first interested in astrophotography. I attached my DSLR to it with a 200mm lens, and I was able to get images that showed the spiral arms of M101 with it (just barely). They're horrible images by my standards now, but they looked like something from the hubble when I first saw them on my laptop. The idea of being able to see another galaxy from my backyard was absolutely astounding to me.

PM me if you want it. It will save you the hassle of building a barn-door tracker.

#6 Intheswamp

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Posted 17 December 2013 - 07:16 PM

:jump: :woohoo: :banjodance: jbalsam, I've gotta say this a few more times...WOW WOW WOW WOW!!!!! THANKS!!! That's over and beyond the top, thanks!!!! I've pm'd you with some of my info. This is so-very-cool!!!!! :)

I was able to get images that showed the spiral arms of M101 with it (just barely). They're horrible images by my standards now, but they looked like something from the hubble when I first saw them on my laptop. The idea of being able to see another galaxy from my backyard was absolutely astounding to me.

^^^^^^ This is what I'm talkin'bout!!! I know I'll never get Life magazine quality images...but one day when I've snagged an image so incredibly far away...amazement will be an understatement!!! Seems similar to when you have a really, really clear black sky with zillions of stars above you and suddenly your vision just kinda "clicks in" and it seems like you start seeing "depth" to the sky and things start taking on a 3-D look(I don't think that that is really possible, is it?). To go a step beyond that and capture images of things barely visible and at incredible distances...now how cool is that going to be!!!<BIG GRIN>

THANK YOU!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Ed

#7 jbalsam

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Posted 17 December 2013 - 07:23 PM

Sent you an email back (hope I deciphered your email address correctly). I'll pack it up and send it to you tomorrow. As I said in the email: when you outgrow it, pass it on to someone else.

#8 Intheswamp

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Posted 17 December 2013 - 07:39 PM

You pegged the email address. I'll definitely keep it moving when I outgrow it!

Thanks again!!!!! :jump:

#9 jbalsam

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Posted 18 December 2013 - 02:22 PM

Package is in the mail. It's going by USPS ground, so it will be a few days (coming from Maryland). Enjoy! and email me if you have any questions about how to use it.

#10 Intheswamp

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Posted 18 December 2013 - 05:36 PM

Thanks Josh!!! I can't wait!!!! Hmm, it might just get here about Christmas Eve....SANTA CLAUS!!!!!!<grin>

I got your emails, thanks for all the information...I will review it as I assemble and don't worry, if I've got a question I'll be asking. ;)

Thanks again!!!!
Ed

#11 Intheswamp

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Posted 03 January 2014 - 09:43 AM

I just wanted to let you know that I still haven't gotten into the box yet but will be soon.<sigh> We had a crystal clear night last night and I considered getting out the camera...the low 20F temperature wasn't that bad but the 20mph winds made it quiet nasty for this ol' southern country boy.

It is supposed to be clear and slightly colder tonight but the wind is supposed to be calm so I may give it shot (I won't have time to prepare the mount, though). We're looking at rain over the weekend and then cold and clear again the first of the week...down to 14F predicted for Tuesday night (16F Monday night). We're not accustomed to prolonged freeze down here (if "burying below the frostline" was mentioned the first question would be "what's a frostline"<sigh>).

Anyhow, ya'll stay warm up that way and I'll report back when I get the mount out of the box, assembled, and hopefully in action! Thanks again!






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