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FORUMS Photography Talk by Genre Astronomy & Celestial Talk 
Thread started 30 May 2014 (Friday) 07:27
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Positional Error (PE)?

 
General_T
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Location: Ottawa, Ontario
     
May 30, 2014 07:27 |  #1

Hi,

I am interested in getting one of the iOptron SkyGuider mounts to get into some basic Astro Photography. I know next to nothing and thought that one of these "tracker" units would maintain precise tracking indefinately once set up.

From what I have read that does not appear to be the case due to positional error (PE). Can someone explain what PE is and is this something that cannot be overcome? Would a SkyGuider track accurately long enough to allow good photos?

I'm guessing the answer is yes, as from what I have read most exposures seem to be no longer than about 3 minutes? Would appreciate any insight into the possible shortcomings of trackers.


Thanks

PS:So can this tracker take 4 to 5 min subs say with a 200mm lens? If you can only get 60 sec subs might as well make a barndoor tracker and save yourself the money.

What is meant by "subs"??


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Nighthound
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May 31, 2014 10:14 |  #2

I can't give an assessment of the SkyGuider since I have not used one but I'll try to add some info that might be helpful. I'm sure someone will be along to give you help with the iOptron.

Achieving tracking accuracy is a multi-faceted task. It involves mechanics and technique/proper procedure. The "PE" I'm familiar with is periodic error, which is a mechanical limitation presented by gear imperfection in the mount. Even high quality drive gears can have imperfections which is why the mid to higher-level tracking mounts have periodic error correction as a feature. I feel confident in saying that at the price point of the SkyGuider you should expect a fairly high level of periodic error, but a low focal lengths like 200mm it shouldn't be a huge factor. Practicing careful polar alignment and rig balance should top your priority list when shooting long exposures with a tracking mount. Also, when extending exposures to 2-5 minutes you'll want to seek out at least moderately dark skies to assure you get the best quality data for your efforts.

"Subs" is short for sub exposure. These are the individual exposures you take each night that will be combined in the stacking process within a software program like Deep Sky Stacker (DSS).

You'll want to be thinking about dew and frost prevention for your lens on moist and/or very cold nights. It doesn't take long to fog or frost up when pointing glass skyward at night. I'll give some suggestions on that if you're interested.

Hope this helps some.


Steve
Canon Gear: 1D Mark IV | 1D Mark II | 5D | 20D | 500L IS (f/4) | 100-400L
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General_T
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May 31, 2014 14:08 |  #3

Hi Nighthound,

Thank you for the info.

Do you think at the price point for the Sky Guider ($550 US) it would be a good starter unit to try some Astrophotography? What would you recommend that has less periodic error for similar money?


Thanks


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Nighthound
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May 31, 2014 19:32 |  #4

You're very welcome. I wouldn't get too concerned about periodic error at 200mm and exposures under 5 minutes. There will be plenty to learn starting out in set up and alignment alone and then there's processing after that. Avoid complicating things to avoid frustration, this is challenging and it's easy to get overwhelmed.

$550 is low end for a tracking mount but there really isn't a whole lot out there an that price point in portable mounts. It would be classified as a starter and I'm sure it'll take some getting used to and time to learn it's limitations. High quality build and precision comes at a price. The Astro Trac is very close in cost for the tracking device itself but after adding a polar alignment scope and other necessities you can break the $1000 dollar mark quick. If portability is important the AstroTrac is a good option, just a bit more money.

You can see the AstroTrac here, tracker on bottom left:
http://www.canadiantel​escopes.com/Shop-By-Brand/AstroTrac/ (external link)

Read more here:
http://www.astrotrac.c​om (external link)

If I was going portable I'd save up for this. Takahashi makes extremely high end stuff, I own a their Sky 90 II refractor and love it. The Teegul SP III is about more than twice what you're looking at spending but it's a very portable mount that you can grow into. It's capable of handling a small telescope down the road if you wanted to go that route. This portable tracker is in a league of its own. Like the AstroTrac it doesn't come with everything so you have to factor in a polar alignment scope and have a sturdy tripod which is essential regardless of which tracker you go with. Like I mentioned, precision and quality come at a price.

https://www.astronomic​s.com …uatorial-mount_p3721.aspx (external link)


Steve
Canon Gear: 1D Mark IV | 1D Mark II | 5D | 20D | 500L IS (f/4) | 100-400L
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SteveInNZ
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May 31, 2014 21:33 |  #5

While there's no doubting the quality of the Tak stuff, the guide port of the iOptron trumps it as being the more versatile option in my opinion.
Something like a 76mm refractor with a finder/guidescope would be a killer travel rig and is certainly something you can start and grow into with this mount. The main downside that I see is that it requires you to take another tripod, whereas the unguided options can use your standard tripod.

General_T, the guideport allows you to use a separate smaller camera/lens and a laptop to control the mount in realtime and eliminates the effect of periodic error. This is called "guiding" as opposed to "tracking" which is just running the motor at the right speed.

With any of these options, you need to be careful about the load you put on it.

Steve (another one).


"Treat every photon with respect" - David Malin.

  
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Nighthound
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May 31, 2014 22:54 |  #6

Good point on the guiding option Steve. I would however not be terribly concerned about guiding as a beginner in the hobby, especially using a 200mm lens or less. Guiding with secondary optics would be very beneficial down the road to achieve consistently-accurate 5 minute plus exposures using a 400-600mm telescope. My mount has guiding capability and I have used it with success but I found that when traveling to dark skies the less complicated set up and lesser electronics made for much more enjoyable nights. I worked hard with fine tuning alignment, rig balance, etc. and ended up being able to get 4-5 minute exposures without guiding at 400mm. That was good enough to get the job done under my skies without a tethered laptop, its power source, inverter, etc. I guess that could be considered lazy, but for me it was just less complicated, less gear to haul and I enjoyed the entire process and night sky much more. I'm not comparing my mount to the model General T is considering because that's not a fair comparison, I was simply making a point about keeping it simple to keep it fun. Once the process of astrophotography becomes familiar and less daunting then advancing to guiding and beyond will be a smooth transition.


Steve
Canon Gear: 1D Mark IV | 1D Mark II | 5D | 20D | 500L IS (f/4) | 100-400L
SteveEllwoodPhotograph​y.com (external link)

  
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General_T
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Jun 02, 2014 18:42 |  #7

Hi,

I have now stumbled on to the Celestron AVX and will probably order this one. Seems to be a bit better build wise - at least I hope!!


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Positional Error (PE)?
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