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Sky Tracker PE, how bad is this?

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#1 Chris_H

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Posted 17 December 2013 - 08:24 PM

Aimed low in the south. I don't really know if this is good or bad :confused: What do you guys think?

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#2 nodalpoint

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Posted 17 December 2013 - 09:02 PM

If that's supposed to be a pinpoint star, pretty bad.

I thought these little trackers might be able to do maybe a minute at best with a lens that long?

#3 Bluejay08

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Posted 18 December 2013 - 12:02 AM

At 300mm for a 20 min exposure, it needs a very goog polar alignment, I think.

#4 orlyandico

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Posted 18 December 2013 - 01:10 AM

The long line is the declination drift due to polar alignment. It is purposely mis-aligned so that the periodic error is visible.

OP: is that image 100% ? an easy solution is to rotate the image so that the long line is horizontal, then measure the vertical deviations which are the periodic error.

What is the declination at which the above image was taken?

My off-the-cuff opinion is that that is pretty bad. But what do you expect for 20 minutes? even at 300mm?

#5 cloudywest

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Posted 18 December 2013 - 08:57 AM

Can you tell how many pixels P-P when the line is rotated horizontally? Then we can estimate the PE.

#6 Chris_H

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Posted 18 December 2013 - 09:13 AM

Thanks guys. This was just a PE test, I wont be using 300mm for imaging ;) I bought Sky Tracker purely for widefield imaging. The worm cycle is between 9-10min, which is why I took a 20 min exposure. I get around 17 pixels when measuring in PS.

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#7 gdd

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Posted 18 December 2013 - 10:39 AM

Excluding the thickness of the line I get about 11 pixels.

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#8 orlyandico

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Posted 18 December 2013 - 11:39 AM

11 pixels at a pixel scale of 3"/pixel is 33 arc seconds peak to peak. Assuming you were at zero declination. If you were at 45 degrees declination then the actual periodic error would be 47" peak to peak, a much less impressive result.

33" is Sky Memo territory.

Not quite Toast Pro or Tak Sky Patrol territory though.

I'd say not bad.

#9 Chris_H

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Posted 18 December 2013 - 12:12 PM

Sorry, forgot to mention dec, I think it was taken around -8 dec

#10 orlyandico

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Posted 18 December 2013 - 12:23 PM

OK the. It would be just around 33" to 35" p-p. If 17 pixels then 51" p-p. Not bad at all for your use case.

#11 Chris_H

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Posted 18 December 2013 - 12:33 PM

Thanks Orlando!

#12 gdd

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Posted 18 December 2013 - 12:47 PM

The 33 arcsec is spread across the 9-10 minute worm period. So very little error over a minute or two. The AstroTrac has a very small 2-3 arcsec error, but "worm" period is a only a minute (give or take).

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#13 orlyandico

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Posted 18 December 2013 - 01:07 PM

Over 1 minute it would then be about 3" so with a 300mm lens and your 1100D 1 minute would be doable. I'm actually more interested in the newer iOptron mount (the one that is essentially an EQ mount with the declination axis removed).

#14 Chris_H

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Posted 18 December 2013 - 01:09 PM

Yes, I saw that one as well. Should have waited! Oh well, this little thing is nice to have in the backpack for when I'm up in the mountains

#15 Yun-Oh

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Posted 21 December 2013 - 12:45 AM

33" is Sky Memo territory.

Not quite Toast Pro or Tak Sky Patrol territory though.

I'd say not bad.


I never tried to actually measure the PE of SkyTracker, but your assessment seems to be dead on based on my experience. I can do about 45 sec exposures with a 300mm lens, and 90 sec exposures with a 200mm without much trouble. One issue though is that the periodic error is not exactly a smooth sine curve, so I often end up throwing out quite a few subs where star trails show up out of nowhere. Most of the time the subs right before and after are just fine.

Even with this issue, I would say it is really a nice piece of equipment. Comparing it to Sky Memo, Toast Pro, and Sky Patrol would be unfair, because all of these Japanese products cost about 3 times more or higher. AstroTrac and Vixen Polarie are more reasonably priced, but they have their flaws, too. With the PE of 33" p-to-p at less than $400, SkyTracker serves its purpose very well. I wish it had a little more payload though...

#16 Maverick199

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Posted 21 December 2013 - 02:48 AM

If that's supposed to be a pinpoint star, pretty bad.

I thought these little trackers might be able to do maybe a minute at best with a lens that long?


And they do. With the Polar scope aligned correctly as per Polar App, easily 1 minute is possible. There is a bit of PE time to time. However nevery tried more than 4 minutes due to light pollution and some of those 4 minutes showed pinpoint stars using a 200mm lens.

#17 GlennLeDrew

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Posted 21 December 2013 - 11:17 AM

An inexpensive barn door tracker can be made which will carry several pounds, and have periodic error of this order. And a simple, effective polar finder can be rigged up for it, too.

Periodic error can be reduced by working in the drive nut on the threaded rod with a fine abrasive mixed in oil. (The rod can be given yet more treatment with additional nuts; when chucked in a drill--before bending--makes for fast work.)

Sorry; as a DIY type I can't hold my tongue, figuratively speaking. :grin:

#18 Chris_H

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Posted 22 December 2013 - 07:07 PM

Ok, a little update!

My SkyTracker had a LOT of rough spots and as I moved the worm wheel around, I could also hear it scraping against something inside. Long story short, I took it apart, got the Dremel out and cleaned all the high points in the casing (and it needed it!). I also adjusted the worm wheel and gear, shimmed both the azimuth and altitude axis (also needed!) so now there is no play. There are still a couple of sticky points in one of the worm wheel bearings, but I'll leave that for now. So far I have only tested it through holes in the clouds but I can already tell the difference. I can't wait to give it a proper go! :grin:

#19 james7ca

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Posted 23 January 2014 - 02:02 AM

Can you give another update? Have you been able to image anything since you made these changes?

#20 Chris_H

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Posted 24 January 2014 - 09:50 AM

Been cloudy since Christmas here so no new light I'm afraid :(

But it has given me time to go over the whole unit again. I primarily wanted to fix 2 things;

#1 Find out where the last bit of "stickiness" came from

Turns out it came from the ball bearings. iOptron said they would replace both but these things look so cheap, I decided to order new ones myself. Still waiting.

#2 The battery compartment issue

Whoever designed this thing never actually tried it with batteries! When you load up the battery holder it wont fit back inside as the walls inside the case are too tight!!! :flame: Got the Dremel out again and cut away what I could. Now it fits! :grin:

I also glued up the azimuth base as it was pretty much useless (WAY too much play). I now use the Feisol PB-70 base instead.

I think the moral of the story is; unless you want to do a lot of modding and tweaking, get a Vixen Polarie. It works out of the box. And you can now buy an off axis holder so you no longer have to remove the camera to use the polarscope. That was one of the reasons I didn't go with the Polarie.

#21 james7ca

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Posted 24 January 2014 - 10:44 PM

Thanks for the update.

As for the battery issue, yes it is tight. However, I've found that it depends upon the type and brand of battery. I tried to use some rechargeable Sanyo Eneloop batteries and it was VERY difficult to get those back into the unit. However, regular Sony alkaline batteries are relatively easy (but you still have to be careful and follow the installation recommendations that are given in the user manual).

I've had two different units (the first basically stopped working after the first night). The first one (that I had to return) had a wide, white nylon or teflon washer around the azimuth bearing surface. It was fairly solid and secure (on the azimuth), but the second unit lacked that washer and it seems a little less secure (but not, at present, very bad).

The main issue I have with my unit is that it seems to have a pretty large amount of periodic error. I've measured it at over 80 arc seconds peak-to-peak and I suspect that I won't be able to use it with lenses of over 100mm in focal length. Even at 100mm it will be something of a struggle, more likely I'll be limited to lenses of 50mm and below with exposures of one to two minutes (maybe one minute at the very best when using a 50mm lens).

#22 Chris_H

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Posted 25 January 2014 - 05:13 PM

It didn't matter what batteries I used as the inside walls had powder-coating on them. I actually had a lot of trouble getting the holder out without any batteries installed.

Mine had the teflon washer but there was still too much play when tightening down the lock screw.

I agree on the bad PE. Hopefully it will improve a little after all I have done, but I'm not holding my breath.

I have to be honest and say I'm more then a little disappointed with the SkyTracker, especially after reading the glowing S&T review.

#23 james7ca

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Posted 25 January 2014 - 06:27 PM

Chris, I agree that the S&T review doesn't represent what seems to be shipping to actual customers. In the long run it probably doesn't do iOptron any favors to have such an overly positive review posted to the internet. My experiences with the SkyTracker have basically "soured" me on any future iOptron products.

You may be interested in this thread (if you haven't already seen it):

http://www.cloudynig...Board=lxd55&...,All_Forums,&Words=&Searchpage=0&Limit=25&Main=5858916&Search=true&where=&Name=199816&daterange=&newerval=&newertype=&olderval=&oldertype=&bodyprev=#Post6333740






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