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iOptron Sky Tracker - Bad Periodic Error

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#1 Cotts

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Posted 13 May 2013 - 10:32 PM

The full frame picture below was taken with a 105mm lens with the Sky Tracker purposely misaligned by about 30 degrees from the pole and the Drive 'on' for 25 minutes aimed at eastern Scorpio which was low in the east.

The fact that the P.E. is so obvious tells me that the mount is unuseable at 105mm focal length. See pic in second post, also 105mm full frame, showing the elongated stars quite clearly.

I would appreciate feedback from other Sky Tracker owners as to what lenses they use and how long the exposures are.

How much P.E. is normal for this mount, in other words....

Thanks

Dave

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#2 Cotts

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Posted 13 May 2013 - 10:33 PM

Full frame on Omega Ce. The P.E. is visible as 'rod-shaped' stars.

3 minutes at f/4

Dave

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#3 Cotts

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Posted 13 May 2013 - 10:35 PM

Crop to show the P.E. more clearly.....

Dave

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#4 n2dpsky

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Posted 14 May 2013 - 12:37 PM

I only recently received mine and although I've not taken anything "pretty" yet, I've conducted a number of tests with both my Canon 18-55 zoom and my 300 f/4. I can manage 2-3 min sub-exposures with the 300mm as long as I shoot in live-mode so the mirror stays up. The mirror-flop became an issue with 5 lbs 2 oz on the ball-head. After 3 minutes, I see the effects of PE, but I never really expected to go that long with that lens, so I'm quite pleased.

Because of my sky conditions where I am testing, I've not gone longer than 5 minutes at 55mm, but I've taken as much as 5 min worth of exposures at 55mm and stacked them in Photoshop without aligning and they look pretty good to me. At 18mm, I'd image I could go 30 min without needing to align, but of course, I'd be doing that in 3-5 min subs. At least in my camera (Canon Rebel T3i), I think dark-current becomes an issue after 5 min.

For anything serious, I plan to induce dithering to deal deal with noise and turn off the drive for 5 secs between subs. Then, I'll need to buy CCDStack to stack and register, but I have to say, I'm very pleased with the drive performance for what this is (and what it cost).

I have a dark sky trip planned for memorial day weekend and I plan to shoot a lot.

Chris

#5 FishInPercolator

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Posted 14 May 2013 - 12:55 PM

What mount if I may ask?

#6 Cotts

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Posted 15 May 2013 - 06:24 AM

What mount if I may ask?


The Sky Tracker IS the mount...

Dave

#7 freestar8n

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Posted 15 May 2013 - 07:40 AM

Hi-

I don't have that mount but I was curious and found a measurement on the web showing about +/- 50" with a 10 minute period. I think that clearly limits the usable focal length unfortunately - and I think it's consistent with your result.

It's too bad that many people are focused on PE with their mounts even when they are guiding - but this is a mount that has no guiding option - yet it doesn't allow PEC. This mount would have huge benefit if they added smarts to allow PEC training/playback. It might even be possible to set it permanently at the factory and greatly improve the performance with the same gear system. Maybe in the next version.

I guess you need to stick to shorter focal length...

Frank

#8 Bluejay08

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Posted 15 May 2013 - 12:25 PM

I read that post. It seems the measurement method was questionable. And still waiting for the clarification.

I've seen quite a few good photos at NEAF. Dennis had done 3 min at 180mm, as he mentioned in his S&T review.

It might be a "lemon".

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#9 Psion

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Posted 16 May 2013 - 12:21 PM

Last night I have tested PE of the SkyTracker with Pentax 500mm and Sbig ST-402. Very good a mount with 200mm lens for me.

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#10 Peds

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Posted 16 May 2013 - 01:41 PM

Psion, that looks very good! I have just received my iOptron SkyTracker today! As soon as I'm able to run some tests on it, I will report back here.

P.

#11 Psion

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Posted 16 May 2013 - 03:00 PM

Before test I have opened SkyTracker and setup worm (pressure and angle) and centre worm wheel - it was bad.

#12 Cotts

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Posted 16 May 2013 - 09:54 PM

Psion, could you post some pictures about this 'centering' process.

To all others in the thread, thanks for your responses. It is beginning to appear I have a 'lemon' and will need to exchange it for another...

Dave

#13 Patrick

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Posted 16 May 2013 - 11:32 PM

It's too bad that many people are focused on PE with their mounts even when they are guiding - but this is a mount that has no guiding option - yet it doesn't allow PEC. This mount would have huge benefit if they added smarts to allow PEC training/playback. It might even be possible to set it permanently at the factory and greatly improve the performance with the same gear system. Maybe in the next version.



This is a glorified barn door tracker, not a telescope mount. I think the criticism is a bit unfair...do you really expect it to have PEC? :question:

See pic in second post, also 105mm full frame, showing the elongated stars quite clearly.



Dave...what was your exposure length in the second picture? I have the Sky Tracker, but have not used it much yet. But I will say that of all the different photo trackers I have used, the Sky Tracker is by far the easiest.

Patrick

#14 freestar8n

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Posted 17 May 2013 - 12:42 AM

It's not really a criticism of the mount - particularly if in fact the PE is not that bad. I'm mainly pointing out that PEC can be very effective and a big benefit to unguided imaging, while it has less benefit for guided imaging. Since this is a mount intended only for unguided imaging, it is a strong candidate for PEC.

If there is any PE present then it would limit the focal length you could use with it - and PEC could be very effective at extending the usable focal length. That would allow it to be get much sharper stars even though it is a form of barndoor tracker. Barndoor trackers aren't usually used with a focal length as long as 100mm, I don't think, but it would be a much more viable option with PEC.

So to me it's a natural extension of this product that could have a market. A PEC trainable and compact barndoor tracker. I noticed that the PE curves for it appear to be very regular at 10 minutes and don't have much non-integer frequency components in it - so that's even better.

Even if the OP's version has unusually high PE, PEC might allow all of them to work more effectively.

Frank

#15 Cotts

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Posted 17 May 2013 - 08:02 AM

Patrick, it was 3 minutes....

Dave

#16 Patrick

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Posted 17 May 2013 - 09:32 AM

Since this is a mount intended only for unguided imaging, it is a strong candidate for PEC.



Perhaps in theory, but practically speaking, it's a completely different mount with PEC. You'd have to add a hand controller, controller boards, and etc. You'd be better off with a small standard GEM mount like the AVX, CG5, or ZEQ25GT.

What makes the Sky Tracker unique is that it will fit in a camera bag and can be carried just about anywhere.

Patrick

#17 Patrick

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Posted 17 May 2013 - 09:38 AM

Patrick, it was 3 minutes....



hmmmm...not very long is it...I see your concern. I like Psions results better. :grin:

If we get any clear weather this weekend, I'll give mine a run, although I don't know if I can get any PE data, just images. If I can mount my finder guide scope on it I may be able to use the Lodestar to capture some data, but that's an unknown at this point (as is the weather...been rainy and cloudy). Getting some data has been on my to-do list since the Winter Star Party... :crazy:

Patrick

#18 Cotts

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Posted 17 May 2013 - 10:40 AM

Patrick, if you could try the 'mis-aligned' tracking picture with something in the 100mm lens range, that would be very helpful.

How I did it - turn tripod 30 degrees or so away from proper polar alignment. Shoot a 20-minute pic in the eastern sky with the SkyTracker's drive on.

I would love to have a comparison pic to mine, especially if I wish to return/exchange my SkyTracker.

thanks,

Dave

#19 Psion

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Posted 17 May 2013 - 12:45 PM

Cotts: The holes for screw on a worm wheel are not centre on holder of the worm wheel and you need recenter the worm wheel (1mm airspace). After that you have to set worm axis (accurate pressure and angle).

#20 Psion

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Posted 17 May 2013 - 12:47 PM

Second image

#21 Psion

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Posted 17 May 2013 - 12:48 PM

Last image
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#22 Cotts

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Posted 17 May 2013 - 09:17 PM

Thanks, Psion. I'll keep the pics bookmarked for now. My next strategy is to send the Sky Tracker I have back for a replacement...... I will test the new one with my 105mm lens.....

Dave

#23 Patrick

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Posted 17 May 2013 - 11:44 PM

Patrick, if you could try the 'mis-aligned' tracking picture with something in the 100mm lens range, that would be very helpful.

How I did it - turn tripod 30 degrees or so away from proper polar alignment. Shoot a 20-minute pic in the eastern sky with the SkyTracker's drive on.



Yes, I'll give it a try. The weather is not cooperating for the next few days though. I'm curious what my star trails will look like.

Patrick

#24 Patrick

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Posted 19 May 2013 - 01:57 AM

We had some clear skies late in the evening tonight. Here's a shot with the Sky Tracker with a 15 minute exposure with the polar alignment offset about 30 degrees from Polaris. I was using a 70mm lens at f/18 and 100 iso. I didn't know the exact settings to use on an exposure that long so I just took a guess at it.

The PE looks pretty decent though...the stars have a consistent arc trail shape.

Patrick

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#25 Spacetravelerx

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Posted 19 May 2013 - 02:55 AM

We had some clear skies late in the evening tonight. Here's a shot with the Sky Tracker with a 15 minute exposure with the polar alignment offset about 30 degrees from Polaris. I was using a 70mm lens at f/18 and 100 iso. I didn't know the exact settings to use on an exposure that long so I just took a guess at it.

The PE looks pretty decent though...the stars have a consistent arc trail shape.

Patrick



Looks pretty decent Patrick! I take it over all the unit has worked well for you?

I am really thinking of getting one of these next month.






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