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Nanotracker - peak-to-peak of 280 arcseconds??

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#1 cuivienor

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Posted 09 February 2014 - 07:42 PM

Well I just measured the performance of my NanoTracker using my Canon 600D and Nifty Fifty 50mm lens. I misaligned the mount in azimuth, and took star trails over a period of 2.5 hours.

What I get is:
- one period per 30 minutes
- 15-16 pixels peak-to-peak

This means (according http://www.astro.sho...lc.htm#details) that the peak-to-peak error of the mount is around 280 arc-seconds??? Anyone care to check my computations?

This is a horrible horrible error. I have seen other sites in Japan measuring it at +-2.5 arcminutes but I couldn't believe it until I tried.

Cheers,

Yannick

#2 Starhawk

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Posted 09 February 2014 - 08:26 PM

Your numbers would support an exposure time of about 2 minutes with that 50mm lens. Sort of worth taking the tracker out, maybe.

-Rich

#3 orlyandico

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Posted 09 February 2014 - 09:36 PM

seems that anyone who can cut a worm can make a tracker these days...

this is the type of mount revolution that mmalik is looking for :D :D :D

#4 cuivienor

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Posted 09 February 2014 - 10:47 PM

Even worse than 2 minutes. The period being 30 minutes, one peak to the other will be done in 15 minutes. So an average deviation of 18 arcseconds per minute - and my resolution at 50mm being 18 arcseconds per pixel... I'm already toast! :) I'd say that close to the peaks, my subs would be good, but in the interval subs will get back. So throw away around half of 1min subs at 50mm? Not glorious :)

Yannick

#5 orlyandico

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Posted 09 February 2014 - 10:53 PM

and that's why people buy the TOAST :D

Seriously. I have an answer to this which is "revolutionary."

Put a $10 Bourns 8-bit absolute encoder on the worm shaft. Now you can index the worm position and apply a PE correction at the factory (since the encoder is absolute). You'd need a processor (rather than a dumb quartz-driven stepper motor controller) to read the encoder and apply PE corrections, but we're talking another $10 in parts here.

et voila. Probably sub 10" p-p periodic error.

Now that's revolutionary, because it leverages cheap electronics to address a horrible mechanical problem.

#6 andysea

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Posted 09 February 2014 - 11:12 PM

I wonder of the gear mesh can be adjusted, I also wonder if tweaking it may improve the PE. You could even have a tiny amount of backlash since you will never be reversing direction in RA. Have you tried that?

#7 cuivienor

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Posted 10 February 2014 - 12:18 AM

I'll try to open it - hopefully I don't break it in the process!!

Cheers,

Yannick

#8 cuivienor

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Posted 10 February 2014 - 05:03 PM

Well I've opened it. No wonder the PE is so bad... The motor assembly is held in the proper position by the cover, there is no way to make sure it is set properly until the cover is closed and screws are set. And this is even smaller than the ETX worm stuff :)

Photos attached

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#9 cuivienor

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Posted 10 February 2014 - 05:03 PM

And a second one

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#10 TONGKW

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Posted 10 February 2014 - 10:33 PM

Last month, just for curiosity I opened up my Nanotracker to have a look at the inside. There is a tiny electric motor driving many small plastic gears, just like a toy car.
I suppose it is just all right for taking short exposure wide field image of the night sky and cannot expect too much from such a small unit.

K W TONG
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#11 andysea

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Posted 11 February 2014 - 12:16 AM

With a wheel that tiny it's no surprise that you get that big PE. I didn't realize how small the wheel was until I saw your photos.

#12 cuivienor

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Posted 11 February 2014 - 01:25 AM

Yeah I was very very surprised by the small wheel - I assumed it took almost all the space but not at all - seems like this is an overpriced toy...

#13 cuivienor

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Posted 16 February 2014 - 09:17 PM

So recently I decided to try and enhance the mediocre performance of this tracker: I remeshed the worm and worm wheel with a light touch, then tried to "age" that connection by running the device at 50 times sidereal speed for over 48 hours.

And in the end, I tried again, and I recorded a PE of...... 280 arcseconds.

OK, I'm dropping the ball on that one. At least it seems this device has everything done to make the PE as "small" as possible at the factory...

Cheers,

Yannick

#14 gdd

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Posted 17 February 2014 - 11:34 AM

Even worse than 2 minutes. The period being 30 minutes, one peak to the other will be done in 15 minutes. So an average deviation of 18 arcseconds per minute - and my resolution at 50mm being 18 arcseconds per pixel... I'm already toast! I'd say that close to the peaks, my subs would be good, but in the interval subs will get back. So throw away around half of 1min subs at 50mm? Not glorious



The extemely long worm period works in your favor provided the other gears do not throw in really large and fast PE of their own. It is possible you might be able to go greater than 50mm and still have only a 50% throw away rate. Have you tried 90 or 120min subs with the 50mm, you may still only have 50% throw away rate if you are lucky. Looks like it will do well around 28mm or less. Should not take long to image the entire sky!

Gale






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