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I've recently replaced my old Vixen Polarie mount with an iOptron Pro for photographing deep-sky objects. However, when trying to make a shot of the M 101 galaxy, all the objects on my pic were motion-blurry due to the Earth's rotation:

Detail with several motion-blurred stars

This is my equipment:

  • Canon EOS 70D (astro-modified)
  • Canon EF 100-400 mm f/4.5-5.6 L IS
  • iOptron Pro with counterweight and Vixen ball-head
  • Slik tripod

These are the parameters of my shot: - 400 mm - f/5.6 - 5" - ISO 2000 - Image stabiliser switched off - 2020-04-12 01:51 CET - 53.06434° N; 8.22302 E

M 101 was almost at the zenith at that time.

What puzzles me, is that the motion blur already appears at 5" exposure time! With my Polarie, 10" @ 300 mm focus (with an EOS 1100D) were not problematic at all when the mount was properly adjusted. The iOptron features a polar alignment lens to adjust the mount, which the Polarie lacked.

What's wrong here? Would the stabiliser in the lens help a bit? And: Is there a possibility to remove motion blur afterwards (I'm using Gimp)? Thank you.

UPDATE: The camera body weighs 755 g, the lens 1460 g. The counterweight on the other side 1500 g. Did I maybe position the counterweight incorrectly?

UPDATE #2: This is how my astrophotographic equipment looks like when in use: iOptron Pro (with counterweight) + EOS 70D + EF 100-400 mm f/4.5-5.6 L IS

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    Is the tracker correctly aligned, right? How much do the lens and camera weigh? Maybe they are too heavy for a ball head. Do you use the counterweight?
    – vsis
    Apr 13, 2020 at 16:49
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    One should never use stabilization with a tripod. It's counterproductive. Your problem seems to be related with polar aligment o weight.
    – vsis
    Apr 13, 2020 at 16:53
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    I see the #2 edit: Maybe it has nothing to do, but I'd low down the head of the tripod. The wind moves that part a lot. The vibrations of the tracker moves it too. It moves less when it's down.
    – vsis
    Apr 21, 2020 at 12:49
  • It WAS down when the broken pictures were made. The pic of my assembly was made during my second try, and I'm now reviewing the pics I've made.
    – Neppomuk
    Apr 21, 2020 at 21:02

2 Answers 2

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I assume you of course locked the mount so that the motor drove it. But you did not mention if your setup first balanced the mount?

In doing this, be very ready to catch it if unlocking it starts to drop (to prevent damage), but with the counterweight shaft rotated to be horizontal (counterweight on one side and camera on the other side), if balanced, it should not move when unlocked. If one side tries to drop, it is heavy and not balanced. Adjust the counterweight position so that it does not rotate if unlocked when horizontal. This reduces the load on the motor by equalizing the load. The motor is designed for a balanced load. You can practice this balance at home, but the proof is in the picture.

Experts advise to make it very slightly East Heavy (the East side of horizontal just barely perceptibly heavy) so that the gears stay fully engaged when lifting the east side. This very light load (almost none) reduces "gear chatter", keeping the gear teeth continually engaged. If you swap sides during the night, then this aspect needs attention then.

Zenith does not matter so much, that's just about which way the camera is aimed. What matters is if the motor is having to lift the heavy camera and lens load. A balanced counterweight makes that be little issue if just rotating a balanced load.

I had to buy and add an extra counterweight to balance a Tamron 150-600mm lens on a full frame Nikon. If it is balanced, it performs well.

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  • In fact, the shaft was in vertical position (as my object was almost at the zenith), so the cam was on the upper end and the counterweight on the lower one.
    – Neppomuk
    Apr 15, 2020 at 18:37
  • Vertical ought to help about unbalanced, at least for a short time. Not sure if you meant 5 seconds or minutes, but f/5.6 needs more than 5 seconds. So was the clutch wheel locked? If it was turned on and locked, it has to try to rotate. You can test balance and lock at home. One hour sideral is very near 15 degrees rotation. iOptron support is very helpful.
    – WayneF
    Apr 15, 2020 at 22:03
  • Yes, the clutch wheel was locked. And: 5" means s sec.
    – Neppomuk
    Apr 16, 2020 at 18:24
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OK, I've finally managed to readjust the counterweight and got fairly good results up to 4" exposure time:

Fragment of a single frame @ 4" exposure time

The outcome still isn't perfect, but at least a significant fraction of the frames I've made are usable.

The pitfall in the counterweight assembly is the following: The camera is located 60 mm away from the pivot. The counterweight then has to be situated 109 mm from the pivot to get an equilibrium. Just the normal lever rule. :)

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  • I use the skyguider pro, and at 200mm I can shoot with 90 seconds of exposure, if I spend like one hour aligning it and adjusting the only counterweight I have. I recomend you this guide for proper aligment, which should allow you more than one minute of exposure: minorplanet.info/ObsGuides/Misc/DriftPolarAlignment.htm
    – vsis
    Apr 29, 2020 at 21:52

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