Issues balancing camera and scope with counterweight
Posted 18 May 2021 - 06:26 PM
I've had a battle with polar aligning this evening. The mount was tracking the spot of the night sky I was aiming at very well. Each light frame of 3 min exposures was pretty much identical in regards to the star patterns etc, much better than my previous night's attempt. That being said, at first look the image is relatively crisp and sharp, but when zooming in all the way on my camera display there's slight trailing. I rebalanced the mount and tripod and re aligned with polaris. Took another couple of 3 min exposures and still there was some trialing. Even though the tracking seems to be working, I'm not thinking my issues lies in one of two things: either the polar alignment is ever so slightly off, or the canera, scope and counter weight isn't balanced. I then tried to rebalanced it but this proved to be tricky. I couldn't find the sweet spot where the rig would smoothly rotate. I'm also wondering whether this is due to the angle the camera is pointing which may be throwing the centre of gravity off ever so.
Too far up the shift with the counter weight and the camera is too heavy, too far down and the weight is too heavy.
For context, I'm using an iOptron Skyguider Pro as my mount, with a RedCat refractor and a Canon T3i.
Thanks for all the continued support, it's been very helpful and informative!
Posted 18 May 2021 - 06:28 PM
Posted 18 May 2021 - 06:35 PM
Isn't 3 minutes a pretty long time for a star tracker? I know alot of people who are happy to get a 3 minute unguided exposure without star trails with an entry GEM. Just saying.
Posted 18 May 2021 - 06:38 PM
Posted 18 May 2021 - 06:39 PM
Take the scope off the mount, leave the camera and all accessories on it. Lay it on a dowel rod or something round, then find and measure or mark the balance point. Be sure your camera is counterbalanced
- limeyx likes this
Posted 18 May 2021 - 06:45 PM
When diagnosing star trails, it is essential to know what direction the trails are. Are they east-west (RA), or north-south (declination), or some other direction? East-west trails would indicate a tracking error, possibly due to balance, imperfect gearing, wrong tracking speed, etc. North-south trails indicate a problem with polar alignment.
- Juno18 likes this
Posted 18 May 2021 - 07:00 PM
You're expecting an awful lot from a tracker. When I used my Star Adventurer with the Lumix G9 and a 200mm lens, getting 80% keepers at 60" subs I considered a good night. I know there are people out there who use trackers and claim 5 minute subs with perfectly round stars. They are absolutely the exception, and not the rule. I have a CEM40 now, a far more capable mount than a star tracker, and I wouldn't trust it to reliably give me 3 minute unguided subs with my current setup of 294MM on a 0.8x reduced GT81. Perhaps with your T3i and Cat, I'd be OK running 3 minute unguided subs with the CEM40. I certainly wouldn't with a tracker.
- Juno18 and limeyx like this
Posted 18 May 2021 - 07:09 PM
And I am in the early days of this hobby so I'm going to run into issues. I don't expect crystal clear images of distant galaxies or nebulae in my first week. Tonight was merely a challenge to get my polar aligning practice in and familiarise myself with it all. In hindsight, the 3 minute subs were very ambitious for sure. I managed 2 minute exposures last night on my first night out with no star trailing despite not fully aligning accurately with the polar scope. Sadly the clouds have now rolled in and my camera battery has died so I've packed up for this evening. Tomorrow is supposed to be clear so I'll try taking some 60" exposures and maybe push it to 2' if all goes well. At least I know for the future that 3 mins are a no go without guiding or upgrading my setup.
I'll also spend some time in the day tomorrow sorting the balancing of the setup out, so thank you for those tips.
Posted 18 May 2021 - 10:10 PM
Posted 19 May 2021 - 12:21 AM
Another troubleshooting technique could be to take a dozen or so images back-to-back and string them together in a short movie or GIF, say, a half to one second per image. This will speed up the motion of the sky so you can see what's going on.
Here's an example from my early days, showing the tracking error in my mount. Mostly it's RA, some coupled to the Dec axis, and some evidence of drift from polar mis-alignment.
Adding an autoguider solution fixed most of it, enough at least that I could keep 80% of the images I took, instead of the 10% that happened to be taken when the mount was steady. That was enough to make progress on actually imaging.
Posted 19 May 2021 - 01:12 AM
Granted these are your first steps and a lot of different things can be causing problems, you cannot expect round stars without guiding. Star Tracker or GEM, you need guiding when exposing for more than 1-2 minutes. Heck, you should be guiding anyway in order to dither
But as Kathy mentioned, guiding might not be your top priority right now, so focus on getting everything dialed in instead of judging your stars
Posted 19 May 2021 - 07:50 AM
I improved my star quality by making sure my counterweights, camera and lens etc, are all balanced in all axis. You cannot have good stars if you dont have a good balance. If the camera has a big nose heavy lens it will screw up balance. I use arca plates and mounts so i can slide my camera back and forth to centralize the weight balance and also side to side on a rotator plate, I also put a polemaster on my rig to elliminate PA error.
Posted 19 May 2021 - 07:59 PM
Thanks for all the support, thought I'd post an update after tonight's more successful session.
Regarding the balancing issues I had, I discovered that I needed to reposition the mounting bracket attached to the redcat. There are numerous configurations depending on the set up you have. I originally mounted the bracket at the top. But then today after playing around I worked out that it needed to be mounted a little further down to get the balance correct. The downside to this configuration is that I have to loosen the redcat and spin it 90° otherwise rotating it on the RA axis collides with the mount. No biggie as there's a feasible work around.
Secondly, I decided to move my tripod off the patio and fitted the spiked feet so it was more securely placed in the ground. I levelled the tripod before placing the mount on, then levelled again with the mount attached, and levelled once more after attaching the scope and camera.
Initially I had a NIGHTMARE with polar aligning, the first mistake I made was aligning the polar scope to THE WRONG STAR...this was down to setting it up before it was fully dark out. I then had further issues after loacting the correct star as the polar finder app I downloaded (PolarFinder on Google store) didn't give a precise enough location of polaris. I had further trailing, and as the trailing was N/S I knew it was likely down to polar aligning (thanks Kathy). I then proceeded to get out my laptop and loaded up sharpcap Pro to see if it could help...it did not, but i think this could've been down to a number of issues that I'll work on at a later date (I know what issues thankfully)
I then decided to try another app to polar align myself once more, (PolarFinder Pro on Google store) and this app I found easier to use and in minutes I was fully polar aligned. Took several test shots of 20 secs, 30 secs, 1 min, and 2 mins and no trailing in any of them.
I'm currently sat waiting for 40 shots to complete of 1 min exposure each, hopefully I'll have a successful shoot to stack some images (callibration frames partially done, just need to take another 10 darks when I'm packing up)
Still a lot to learn, but I nearly gave up tonight before deciding to give the manual aligning one last shot!
Glad I stuck it out and have something to show for my evening.
So again, thank you for all the tips, this is a great resource to have for a beginner.
Posted 19 May 2021 - 11:22 PM
When you do eventually dip your toe into that pool, you will be astonished at the difference it makes.
Even in my Star Adventurer 2i, I can achieve perfectly round stars on 5min+ exposures. Not only that, but my stars are more circular than I've ever managed to achieve with 30sec subs.
I've only guided a handful of times, but I'm sold. The minor extra cost is absolutely worth it in my opinion.
Sent from my NOTE 10 using Tapatalk
Posted 01 June 2021 - 12:13 PM
No matter how good you balance or polar align, you will still have periodic error. This is a natural property of worm gears. The only ways to get around it are:
Shorten your exposure time,
Use a focal length short enough that it doesn't show.