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Old 30-04-2020, 05:09 PM
Emuhead (Andrew)
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Star Adventurer average tracking - ideas welcome (example attached)

Hi guys,

So looking at the attached image, this happened a lot last imaging session and there was no wind. I had to throw out about about 60-70% of the 2 minute subs due to this. The others were acceptable to use and had pretty round stars. So that in mind, what might be the most obvious reason for this?

Gear used was the Star Adventurer with dec bracket & counterweight, Redcat51, Fuji X-T2. No guidescope or autoguiding. I balanced the gear as best as possible for the image shot, and polar alignment was also good. Im guessing its either the lens & camera were too heavy for the Star Adventurer, or perhaps something odd with the SA gears themselves.

Anyone had these and found a fix?
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Last edited by Emuhead; 30-04-2020 at 08:25 PM.
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Old 30-04-2020, 05:52 PM
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blink138 (Pat)
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A double up like that could mean a bumped mount
Also a 2 min exposure is hard going for a focal length of more than 50 to 100mm on the SA mount unguided
We have only just had a number of successful exposures with a 200 canon at 1.5 mins
Drop your exposure down to 1 min and raise your ISO? Maybe even 30 secs?
It is a good mount but has serious limitations for anything over 100mm f/l
It has a guide port though!
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Old 30-04-2020, 06:53 PM
raymo
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Two minutes @ 250mm is definitely too long for consistent results with this
mount [you might get some usable subs], but obviously this is not your primary problem. Assuming that the elongation is in the R.A. tracking direction, it is most likely that it exposes correctly for a period of time, the
driven mass then slips quite quickly to a new fixed position, and continues exposing correctly at the new position for the rest of the duration of the exposure. If this is the case, then most likely something is not properly
firmly locked, such as the scope to the mount, or the drive clutch if the adventurer has one.
The other possibility is that you have not set the scope up so that it is slightly unbalanced against the direction of the drive, thereby eliminating the backlash that all gear trains have. This can frequently happen when imaging is commenced not long before the scope becomes vertical, and then flops forward a tiny bit when the scope passes through the vertical position.
raymo
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Old 30-04-2020, 08:06 PM
Emuhead (Andrew)
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Ok, I have read a little about loading the SA up so that its east-heavy.. Guessing that means that if you are standing behind the mount looking through the camera that you lean the gear over to the left side so that the mount brings the gear 'up', rather than lowering it 'down' on the right side.

I didnt have the screws that control the swivel (that require the allen key) tightened fully either. Rookie mistake.

And as you say, 250mm is long for the SA, however ive seen youtubers like Astrobackyard & Peter Zelinka use this combo fine, at least not having mentioned that issue i have.

Also, I can rule it being bumped out as there are so many subs like this. If it was one or two, perhaps, but 70% like this, i know its not that. Actually be good if it was.
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Old 30-04-2020, 09:10 PM
raymo
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O.K happy snapping.
raymo
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Old 30-04-2020, 09:31 PM
Emuhead (Andrew)
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Thanks for the responses. I am curious though how common it might be that the gears or teeth themselves could be less than perfect, which could in turn cause this kind of thing.. is this likely?
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Old 30-04-2020, 10:44 PM
raymo
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Individual gears and gear trains are always less than perfect, the more gears in a given train the greater the imperfection. Even horrendously expensive
gear systems are not perfect, that is one of several reasons that guiding of long exposures is necessary. The imperfections in gears causes a phenomenon called periodic error which naturally is seen with each revolution of any given gear[hence periodic]. There are PEC features which can be trained to largely
nullify PE by causing the tracking motor to adjust to the error, but even these cannot completely eliminate the error. Because of the imperfection the gears
cannot be in constant mesh, so a tiny amount of backlash is necessary; the
higher the quality of the gear train, the smaller the necessary backlash, and the better the tracking will be.
There is a point where trying for gear perfection becomes pointless because other things which affect tracking accuracy can cause larger tracking
inaccuracy than gear imperfections do, so back to guiding.
raymo

Last edited by raymo; 01-05-2020 at 10:40 AM. Reason: correction
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Old 01-05-2020, 05:40 AM
Pepper (Steve)
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Check how much slop is in the axis.
There is a Allen screw on the under side of the mount that adjusts this.
I’ve also noticed the alt adjuster can ba a bit sloppy. This can be adjusted slightly too.
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Old 01-05-2020, 10:47 AM
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Sunfish (Ray)
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I have a star adventurer and I agree that more than a minute for a heavy lense and camera is too much unless you you tweak the whole assembly with perfect balance and weights and have a super heavy tripod on concrete. There are many examples on cloudy nights of tweaked balanced assemblies with some success. Mine works better bolted to a wedge on a pier.

Also the azimuth adjustment bolts which allow alignment also need tightening once you have aligned and the altitude plastic tightening handle can be a problem along with the clutch plate.
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Old 08-05-2020, 07:19 PM
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I don't have a Star Adventurer but I do have part of their components.

I have not found I was able to get decent exposures with a Redcat 51 and a Sony or Canon mirrorless camera using a Lightrack ii mount.

I also did not do an accurate polar alignment nor was it counter balanced.

But I think you are finding that you are at the extreme end of these mounts performance.

Greg.
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