I wonder what people think the limit of an unguided star tracker is. I regularly take 60 second exposures with a 300mm lens and get a reasonable rate of frames without trails. I have recently tried to extend this to 2mins when using a narrowband filter, but find my sucess rate is pretty low. So I think I have found my limit, but wonder if that’s reasonable for the gear, or should I be able to do better?
Limits of my star tracker
Well Written Engaging
Hello,
With a 300mm lens the Star Adventurer needs to be polar aligned at high precision, which ain't always easy.
When using the Samyang 135mm, I always autoguide with a small light-weight 120mm guidescope. With under 100mm tracking is okay for me.
In part this is because my alignment skill is lowest drawer, but the reward of autoguiding is great. Though it may sound crazy, I am planning to
use AG with a 28mm lens. My last tracking with 28mm was soso. I don't have the guts to track two minutes with 300mm. 🤡
CheerS,
Robert
With a 300mm lens the Star Adventurer needs to be polar aligned at high precision, which ain't always easy.
When using the Samyang 135mm, I always autoguide with a small light-weight 120mm guidescope. With under 100mm tracking is okay for me.
In part this is because my alignment skill is lowest drawer, but the reward of autoguiding is great. Though it may sound crazy, I am planning to
use AG with a 28mm lens. My last tracking with 28mm was soso. I don't have the guts to track two minutes with 300mm. 🤡
CheerS,
Robert
Helpful Concise Engaging
Hello Peter,
I use the Sky Watcher Star Adventurer Pro with a 250mm lens and an aps-c canon, and found that i can take 2 mins exposure without trails. I think you can also push to 2 mins of exposure.
Here is what i suggest:
1- Always make sure your tripod is leveled
2- Balance the counter weight when you put everything you need on the camera ( finder scope, intervalometer.. etc) also after you take focus with the lens.
3- I polar align twice, after balancing and before shooting, just to make sure that im perfectly polar aligned.
I hope this will help you.
Mohd
I use the Sky Watcher Star Adventurer Pro with a 250mm lens and an aps-c canon, and found that i can take 2 mins exposure without trails. I think you can also push to 2 mins of exposure.
Here is what i suggest:
1- Always make sure your tripod is leveled
2- Balance the counter weight when you put everything you need on the camera ( finder scope, intervalometer.. etc) also after you take focus with the lens.
3- I polar align twice, after balancing and before shooting, just to make sure that im perfectly polar aligned.
I hope this will help you.
Mohd
Helpful Supportive
Hi,
Each mount has a periodic error. This represents small bumps and other impefections in the gear which result in a predictable movement with every cycle.
As an example think that one particular tooth in the worm wheel is a little longer than the others. Every time that tooth touches the worm screw, the increased resistance makes things slower and tracking lags by a few arcsec. Always the same few arcsec.
The total error in tracking that occurs during a period (a full rotation of the system) is called the periodic error, PE.
The duration T of the period itself is the other important number.
The rate e= PE/T gives you how much your mount will err per second. If you track for t seconds and et>your resolution in arcsecs, you will have star trails.
Example:
period = 500 secondsperiodic
error = 10 arcsec
resolution = 2.96 arcsec per pixel (Canon EOS 40D with 300mm lens).
Your error rate in the example is 0.02 arcsec/pixel so you should be able to achieve about 150 seconds without star trails, assuming of course no wind, perfect balancing, polar alignment and total weight well within your mount's payload capacity (all of those things can cause star trails and are more likely to cause them with longer exposures).
This is an average value, mind. It could be that your worm is almost perfect for the first 80% of it's cycle while having many anomalies in the remaining 20%. In that case you would be able to achieve up to 400 seconds when in the "good" part of the cycles and not even 2 seconds during the "bad" part. But the average is what you are able to do, well, on average.
So to answer your question, you need to look up your mount's worm period and periodic error. I believe for a SA the numbers are close to 50 and 500 respectively so approximately 30-45 seconds is the best you can expect at 300mm on average. This matches my experience at the slightly larger 360mm, where 30 seconds is good while 45 seconds gives about 50% keepers.
Cheers,
Dimitris
Each mount has a periodic error. This represents small bumps and other impefections in the gear which result in a predictable movement with every cycle.
As an example think that one particular tooth in the worm wheel is a little longer than the others. Every time that tooth touches the worm screw, the increased resistance makes things slower and tracking lags by a few arcsec. Always the same few arcsec.
The total error in tracking that occurs during a period (a full rotation of the system) is called the periodic error, PE.
The duration T of the period itself is the other important number.
The rate e= PE/T gives you how much your mount will err per second. If you track for t seconds and et>your resolution in arcsecs, you will have star trails.
Example:
period = 500 secondsperiodic
error = 10 arcsec
resolution = 2.96 arcsec per pixel (Canon EOS 40D with 300mm lens).
Your error rate in the example is 0.02 arcsec/pixel so you should be able to achieve about 150 seconds without star trails, assuming of course no wind, perfect balancing, polar alignment and total weight well within your mount's payload capacity (all of those things can cause star trails and are more likely to cause them with longer exposures).
This is an average value, mind. It could be that your worm is almost perfect for the first 80% of it's cycle while having many anomalies in the remaining 20%. In that case you would be able to achieve up to 400 seconds when in the "good" part of the cycles and not even 2 seconds during the "bad" part. But the average is what you are able to do, well, on average.
So to answer your question, you need to look up your mount's worm period and periodic error. I believe for a SA the numbers are close to 50 and 500 respectively so approximately 30-45 seconds is the best you can expect at 300mm on average. This matches my experience at the slightly larger 360mm, where 30 seconds is good while 45 seconds gives about 50% keepers.
Cheers,
Dimitris
Helpful Insightful Engaging
Thanks for tips and advice. Yes I am using the SA pro so, from Dimitris’, comments it seems I am certainly pushing the limits. Thanks for the useful tips. I seem to be able to align okay but the SA illumination is very bad so it can be tricky. Probably worth re-checking just before shooting and trying autoguiding.
Wholeheartedly agreed👍
I've also found my limit with a Star Adventurer Mini with a 300mm lens (the 70-300mm Nikkor kit lens @ 300mm) has been 60 seconds, after taking A LOT of care with polar alignment and balancing the counterweight. Last time I did that I ended up dumping around 1/3rd of my frames (and decided to save up for a proper EQ mount!) but still was able to get enough data to work with (my shots of M13, M51 and the Leo Triplet in my gallery were made with this setup). I've made a couple of attempts to go longer but decided it wasn't worth the effort as the majority of frames were unusable. I've recently started using a heavy Nikon 180mm f2.8 + D3500 on the SW SAM and found 30 second exposures to be fine, with all the frames I collected having no noticeable trailing (the extra light gathering over the kit lens helps too!) - haven't pushed it to 60 seconds yet
Helpful Engaging
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