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Star Adventurer-Max achievable unguided exposure time? (deep sky)

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#1 Naraya

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Posted 09 September 2019 - 10:39 AM

I'm wondering exactly how far I can push my star adventurer without Autoguiding

 

From what i know, i think these 2 are the most important:

  • Polar alignment
  • Balance

Let's say i perfect my polar alignment using the star drift technique, and get perfect balance on my mount (east-side heavy, telescope as close to the mount as possible).

 

What could be the maximum obtainable exposure i can get, and what would be the keeper rate?

 

 

 

 

 



#2 Jim Waters

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Posted 09 September 2019 - 10:53 AM

You forgot to include one item - F.L. of your imaging lens / scope.  I have gone up to 3 minutes with my 200mm lens.



#3 Naraya

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Posted 09 September 2019 - 11:04 AM

You forgot to include one item - F.L. of your imaging lens / scope.  I have gone up to 3 minutes with my 200mm lens.

Oh whoops! How could i forget about that! Im using an APS-C sized sensor with a 80mm f5 refractor (400mm focal length & 1kg weight)

 

I'll be upgrading to a 72ed soon, it's only a 20mm focal length increase although it is double the weight of the 80mm frac.



#4 Jim Waters

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Posted 09 September 2019 - 11:42 AM

I would search www.astrobin.com and look for examples.  I think you will be limited to about 45-60 seconds max.  What's the F.L. of the 72mm?

 

I polar align using a QHY PoleMaster.



#5 Kevin_A

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Posted 09 September 2019 - 05:28 PM

I would search www.astrobin.com and look for examples.  I think you will be limited to about 45-60 seconds max.  What's the F.L. of the 72mm?

 

I polar align using a QHY PoleMaster.

+1



#6 Naraya

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Posted 09 September 2019 - 07:55 PM

I would search www.astrobin.com and look for examples.  I think you will be limited to about 45-60 seconds max.  What's the F.L. of the 72mm?

 

I polar align using a QHY PoleMaster.

F.L is 420mm, also most of the ones in astrobin's guided with a few exceptions



#7 Kevin_A

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Posted 10 September 2019 - 07:00 AM

Good Polar alignment and balance is critical on the unit especially at 420mm FL.

I manage about 60-90 seconds max at 300mm at a 15% throwaway rate.

Balancing is the most critical and unforgiving item I have found that directly affects the star quality (roundness).


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#8 aatdalton

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Posted 10 September 2019 - 02:18 PM

I was getting 2'30" exposures at 200mm with an APS-C DLSR with about a 75% keep rate. 

 

This was just using the built in polar scope and being very careful to re-verify my PA as the last step before letting it run. 

 

The biggest issue I had was it seemed like the motor skipped? on a pretty regular basis - about every 4th exposure. Probably a burr in the gearing or an imperfection in the bearing. It would show up in my subs as a very consistent "C" shaped star trail that would sort of reset it back to a neutral position. That said, I was very happy with the performance from such a little mount. 

 

I wonder if anyone has tried a custom Hypertune? 



#9 OldManSky

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Posted 10 September 2019 - 06:55 PM

There's a dirty little secret you should know...

Even if you had absolutely perfect polar alignment, and absolutely perfect balance, and absolutely perfect drive motors driving absolutely perfect axes moving at absolutely perfect sidereal rate...you still wouldn't be able to go more than 8-10 minutes unguided without trailing.

 

'Cause the stars don't move at sidereal rate everywhere in the sky.  Or, at least, they don't *appear* to, and that's what matters (yes, actually the earth is moving and the stars are still...but the rate they appear to move because of the earth moving varies by latitude, local conditions, seeing, altitude of the stars above the horizon, upper atmosphere currents, and more...).

 

I don't mean to rain on anyone's party.  But without software that's taking all of those things into account, and adjusting the mount rate in real-time, very long unguided exposures are out.  Sometimes you still can't do them ever WITH that kind of software (though sometimes you can).

 

:)


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#10 Hesiod

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Posted 11 September 2019 - 10:41 AM

There is another fact you should ponder well: the possibility to take really long exposures is often too much overlooked.

Really long exposures are useful for the most if shooting narrowband with a cooled camera; otherwise the camera/sensor behavior and the sky's brightness will likely limit your exposures way before the tracking issues start to become dominant.

If shoot amidst heavy light pollution with a somewhat fast (e.g. f/5-f/6) lens could find that already 60" may be too much; even from more decent environments (e.g. those Bortle called "rural", or better sites) your histogram could be the parameter to determine the exposure.

Therefore, rather than asking how much the SA could manage to achieve (which is rather pointless given the variability between samples, and the fact that each of them is not granted for a minimum level of PE, etc...) I would try to see what is the longest exposure my histogram allows me to take.

Only if this exceed my own Star Adventurer's possibility I would think how to address the issue (an example could be to get a photographic lens instead of the 72ed)


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#11 Kevin_A

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Posted 11 September 2019 - 05:10 PM

There is definitely a truth to the saying "The Sky is the Limit" but for some of us its meaning is not good!

In my last home it was Bortle 8 and 60 seconds was pushing my histogram to 1/3 at ISO 200... In my new home it is Bortle 4 and 300 seconds is 1/4 histogram.



#12 Betelgeuze

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Posted 19 September 2019 - 02:47 AM

4 minutes, (haven' t tried longer) with 200 mm lens.

Carefull polar aligned with standard polar scope.  waytogo.gif




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