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Long exposures and worm period?

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#1 Stew57

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Posted 03 October 2012 - 05:57 PM

If one can get good exposures past the worm period (say 6 minutes on a CGEM, what would make longer exposures bad? i guess flexure(but an off axis guider would take care of that) and non-guidable gear train errors. Is there something else I am missing? I am thinking the gear train errors are the limiting factor for my CGEM.

#2 Mike X.

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Posted 03 October 2012 - 06:11 PM

wind,cat's and dog's that like to do stuff around the scope..like chewing cables...play around.etc...curiosity of strangers that like to touch things....and last but not least..a nice old time classic simple bump on it :)

If your manage to get exposures as long as you like regardless of the worm period i think that you have nothing more to be afraid of except the stuff you mentioned ;)

#3 SteveInNZ

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Posted 03 October 2012 - 06:51 PM

Refraction.

#4 orlyandico

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Posted 03 October 2012 - 07:56 PM

Declination backlash. Sometimes the mount will go off in one direction (e.g. due to imbalance, wind, a bump) and can't correct in time because of the huge backlash, this causes egg stars.

Not exclusive to longer exposures, but the probability increases the longer you go.

#5 Stew57

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Posted 03 October 2012 - 08:04 PM

I use a string around the RA axis to keep a constant slight east torque no matter the RA orientation. Wind and refraction are something I haven't accounted for.


If your manage to get exposures as long as you like regardless of the worm period i think that you have nothing more to be afraid of except the stuff you mentioned




Well I would like multi hour long subs, but that won't happen until I get a cloudless moonless night with a complete power failure for half of TX and LA and still have power at my place.

#6 Mike X.

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Posted 03 October 2012 - 09:07 PM

I understand that. Well i guess clear and dark skies is the dream of most of us here afterall :)

Mark,may i ask at what focal length you are using the CGEM for long exposure?
I'm going to shoot the trigger on a new mount soon for AP and my wish is to mount an RC8+an ED80 side by side..so the maximum FL i'm probably going to use it would be around 1600mm.
I'm just not sure which to choose between the CGEM and the DX version...Oh...total weight should not go higher than 13-14kgs with all the stuff on.

You think i could stick with the normal version of it or the CGEM DX would provide a more solid platform due to the tripod?
During summer i do photograph on very windy places and the poor CG5 is at the limits even at 500mm-945mm (10kgs) so i am hoping the CGEM could solve that...at least partially.

#7 Stew57

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Posted 03 October 2012 - 09:31 PM

I have tried a c11 with the 80mm F7.5 guidescope. It is hit or miss. The AT6RC works better but it is half the FL. The standard version with the tripod as collapsed as it can be is pretty stable. I am not sure if the added weight penalty is worth it for the DX. What would be worth it is the heavier counterweight shaft. I added an extention so I could use just 2 weights with my C11 fully loaded. It Runs right at the 40lb limit. I get some vibration from the cw shaft at times when slewing that takes a few seconds to settle.

#8 Mike X.

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Posted 03 October 2012 - 10:07 PM

I see, thank you very much for the feedback Mark ;)

#9 lawrie

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Posted 03 October 2012 - 10:32 PM

Polar alignment, if your alignment isn't that good you will have field rotation. This is when stars look like they are circling the target
Focus issues, if you don't check your focus thru the night, that will effect your image. Stars become more and more bloated or bigger.
Guiding itself, you have to determine if your guiding program has been set too aggressively, this causes you to chase the seeing. This can cause big stars, or sometimes stars that are elongated.
You would need to post an image to see which specific issues you are thinking about.

#10 jerryyyyy

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Posted 03 October 2012 - 11:11 PM

Helpful stuff. I am running a C8 with an Atlas and a new camera. Looks like the guiding will work at the 2032mm focal length.... I have moved from PhD to Maxim and now have to learn a whole new settings dialogue. The C11 must be a challenge.

#11 tjugo

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Posted 04 October 2012 - 10:40 AM

Polar alignment and dec guiding. You might have a unnoticeable drift in 10 minute subs but the drift could be very noticeable in 20m or 30m subs.

Cheers,

Jose

#12 Mike X.

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Posted 04 October 2012 - 04:06 PM

I guess a nice drift alignment can solve that.Specially if we need to do 20 o 30 minute subs and in such focal lenghts,(and i guess this is the case we are or under a nice dark sky or we shoot with narrow band filters)..In that case drift alignment is a must in my humble opinion.

One type of problem i currently have (and i hope it's not the case of CGEMs) with my CG5 in Dec axis guiding is that..in some positions is stiffer than others.That gives a bad time to the autoguider and i lose resolution if not the sub sometimes.
That occurs when i don't drift align and only in some positions of the Dec axis.
Consider that i do balance the setup as good as possible and i leave it permanently mounted after for long periods as i use it almost every night during the periods i'm photographically active...Also the mount has backlash registered twice a year (every change of season).
So ..i do things with a lot of calm for achieving as much as i can during the time i have free.
I write those things only to give ideas of possible problems (meccanical) that could influece the exposure.
For beeing more specific.
I usually do subs of 10 minutes most of the time (600 seconds) and if i remember well the CG5 has a worm period of 597 seconds...so i do surpass that limit.

What is strange is that unless i'm prefectly alinged (well as much as possible) even if i surpass the worm period when the Dec axis arrives at a center point it starts giving a really bad time to the autoguider till it surpasses it.Generally i solve that by moving a few arc seconds the framing...but for sure it makes me think that this is related with something that has to do with the axis itself
So..or it is a problem of stiction,grease,(backlash is fixed as good as possible),or it has to do with the lack of ball bearings in Dec?..or..even with construction quality itself!For saying the truth..i find a really hard time to fix backlash meccanically in Dec axis while not in RA.
The Dec axis gives me the idea it is.."oval" (i don't say it is).I try to explain myself better.After i registered the worm gear i check the smoothness by moving the gears with my finger and rotate the axis for 360°.Well..it does become really stiff in some positions and really smooth in others... :confused:...So..registering the backlash in the worm gear is allways a compromise there...something that has to be ok even for the "stiff" areas to become smoother and the "smoot" areas to become just a litle stiffer..with less backlash possible.That gives me a responce more or less of under a second in rate 3 but still it doesn't helps.

I don't know guys..probably after reading this you need an aspirin..and i'm sorry :o..but i thought they could be usefull for giving an idea of possible meccanical issues that could present during the night even with a well balanced and maintained mount..well in the possibilities i have as an amateur and by doing it myself.

Hope it helps..

PS:Please don't get me wrong..i dont' say the mount is not good..i actually love it ...and if there is a reason i am really considering to get a CGEM or a DX is actually the fact that i'm having alot of fun with my CG5..with photos that please me ..even with a litle bit of tinkering... ;)

#13 TxStars

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Posted 06 October 2012 - 12:45 AM

1)Drift twice the exposure length
2)Balance heavy R.A & Dec (depending on mount)
3)Cables & cords (cant drag or hang up)
4)Pets & people
5)No Concrete slabs (Small one can shift & thermals)
6)No lights near scope (^&&$ Fire Flies)






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