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Can you do 20min unguided subs at 1 arcSec/pixel?


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#1 pesa

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Posted 19 January 2016 - 02:11 PM

Sorry about this double posting. I have DDM60 but is very interrested in what DDM85 owners can report!

 

 

I urge all of you that read this to take 30s and answer!

2 cloud free months to anyone that answers! :D

 

I cant reliably.And I feel I should be able to do even longer with my setup.

 

 

( 1 arcSec/pixel is scaled down or binned - ofc - for you guys with monster resolution)

 

 

My system is dome with permanently mounted DDM60Pro+N10 and atik 460+filterwheel.The extra big ASA focuser.

If you can do 20min unguided, please give a short description of your hardware.

 

Pelle

 

 

PS. A long description of my workflow, I'd appreciate any tip on bettering:

 

Mount has internal USB (DDM60Pro) but I still have 3 cables (thin,flexible, carefully arranged not to create tension).power to camera and wheel,since focuser wants to start before camera.
Also usb to camera (Atik469) can not go thru DDM60 Hub, Maxim looses contact after a while if I go thru DDM60 usb hub..
 
Scope: ASA N10 on DDM60Pro. mount sits on metal pier with concrete inside tube. Pier is attached to solid rock ground with concrete with lots of armour irons drilled into rock.
Polaralignment is very good, <0.5 arcMin on both axis.
 
Pixel scale is 1.07 arcSec/pixel (unbinned)
Camera and wheel are light and are connected by ASA's big focuser.
 
Model:
I have 34star model made with autopoint altitude 25 degrees up to 90.
I wait about 30s before each exposure to minimize flex (let system settle), up to 4minutes if meridian shift. Have too since dome is very slow.
 
Pointing is very good, after a first synch, goto always lands cross inside star "blob" even after meridian shift.
There is some flex but the system settles very good after some minutes.
 
All my images with MLPT I now take like this:
Move to object.
Find (in planetarium software) good star very closeby to focus (if not in object)
Focus manually (takes 2minutes since some little astigmatism makes it very easy to find sweet spot). Autofocus doesnt give good result
(Astigmatism, I'm waiting for comment on that from ASA, I like it cause of focusing but need to know that scope still is diffraction limited)
I dont start MLPT-run untill mount have been on same object for at least 15minutes.Usually 30minutes
 
I use 6 exposures in MLPT covering 22min (2 extra for fiddeling with names before start)
then I take ONE 20min exposure.
 
Sometimes it works, often not. chance of creating a working MLPT to let me do 3*20min in sequence is very bad, I dont even try that anymore.
with bad I mean about 3 arcSec drag on stars. Not visible on big stars of course but all small stars gets a very obvious elongation.
 
Log reports no current spike
Seeing is sometimes excellent for sweden, can get below 2 arcSec. Very dark sky.
Scope sits inside a scopedome 3m (unfortunately).
Pier and dome platform/dome are not connected of course.



#2 maxmirot

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Posted 27 May 2016 - 08:41 PM

I can get  20 min with MLPT.   A lot depends on the scope.

 

However, I don't use Sequence because lacks a lot a features.  I use SGP but this forces me to guide instead. 

 

To bad we can't get a script to use this feature with other programs



#3 MarkS

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Posted 31 May 2016 - 03:06 AM

I get essentially the same results with the DDM85 as the DDM60. Both will easily do 20 min subs. I ran a test last night with the f/2.75 set-up (1.6"/px) and on two consecutive 20 min subs there no detectable drag. When examined at 300% size, tiny faint stars showed seeing variations, but no systematic drift.

 

PA was 1.7arcmin RA and 0.5 arcmin DE. Balance reasonably OK (maximum current draw shown by ammeter ~0.55A). 10 star config; MLPT used.

 

(Accurate focus is the issue for me at f/2.75 - I went back to manual focussing, which remained accurate for > 4 hours including meridian flip and 12 degree C temperature change)

 

Mark






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