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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 - 02:04 PM

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 RamaSpaceShip

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Posted 19 January 2016 - 04:18 PM

Hi Pelle,

 

If you have some flexure, then it could be very hard to get 20 minutes exposures, in particular if not all cables go through the mount.

 

You need to identify the source of the flexure. And to solve it, or at least reduce it.

You need to try other USB cables to find one that is usable through the mount hub, or to manage such that the cabling has a very little torque on the OTA, else it can be a good source for your issue.

 

 

Good luck.

 

Bernard



#3 pesa

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Posted 19 January 2016 - 06:31 PM

Thanks Bernard but how about my question??

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

 

pelle



#4 MarkS

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Posted 19 January 2016 - 11:50 PM

Hi Pelle,

 

Please see my thread on this forum. (second page).

The set-up was DDM60standard/ASA10N/STT8300 with trailing wires etc.

The run was 1hr, subs were 5min, no dither.

The 12 subs were integrated without registering to give the image shown.

 

So the answer is yes at 1.24"/px. I don't make a habit of doing this, but I have used 10 and 20min subs often when narrow band imaging without trouble.

 

Regards,

Mark



#5 lukepower

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Posted 20 January 2016 - 08:00 AM

Hi pelle,

 

with my setup (20" Cassegrain at 3414mm focal lenght, 0,54"/pixel resolution), I have a few times been able to go for 20 minute subs unguided, only with MLPT.

It worked once temperature equilibrium was reached, so basically a bit further into the night. I personally think that with a perfect setup (no flexure, no hysteresis) and ideal skies (perfect seeing, no temperature changes) one could basically push this to infinite as it's not a matter of mount...


Lukas Demetz
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www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#6 wessel

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Posted 20 September 2016 - 07:45 AM

Here you have one of the  examples for 1.27 arcsec/piksel and 20 mins of exposure - all subframes.

9d589e750aa011441118ee701e01716f.1824x0_


Clear skies!

Visit www.astrofun.pl


#7 pesa

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Posted 27 September 2016 - 11:52 AM

Thanks all for responding!

Your results is a wet dream for me :)

 

I wanted to do a lot of experimentation before I got back and so I have done now!

After a cpl of weeks I can say this, I have absolutely no idea whats wrong :(

The only thing that gives temporarily better results is to make a big slew to Park2 and then back again.

Then the next 10min frame (at exactly the same place) is perfect, after that results deteriorate fast.

The fact that big slew always helps makes mechanical problem less likely (in my mind) So what the heck is the problem???

I have a model with 17+17positions. Pointing is fantastic,even the biggest slews are rarely off more than 0.3'.

But tracking is of course not better than pointing andfor tracking 0.3' is big ofc.

MLPT only makes things worse.

I use 5.2.2.8, changed from 5.2.2.3 (Win10 64bit)

same problems with 5.2.2.3

Also I have 2 new problems after abot 30-40 min sidereal without slew the tracking just stops. No error reported in AS or ACC. The "tracking on" symbol is still chosen in AS.

I also get "pointing error" in AS with the red key showing up. (Snag is impossible since all cords are internal.)

 

Any pointers would be welcome

 

pelle

Edit: Forgot to mention that first 10min exposure (no mlpt) is always perfect.


Edited by pesa, 27 September 2016 - 12:52 PM.


#8 lukepower

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Posted 27 September 2016 - 12:38 PM

Pelle,

either you have some flexure (the fact that if you slew to a predefined position and back makes it better would support this theory), or something is wrong with AUtoslew in combination with high altitude... I really dunno...


Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL


#9 pesa

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Posted 27 September 2016 - 01:03 PM

Thanks for the respons luke

 

I forgot to mention in my previous post (I edited it now)

​"Forgot to mention that first 10min exposure (no mlpt) is always perfect."

 

To my mind that rules out flexture.

If flexture it would be a very strange flexture:

1 slew to a position

2 10 minutes exposure thats perfect (so no flexture)

3 a cpl more exposures that turns out decent

4 really bad 10min exposure (why would it flex now and not from beginning?)

Besides the pier is filled with concrete and bored/concrete into solid rock so its not the pier.

the balance is close to perfect

The tube is a N10 connected to mount with ASA clamps.

All screws are tightend.

I agree that flexture could still occur with that setup but why would then the first image be perfect?

 

pelle

 

pelle



#10 lukepower

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Posted 28 September 2016 - 05:33 PM

Maybe its temperature related? Like very fast temperature drop...?
Could you try to make a series of unguided frames with no model/corrections at all? With such a good polar align the results should be quite good on their own...

Lukas Demetz
------------
www.skygemsobservatories.com
Astro Dolomites Observatory, Skygems Network
Santa Cristina Valgardena, Italy
20" Cassegrain-Newton on ASA DDM85XL





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