what is your OTA, balance method and typical unguided exposure time?

[prefetch]

i'd like to start a thread to better understand the typical DDM85 user experience.

 

OTA: C14 HD

mounted accessories: laser finder

installation: permanent observatory

balancing: standard ASA counter-weights and MDS OTA counterweight on underside of OTA

pointing file: 46 points

typical maximum unguided exposure: 400 seconds

 

 

what are your stats?  

Edited November 11, 2014 by prefetch

[Jay]

Maybe you should ask about the focal length and image scale (resolution arc sec/pixel) figures also because that's what matters the most.

 

Anyway, my scope is Veloce RH200 and the mount is DDM60 Pro. FL is 600 mm and image scale is 1.86 arcsec/pixel. I'm using a permanent set up with 200 points skymap. However, I always use also MLPT because it's an easy and fast way to get rid the residual errors. 

 

My unguided exposures are typically 1200 sec (20 min) but that's not a maximum upper limit just somethings I've found to be practical. 

[LucMurphy]

Hi,

 

To compare apple with apple, you should also add the length of the tube (moment of inertia) and the equipment weight ratio.

 

OTA: C11 HD; fl = 1800 mm; 067"/pix; in my part of the world, the scope must have dew shield. It doesn't seems much but the shield  those increase significantly the moment of inertia.

Mount: DDM60 non-pro mounted on a "permanent" tripod (moving to a permanent pier soon)

80 points file

Weight ratio = approx. 80% (equipment weight to DDM payload capacity)

 

I get very consistent 10 min sub and about 75% success rate for 15 min sub. I did achieve 20 min, but the results are not consistent enough. I define "success" with a roundness of 0.1 or less as measure by Maxim.

 

But what I found out that had the most influence on my setup was the rigidity of your whole setup. It is harder to compare apple with apple when talking about setup rigidity but here what I did to improve my setup that may help you with your C14.

* Added a second dovetail on the top of the tube. Didn't use Celestron dovetail, they are crap.

* Remove the ASA female dovetail and screwed scope bottom dovetail directly to the head of the DDM60.

* Even though the EdgeHD has two mirror lock, they are not sufficient to prevent mirror sag over 10 min, Therefore, I added 3 "fingers" inside the tube to prevent mirror sag.

* The secondary mirror alignment was done in such a way that it is no longer floating ( (always by tightening the screws).

* Cable management: repalce the FLI power cable which was becoming rigid as a rock in sub-zero temp and inducing drift; i have one power cable bundle on one side of the tube and one data cable bundle on the other side of the tube.

 

 

Regards

Luc

[Jay]

I second Luc,

The most important "component" when trying to get (long) consistent unguided exposures is the rigidity of your setup. Most people don't realize how much flexure will be included in every setup. Even a simple short focal length refractor has surprisingly large amount of flexure.  

Jay

[prefetch]

i just ordered some parallax rings for my C14 so i can ditch my celestron dovetail mount.  i hope this makes for a more rigid setup.

 

also, i think i need to do a lot more work on my cable management, which could be causing problems as well - especially as the temperature drops.

 

it's nice to hear that a C11 can get 10 minute subs consistently.  that's my goal, but i honestly believe i am going to have to use an on axis guider for my system.  i just don't have confidence that i can get to 20-30 minute non-guided subs required for narrrowband imaging.

[Jay]

prefetch,

I'm afraid using parallax rings (and better cable management) won't be enough if you're imaging with the full 3900 mm fl.

Jay 

[prefetch]

jay,

 

i'm using a .7 reducer, so i'm at a 2737 mm FL, which i think helps a bit.  also, my CCD is a 16803, so i'm at .68 arcsec/pixel w/ a FOV of 46'.

 

if parallax rings and improved cable management won't do it, what else do you think i should be doing to help my setup?  or do you think the DDM85 just isn't capable of handling a long focal length OTA?

Edited November 12, 2014 by prefetch

[Jay]

prefetch

 

FL 2700 mm will be a lot easier to handle than 3900 mm but it'll not be easy. I don't think it's the question if the DDM85 can handle it or not (because I'm pretty sure it can) but how rigid system you're able to build.

 

There are many sources of flexure and you should be able to get rid of them all before consistent 20-30 min unguided exposures can be achieved.Not easy but possible...

E.g. if you read what Luc said about the standard mirror locking. It won't be able to handle the mirror flip very well.... etc.

What kind of focusing system you're using ?

 

Jay

 

P.S. Sequence has some pretty nice tools to measure flexure, and using MLPT will help you to get rid off some....

Edited November 12, 2014 by Jay

[LucMurphy]

I second Jay's recommendation to use Sequence hysteris and time flex measure tools. If you already are able to acheive subs of 400 sec undguide, it probably means that the hysteris is pretty much under control. To improve, first you need to measure timeflex before doing anything (that will be your baseline), try changes to your setup (hoping it will improve the ridigity) and measure timeflex again to see if the change had any effect or not. This is the only way because time flexure cannot be cancel with MLTP.

 

In the world we are leaving, more expensive usually means better... but not always. In the old yahoo forum, there is an email exchange between P. Keller and a Tak FSQ-106 owner that couldn't get long un-guided subs (do not remember exactly how long, but it was much shorter than what you would expect for a short fl tube). In the end, they concluded that the Tak focuser was the issue.

[LucMurphy]

I kind of recollect that one DDM85 user had reported 45 min unguided subs with a C14 but i can't find the post in this forum. maybe it was in the old yahoo forum

[prefetch]

jay, i'm using an FLI PDF focuser.  mirror is locked down tight, but i'd like to know more about what luc means by "adding 3 fingers" to the mirror to prevent sag.  sounds interesting.

 

also, i've decided that i need to get the power/data cables to route via the GALV and the USB hub in the mount.  i think that will help a lot by taking off all that pressure from the image train by having a long woven cable rope hanging off it.

 

i've also went ahead and ordered a monster MOAG guider.  i just think that i can either spent an enormous amount of time and energy attempting to perfect my C14 balance and flexure, or i can just cut to the chase and go with guided images.  sure, i'm kind of sad that the vision of the unguided DDM85 can't be realized with my setup, but it's still a great mount, and the software is really good too, and i'm all okay with it.

 

it's below freezing right now, and it's cloudy - but when things clear up i'll post some flexure results and some pics of my setup.

[Jay]

prefetch,

 

Good luck with your project and ASA DDM truly is an amazing mount. Just requires some ("not always so easy to achieve") features from your setup to perform its best. However, It's good to remember that those preconditions apply to any mount if taking long unguided exposures is your goal. The only (sometime annoying) "ASA feature" is the need to have such a perfect balance of your setup. It's doable but will require some effort, especially when having a very heavy complex system.

 

But I would also argue that no other mount manufacturer (e.g. 10Micron, AP, SB) is even close to ASA at the moment. But that's just a bias opinion from a happy DDM user    

 

Jay

Edited November 13, 2014 by Jay

[prefetch]

just to followup: i've installed the parallax rings, and now i'm finding i'm hopelessly out of balance with this new setup.

 

it requires over 5kg of counterweights to the corrector-side of the OTA. 

 

i don't know how to add that much weight to my system without making the center-of-OTA-axis completely out of balance.

 

i wonder if i'm the only C14 user with a DDM85 out there!  i don't know how other people have solved this problem.  i just have too much weight on my image train.

 

[RamaSpaceShip]

Hi Prefetch,

 

I am not sure I understand your issue and, if it is not the case, please ignore the following question:

Instead of adding weight on the other side of the tube, can't you simply slide the dovetail to put your current center of balance exactly on the DEC axis?

 

This is what I do to balance my OTA on my DDM85. I added a mechanism on my dovetail to make this easy and precise by only turning a knob to adjust the balance. (see this post: http://forum.astrosysteme.com/index.php?/topic/245-dec-balance-precise-adjustment/).

 

 

Bernard

Edited November 29, 2014 by RamaSpaceShip

[prefetch]

bernard, the problem is that i'm sort of out of room.  if i slide my dovetail any more, then i start to lose contact with the DDM85 and i'll be back to having flexure problems.

 

(btw, i really like the setup you've made for adjusting the dec - very nice!)

 

here's the dilemma:

 

[RamaSpaceShip]

Hi Prefetch,
 
Ok, I understand now.
 
But adding such a weight as you indicate, appears to me as a bad idea, as it can create other issues.
 
I think that you need a longer dovetail to replace the ASA 15" one, probably a 24" dovetail would be ok.
This is not a complex piece of metal and you should be able to find someone in Salt Lake City that can build it precisely at a very reasonnable price.
In addition, it can be designed stronger enough to highly reduce the flexure issues.
 
You can also make it even longer to add a handler that will also support your imaging train, thus limiting the flexures in it.

 
Bernard

[prefetch]

bernard, interesting idea.  i think they key here would be to figure out how to design the "handler" portion of the system.  i'll think on it and see what i can come up with.