balancing questions

[prefetch]

so i've been wrestling with balance on my DDM85 with my C14HD for a few months now.  the stars in my images aren't super round - mostly slightly egg shaped, and it's driving me nuts.

 

i assume it's because i'm not quite as balanced as i should be.  based on everything i've read, and experienced, imbalance is the root of all evil for the ASA DDM system.

 

i just thought i'd check with everyone to see what they think about the process i'm going through.

 

here's what i've been doing:

 

 

and step 2:

 

 

and finally, do the RA balance.

 

i've been setting the balancing tool to measure at a scale of 1.5 A and when i get step 1 lined up on the east side, i then move it over to the west side and verify that step 1 is still balanced, and it invariably is not.

 

at this point, i don't bother going to step 2 because if i can't get step 1 balanced on east *and* west sides, then there isn't much point in continuing.

 

today i got desperate, and i tore my image train down to nothing and went to step 1 for east, then west.  next, i added my OAG, and repeated step 1 on east then west, rotating it into place to find the perfect balance.

 

after that, i added my focuser, and repeated everything.

 

then my filter wheel, and at this point, i simply couldn't get east and west to match.  i use an FLI centerline focuser.  i guess it's not balanced.  it has two wheels, 4 filters in one, 3 in the other - and perhaps that extra filter on the one side makes it imbalanced.  the DDM is so freaking sensitive it drives me crazy.

 

anyway, my question to everyone is, does this method make sense?  and how close do you match the red lines at 1.5A?  i try to get them to settle exactly on top of each other.

 

and another question - if you slew the mount around and run the balance tool at random places, do you actually get it to show balance in arbitrary positions?  i believe this is the holy grail, but this seems nearly unobtainable for my setup.

[prefetch]

40 views so far and no replies.  

 

well, i kept at it, and after rotating just about every piece of my image train, over and over, and then laying cables down one at a time, testing one side, then the other side, i was still unable to obtain a perfect balance.

 

i got about 100 mA or so from a perfect balance in the vertical DEC position east pier side and west pier side and because of this i was of course unable to obtain balance over all.

 

i went ahead with it, did an auto tune and then did a random 32 star autpoint file so as to average any sagging out and was able to track at 5 minutes, then 10 minutes in a particular position, but later that night in another position i got trails at 5 minutes.

 

i ran a flexure test, and last time i ran it i was at about 5" error, but this time i was at 30" error.  i have no idea what caused the difference this time.  it's black magic.

 

i'm about ready to give up on this setup.  i can probably have things working with a smaller, lighter OTA or perhaps i just need to go with an AP1200 mount instead or just know that i can't do anything without guiding.

 

i'm pretty frustrated.  

[LucMurphy]

I don't think you have a balancing issue, or at least not a big one. From my experience with a similar albeit scaled down setup (DDM60+C11), if you are getting the red lines reasonably close (let's say

 

From what you wrote, I think the issue is flexure in the imaging train, not mount inbalance. You live in Utah which get freezing temperature at night. Is it possible that you went from 5" flexure to 30" from summer to winter? Don't laugh, that is what happen to me. I made several mod to improve the rigidity of my setup (documented in another email trail) but the biggest contributor was cable management that were getting hard in cold temperature and "pulling" on the tube therefore inducing flexure. In particular, the power cable of the FLI camera was getting rock solid in freezing temperature. I tried 2-3 setup but I ended up replacing the power cable (I know,I know, this invalidate the warranty) that went from the transformer box (which in sitting on the mount base) to the camera with a cold resistant cable that stays flexible down to -50 C. Problem solved. I am getting consistent 10 min unguided subs in any position. 15 min unguided is less successful but still in the 75% range. For my 20 min Ha, I have to guide though. 

 

 

The AP1200 mount will not remove the flexure issue. It will simply hide it by the off-axis-guiding.

 

I hope this help a bit.

 

Luc

[prefetch]

Luc, i very much appreciate the sanity check.  i really want to love this DDM85, but it's kind of driving me nuts.  maybe it is the cables.  it was definitely sub zero last night, and will be again tonight.  the 5" flex was a few weeks ago, but i don't remember if it was freezing at the time or not.  that could be the difference.

 

perhaps i should look into get sub zero cables for everything.

 

so i've collimated my scope tonight, and it's much nicer.  still can't really do 5 minutes without getting little eggs, but i'm getting the hang of maximDL guiding and it seems to work better for me than PHD for guiding.

 

here's a pic of my system:

 

 

[GeorgeCarey]

Is that an extending pier?

Maybe that is the problem if there is some wobble in it?

[LucMurphy]

I concur with George. Even if you do not feel any wobble, the mechanical device that moves the mount up or down (being an electrical motor or hydro-mechanical piston) will take time to settle to a final resting position. Even worst, you could have minuscule slippage in the up position. I suggest that you do a flexure measurement with the mount in the most stable position (down) after several hours on being in that position and see what happens. 

 

Which brings me to a totally different subject. If I interpret the picture correctly, the imaging train is:

Focal reducer

OAG

Focuser

Filter wheel

Shutter (the 65mm i believe)

Camera

 

Have you made sure that the back focus is 146 mm as per Celestron specs? Celestron has designed their focal reducer to have the same back focus with or without it: so it's 146 mm starting from the reducer flange.

[prefetch]

i did some testing a few months back with my C11, and up vs. down did not show any significant difference in flexure.  it's a double lift system, mated with 1/4" steel plate, but still it doesn't take much wobble make a difference.  

 

i will test it again tonight up vs. down and report back on what i find.

 

and yes, i'm at 146mm on my backfocus, starting at the top end of the flange threads of the reducer.  that took A LOT of work to get right, and a bunch of custom adapters from preciseparts.com.  thanks for bringing that up, i appreciate any advice or insights.

 

i'll post a report later tonight on flexure up vs. down.

[prefetch]

oops - didn't work out last night for running the flexure tests (they take so long!)  i got distracted with jupiter.

 

maybe this weekend i'll get to testing that.

 

but i did make some adjustments and got a 10 minute perfect round star image.  when it works, it works.  black magic i tell ya.  

[RamaSpaceShip]

Hi Prefetch,

 

Sorry to answer lately, but I was away all the week and unable to post here due to local internet restrictions.

I usually do only your step 2 of Dec balance and RA dec, but with a 0.7A scale.

I manage so that the red lines partially touch each other, and don't try further precision.

May-be it is not well balanced in other positions. I will check.

On my setup (DDM85, OO ODK16. Moravian G4-16000 with a 7 positions filter wheel), it appears sufficient.

For me, your star shapes are due to variable flexure of your imaging train, as I doubt that the culprit is the balance, unless you are at the weight limits of your mount (but it would be surprising with a C14).

Bernard
 

[GeorgeCarey]

What is your current consumption like when tracking (using the data logging facility)?

If the balance is out you should see a rising current. If the current stays fairly level and you are not getting round stars then balance may not be the cause.

Errors in the pointing model are more likely.

Pointing model errors are probably due to flexure or wobbles somewhere.