first light (sort of)

[prefetch]

so i've been tinkering with this new mount for a few weeks now.  i absolutely love it!  

 

i finally got to the point where i felt like i could try out a reasonably long exposure of 8 minutes.

 

here's how it turned out (uncalibrated, minor stretch):

 

 

so i thought "wow, i think i got this down" and so i went a few degrees over and did a 5 minute exposure of C39 the clown face nebula:

 

 

and yeah, it looks like crap.  so...not sure what happened here.

 

i polar aligned with 3 stars, and then cleared my configuration and then did a 5 star point file, then did the good 8 minute exposure, and immediately after my short slew did the awful 5 minute one.

 

so what is the best way to diagnose my system?  what should i be looking at here?

[prefetch]

i've been tinkering the last hour.  i cleared my config, did a 3 point polar alignment (picked option #1) and then i did a auto pointing file, and loaded it and ran calculate on it and this was the result:

 

 

after this, i didn't slew, i just went ahead and did a 5 minute exposure and this is what came out:

 

 

it's so-so, but the stars are really a bit too egg shaped.

 

so, any advice on how to improve?

[lukepower]

HI Prefetch,

 

my first guess would be two-fold: Increase the coverage of your pointing file (seems relatively small to me), and make sure everything on the mount is firmly fixed (all screws tightened). ANd make sure you do not have any mirrors moving or similar. I would also check the PID settings of the motors, if they are not well-tuned the mount can "dance" around the target coordinates depending on where in the sky it is pointing...

[prefetch]

thanks lukas.  yes, the pointing file isn't that big.  i suppose i should make a larger one.  i'll do a check for looseness on the mount and OTA.  the mirror is locked.  the PID settings are really quite good right now.

 

i guess i'm curious about how to read the output of autoslew here.  the one window on the left says "Pointing Error Azimuth: -9.64" and "Pointing Error Altitude: 28.97" which seems kind of bad to me, but then in the window on the right it says "RMS RA Error after fit: 00.46 arc min" and "RMS DE Error after fit: 00.21 arc min" which seems really good to me.

 

i'm curious what a "good" configuration looks like as far as the numbers go.  also, what the heck is "collimation"?  sounds like they are using the term to me "how level is your mount" or something as opposed to the mirrors lining up in your OTA.

[lukepower]

Polar error altitude/azimuth refer to your polar alignment! The rms values on the other hand specify how good the calculated model is (lower is better). Autoslew calculates a multitude of errors like collimation, flexure and so on, so cool

[GeorgeCarey]

Your polar alignment could be better. The values at the left are -9.64 and 28.97 which is the raw polar error.

Try to get it down to about 2 arc minutes or better.

The values on the right are the best values Autoslew can come up with after trying to compensate for all the errors.

'Collimation' here is the mount/telescope collimation error. The axis of your telescope is tilted 10.32 arcminutes relative to the mounting plate.

If you lift the front end of the scope up a bit it could be improved, but it is not essential.

[prefetch]

okay, so it looks like the 'raw' polar error could be improved.  i can keep iterating on that.  i'll go for 2 minutes or better.

 

but if the RMS correction is sub arc-minute, i would think that would be good enough to get round stars at a 5 minute exposure.

 

am i mistaken here?  what do you guys typically achieve on RMS correction?

 

george, so does this 'collimation' mean that where the OTA connects to the mount plate?  like it's not square all the way?  if so, i guess i could use some shims to try and square it up.

 

also note that the mount is currently on a non-permanent setup (on a CGEM tripod, which is on rollers with wheel locks) and so it's not ideal!

 

thanks again for all your help everyone!  the learning curve is steep, but this is a lot of fun and i love this new mount technology!

[lukepower]

Prefetch,

 

A stable pier/tripod is essential

But anyway, i would try to make everything stiff and a bigger (= over a wider Area of the sky) pointing model...

[GeorgeCarey]

Yes, mount collimation is where the scope is misaligned on the mount plate. Some thin metal shims would do the trick. However, if you are taking the scope off the mount on a mobile setup it probably will be different each time. There is a way to check mount collimation which is something like....

Clear configuration so you are not using a pointing model.

Align on a star DEC close to zero and on the meridian. Sync on the star.

Slew the scope to the opposite side of the pier.

Do a goto to the star.

If the mount goes to the star without doing a meridian switch then you will find the star to be off centre in RA, but centred in DEC.

This is due to the collimation error.