PDA

View Full Version here: : What am I doing wrong?


Nikolas
03-07-2017, 10:16 AM
I have a small issue when taking planetary photos where no matter how close I polar align I keep having to make small adjustments to the dec controls on my skywatcher neq6. The image always drifts and the only adjustments needed are in the dec (up and down keys)
What is needed to eliminate or at least counter it as much as possible?

billdan
03-07-2017, 10:54 AM
Guiding helps a lot in keeping DEC under control. Also bad seeing can make DEC jump around. The movement in DEC is also exaggerated with planetary because of the high magnification normally used ( barlows etc). Plus planets don't move at sidereal rate but that would affect RA more than DEC.

rrussell1962
03-07-2017, 11:02 AM
My knowledge of AP is very limited, but I do wonder whether you have some cone error between your OTA and the mount.

Nikolas
03-07-2017, 11:04 AM
hmmm cone error, its an sct properly mounted and balanced as far as I know, how do you check?

g__day
03-07-2017, 11:12 AM
How well have you polar aligned? What method did you use and in what part/s of the sky?

If you say polar aligned using a drift method in the area of the sky you are imaging I would not expect to see the drift you are encountering. Given atomspheric refraction varies slightly at different elevations - it is kind of impossible to have perfect polar alignment in all parts of the sky. Whether you are perfectly zoned in on the Celestial pole or the refracted celestial pole - you will get some Dec drift in various parts of the sky. I have read many debates about this between the Bisque Brothers pushing Tpoint vs the five or so most common drift alignment techniques.

Two suggestions - check and report if drift is consistent in all quadrants of the sky and at all elevations and try to see how much atomspheric jitter you are seeing on a still night versus a turbulent clear night. You don't want to be chasing the seeing.

Hope this helps!

Nikolas
03-07-2017, 12:48 PM
where I live I only have access to the Northern sky so I can't really do a real decent polar alignment. It is consistent within the camera frame that the image drifts always up or down depending on how I align the camera and I only ever have to adjust the dec buttons, never the RA buttons. I'm not much use with drift alignment etc. I roughly polar align as best I can under the circumstances.

glend
03-07-2017, 01:53 PM
Well how long is your focal length? If its an SCT and your using a barlow, your probably dealing with at least 4000 mm of focal length. The longer your combined focal length the greater any error is going to appear, in terms of drift in the frame. That is doable for planetary imaging assuming your using high frame rate video and stacking frames. Try keeping your video frame grab to 30 seconds, you should get alot of frames to stack in that period and virtually no drift. Your software will align the frames.
What equipment are you using? What software are you using?

Wavytone
03-07-2017, 04:38 PM
Nik, a few thoughts from my astrophotography back in the film days.

Mounts like the EQ6 might seem solid but at the level of magnification used for planetary photography all sorts of things show up.

1. Asymmetric flexure in the mount and/or tripod that varies as the scope moves. Add to this changes due to contraction as the temperature drops.
2. Within the polar axis, if it has ball or roller bearings, these can flex slightly as the rollers or balls pass top-dead-centre (or bottom) where the load is maximum. The result is a slight periodic wiggle in dec and it will drive you nuts trying to correct it manually especially if there is backlash in the dec axis.
3. Variable forces between the RA worm and worm wheel as the worm rotates. Given where the worm is on the EQ6 this could easily result in a wiggle in dec correlating to the period of the RA worm. This one is the reason why old-school mounts did not put the RA worm wheel up between the dec axis and the first bearing - the best place for it is at the bottom end of the RA shaft where this can't influence the dec axis. But mounts like that are big, awkward and expensive.
4. Drive rate errors (affects RA only).
5. Atmospheric refraction - especially for objects less than 60 degrees altitude. Worse, atmospheric reflection can show slow semi-periodic movements of several arc secs. Affect its apparent position in RA as well as dec.

Solutions:

A. Instead of aspiring to perfect polar alignment I used to deliberately misalign the polar axis so that while guiding the scope would always require declination in one direction only. This avoids the backlash.

B. Use an autoguider.

Nikolas
03-07-2017, 06:32 PM
It's all in my sigfile
ZWO 240mc c9.25 2x barlow etc

Nikolas
03-07-2017, 06:34 PM
ok thanks for the suggestions folks I'll try them out in the upcoming week as soon as the clouds clear
cheers

Peter Ward
03-07-2017, 06:47 PM
"My Great Wall Ute gets really twitchy above 160km/hr on the straight at Eastern Creek..."

Excuse my drole attempt at humour...but...Might be time to upgrade the mount. ;)

John K
03-07-2017, 09:53 PM
Hi Nik,

You dont need a better mount and you don't need perfect polar alignment.

I take images with a focal length approaching 10m and don't have proper alignment or a high end mount.

Just set Firecapture up so that it guides for you and it will automatically make corrections in both RA and Dec. The resulting files will look like you have perfect polar alignment. Works a treat and beats spending hours polar aligning, balancing etc.

John K.

Nikolas
04-07-2017, 08:30 AM
Hi John
Yep I'll have to set up guiding I think, I'm not a huge fan of firecapture but I'm gonna try guiding and see how it goes.
Now if Melbourne would cooperate and give me a decent night. Why is it that when I can set up and image the weather goes to crap?

Nikolas
04-07-2017, 08:32 AM
Ha ha one day when the money is available I'd like to have a permanent setup on an acre or 2 out in the sticks but until that day............

John K
04-07-2017, 10:16 AM
Hi Nik,

I think in my experience for capture, Firecapture is a winner which in my case includes automatic filter changes and guiding.

As well there is an image stabiliation (Auto Align) function, so if you tick that and Autoguide (and play around with the auto guide parameters) then you'll be on a winner and never look back - steady as a rock!

Yep planetary imaging in Melbourne is totally dictated by seeing conditions and not much point trying to image when the seeing is poor.

Clear skies.

John K.