Hi,
I have a Celestron 9.25" + Canon 1100D setup and last night I was able to try out another piece of Camera software called entangle on Linux.
I was able to do my alignment quickly...
2 star
+ 4 calibration stars.
Then I did my Polar alignment using live preview. It was quite a good experience compared to many years ago.
I then decided to try and take a 200 second exposure of sombrero galaxy but I am getting some movement in both RA and DEC. Just doesn't make much sense. I would have thought I'd be getting perhaps some north/south movement due to minor Polar alignment discrepancies, but RA, not sure why.
(See attached image)
Looking at image it LOOKS like it does a "jump" between...rather than a trail.
Sombrero galaxy at the time of taking was in north West Sky.
I did try setting a timer and walking out of the shed...incase I was "somehow" making vibrations on the floor, but no help.
I am thinking I'll try the drift alignment...200 seconds track RA EAst then 200 seconds track RA WEST...but not too sure this will help too much...well not the RA component of the drift.
It’s highly likely, looking at your image, that it has nothing to do with PA and everything to do with the smoothness of your mount. I also have a C9.25 and I can tell you that even guiding that sort of focal length is a nightmare let alone unguided. Easiest way to describe it to you is you’ve got a focal length of 2350mm. Grab a stick 2.5m long and hold it out in front of you and try and keep the other end of the stick steady. Nigh on impossible. This doesn’t mean you can’t image with this setup. It just means setting some limits. If you try for 30 second exposures, and lots of them, you’ll probably find you can throw out the ones where the movement has impacted the sub and stack the remaining ones. You should then be able to do some pretty good galaxies or planetary nebulae.
What kind of mount.? I agree 200 sec is too long unguided. Also mirror slap on a DSLR if you do not set up for the mirror to lock at least a few seconds before the shutter fires with a remote. Altitude can be tricky and for some reason it moves around more than RA . Perhaps something to do with refraction. Drift guiding is definitely a good way to fine tune.
Ryan - Thanks for that visualization. That give me something to think about.
Yes, I started to wonder if it is an intrinsic limitation of the equipment. Keeping your description in mind, there might be some "positions" that have less errors? like if the object being photographed is closer to the meridian?
Perhaps if I imbalance the weights a little might reduce the chance of the "jumps"? i.e. something for motors to work a "little" against...to dampen things? Perhaps?
I think Alex has a point, I need to do a bit more testing, to try and understand what makes it more likely, less likely.
Sunfish, the mount is a "Celestron CGX German Equatorial Mount and Tripod". What do you mean by "Also mirror slap on a DSLR if you do not set up for the mirror to lock at least a few seconds before the shutter fires with a remote." Vibration of camera when it starts the photo and ends photo?
Yes, I should probably look at a guide. Once I understand issue a bit more, I may look at getting one.
At ISO 100 you should be able to expose for 15 minutes maybe more...no doubt the star trails will be long but some patterns should appear.
Do three runs.First with weights where you balancing tells you is ideal then move them up say an inch or two take another 15 minutes and then down an inch or two...check your clutch is engaging properly ..I was caught once where I thought it was tight but it was running into the mount and was not tight at all...also if you decide to post your photos mark direction and start..ie start exposure with say one minute, shut off, wait two minutes then start 15 minute exposure..this way you get a dot showing where the trail started...also listen to the motors and note if you hear any clicks..that will be jumping gears..could be worm gear not meshing..if you hear regular clicking switch off and pull it down and inspect worm gear.
However...you could try to do the 15 minute run and find at the end have a perfectly round star..could happen.
Last night I switched on and selected ( as I thought tracking) then trails..mmm yes lights on..what the ??? ..I had not pressed sidereal rate so the mount never was tracking...and power..check that you have good power as that just possibly could be your problem.
At ISO 100 you should be able to expose for 15 minutes maybe more...no doubt the star trails will be long but some patterns should appear.
Do three runs.First with weights where you balancing tells you is ideal then move them up say an inch or two take another 15 minutes and then down an inch or two...check your clutch is engaging properly ..I was caught once where I thought it was tight but it was running into the mount and was not tight at all...also if you decide to post your photos mark direction and start..ie start exposure with say one minute, cover the scope and wait two minutes then take off cover and go15 minute exposure..this way you get a dot showing where the trail started...also listen to the motors and note if you hear any clicks..that will be jumping gears..could be worm gear not meshing..if you hear regular clicking switch off and pull it down and inspect worm gear.
However...you could try to do the 15 minute run and find at the end have a perfectly round star..could happen.
Last night I switched on and selected ( as I thought tracking) then trails..mmm yes lights on..what the ??? ..I had not pressed sidereal rate so the mount never was tracking...and power..check that you have good power as that just possibly could be your problem.
Yes I do mean vibration at the beginning and end of a dslr shot. Looks a bit like that. If you do not have any means of locking up the mirror and waiting a few seconds before taking the shot, turn off live view before exposing , wait ten seconds and use a remote.
Ryan - Thanks for that visualization. That give me something to think about.
Yes, I started to wonder if it is an intrinsic limitation of the equipment. Keeping your description in mind, there might be some "positions" that have less errors? like if the object being photographed is closer to the meridian?
Perhaps if I imbalance the weights a little might reduce the chance of the "jumps"? i.e. something for motors to work a "little" against...to dampen things? Perhaps?
Joe
Yes there will be areas of the gear that suffer more from stiction etc than others. Also counterweight loading ( usually East heavy ) to hold constant load on one side of the gears is common practice in astrophotography. I’d still suggest that even with those things considered, you will struggle to get long subs with this setup without guiding and pretty good guiding at that. The short length subs really might be a more reliable way to go.
I tell you what it must be catching.
Last night I changed my 80mm to 115 mm on the HEQ 5 mount.
Now I have had it before on the HEQ5 ..took a reasonable M31 using the same set up...last night however..trails..moved the counter weight until they got a lot better..did a few captures and on inspection some trailed some not...I have no idea why.
The only thing different is the scope is on the West side whereas it was on the East side for M31.
Alex
I would do the polar alignment first and then do the star alignment.
Reason is if you do a star alignment and then do the polar alignment, the star alignment will be out after making the physical adjustments for the polar alignment.
Polar alignment is for the mount, star alignment is for the scope.
You should be able to get 30 second exposures unguided until you get a guiding setup.
The fact that the scope is moving in DEC and RA can point to not the best polar alignment. How did you actually do the polar alignment and how could you tell is was good?
The worm gear or whatever type you have is not that precise and can cause the jumps that cause star trails on short exposures. PEC can help with that. The scope is tracking, not guiding.
Limit your exposures to 30 seconds or less and see how you go. Take lots of them. No point going longer than about 30 seconds or you will have issues without guiding.
