Help diagnosing tracking problems

[neilcreek (Australia)]

Hi all!

I'm relatively new to telescopic astrophotography and I'm still learning all of my gear. I keep coming up against a problem with my tracking which causes me to throw out a lot of my subs. Last night I was out testing things to try and work out my problems. There was cloud, but that didn't affect the testing, so please ignore it.

Here's an animated gif I created from a set of 44x2 min unguided exposures: http://gph.is/1OpxQL9

I'm using a 200mm f8 carbon fibre tube newtonian reflector on an NEQ6 pro mount, no guiding and no PEC as yet. There was no wind and I didn't touch the scope through the whole time.

The movement seems to be in a general direction, which I'm guessing is polar misalignment, but there's a high frequency movement in random directions, and one very significant movement in the middle.

Is this normal? Shouldn't I be able to get mostly good exposures with no trailing at 2mins on this mount? Is the only solution to start autoguiding?

Your advice would be much appreciated

[codemonkey (Lee)]

You're photographing at 1.6m focal length if the information you gave is correct, which is very long, especially for a beginner. Generally people don't recommend focal lengths over 600mm for beginners and you're way over, so you're not making it easy on yourself.

You won't get good subs at 2mins unguided at 1.6m with a mount of that class. You really need to start guiding.

As for the gif, looks like polar alignment drift along with typical period error. That big jump could be any number of things: cable snag, grit in your gears etc.

[raymo]

If your 200mm scope is f/8[=1600mm focal length], you won't get 2 min
unguided subs, even with PEC. My 200mm f/5 gets consistent 75-90 secs
[depending upon the declination of the target], and a small percentage of
usable subs out to about 100-110 secs.
Scope movement obviously caused the blip in your animation. Could be
caused by R.A. clutch slippage, or more likely the scope was too well balanced, rather than balanced slightly against the drive direction, so the
scope could flop around within the confines of any backlash present in the
gears or the worm.
raymo

[gregbradley]

No guiding = no round stars except at 15 second exposures. Even a high end mount requires guiding.

Yes good polar alignment is needed but you still need guiding.

The target is to get round stars at 10minute exposures for cooled astro ccd cameras.

To achieve that you usually need an off axis guider and a guide camera.
An off axis guider attaches in front of the camera and filter wheel and has a pick off prism that shoots some of the light looking through the telescope up to a small guide camera.

Guide cameras are usually something like:

Starlight Express Lodestar X2 (the most sensitive)
SBIG Sti (has a built in shutter which I like)
QHY 5

There are others but those 3 are probably the most popular and they are small like an eyepiece.

Some use a guide scope mounted on top of the main scope with a guide camera in it. That works but is more likely to cause problems as the guide camera is no longer looking through the imaging telescope so if there are differences in flexing between the scopes you will get elongated stars from that factor - usually its slow to build up to elongation but will damage 10 minute subexposures.


Some SBIG cameras have a guide camera chip installed inside them or fitted to the filter wheel. This is popular.

QSI, Moravian offer models with filter wheel and off axis guider built in which makes it all a lot easier. QSI being the leader there. QSI 2nd hand prices are falling as a result of newer sensors coming out that offer an upgrade to existing QSI owners so you may be able to pick up a nice QSI 683 WSG 8 for less than you would've a year ago.

That would be my pick. A QSI 683wsg 8 or the 5 model (5 positions in the filter wheel or 8). 1.25 inch filters which are the cheapest.

Greg.

[neilcreek (Australia)]

Thanks for the replies! I made a huge mistake in the original post, and I feel like an idot! My telescope is an f4 not an f8. Not sure why I typed that. So my focal length is 800mm.

Even so, from the sounds of your replies, it seems like what I'm seeing is typical of periodic error. And come to think of it, the big movement would have occurred around the time my subject passed the meridian. I probably saw the scope flopping about from one side of the gear slack to the other as gravity took hold of it.

I bought the scope second hand as part of a complete lot from an astrophotographer, and it came with an Orion auto guider camera and mini scope, so that's what I'll be using to guide with. I haven't been guiding yet because I wanted to learn one thing at a time, and start guiding when I reached the limit of what I can do with my skills and equipment. Looks like I've arrived! I'm reluctant to take that leap because of the learning curve and all the extra gear I'll need to take with me, but I do want those round stars!

Do you have any additional advice in light of this new information?

Thank you again for your helpful replies!

[multiweb (Marc)]

The consistent drift and the sharp drop hints to a balance issue. Make sure your mount is east heavy. Polar alignment will cause field rotation on long exposures depending on which area of the sky you are imaging but general drift in RA is almost always poor balance.

[raymo]

I would get a few things right before progressing to guiding, as you said, one
thing at a time.
Get routinely good polar alignment, essential whether guiding or not.
I strongly suggest the DARV method of polar alignment as being easy to
understand and execute. Google DARV for tutorial, it's easier and quicker
than traditional drift alignment. There are other methods once you start
using software when imaging, but until then DARV is brilliant.
Get a cheap intervalometer on ebay so you can leave the rig to do its
thing when taking lots of subs. Get your imaging routine down pat,
including using darks, flats, etc: ; learn sub stacking in DSS or other
stacking software, and basic image processing. When happy with all that,
move onto guiding. Once proficient at guiding, you can then decide whether to stay with a DSLR, or move on to a One Shot Colour camera, or a mono
CCD camera.
raymo