Jump to content

sgl_imaging_challenge_banner_31.thumb.jpg.b7a41d6a0fa4e315f57ea3e240acf140.jpg

Guiding Problems... (WARNING: Big-ish Images!)


Recommended Posts

I can't get my small brain around this, because there are too many variables for me to figure out, not to mention the whole flip-left-right-up-down-backwards-inside-out thing that seems to happen with my images between Artemis and Maxim.

I've got my 314L+ underslung on my MN190, and my Titan attached to my 80ED DS P through a diagonal. The two scopes are mounted side-by-side on a dual bar, with ADM dovetails throughout (no flimsy SW ones). With the scopes parked, the MN190 is to the east, and the 80ED to the west. The Titan is serving as the guide camera through PHD, set to 3 second loops.

Here is a single 240 second sub through the 314L+ (HA filter on it) Please excuse the image size, but I wanted to leave them as large as possible to show what's happening:

GuidingProblem240.jpg

Here's a 480 sec:

GuidingProblem480.jpg

And here's a 960 sec:

GuidingProblem960.jpg

I didn't touch the scopes or mount at all throughout.

Here's the three subs overlayed so you can see the general trend upwards at about 30 degrees:

GuidingProblem.jpg

So what's wrong?

For one thing, after all the flips and stuff I have completely lost track of where N,S, E and W are in this image (it's NGC281 if you haven't already identified it), which is making it difficult to determine which direction the drift is happening.

Next, how is it even possible?

The scopes are pretty well balanced, so I don't think it's being caused by imbalance.

Surely this is the sort of thing that PHD and the guide camera should be correcting, no?

Is it because the guide scope is shorter focal length than the main scope? I figured the smaller chip of the Titan would offset that - the field of view of each scope/cam combination is roughly comparable.

The mount's polar alignment isn't hopeless. Each session, I start by checking the polar scope. Polaris has remained on its circle for months without any correction for months. I figure that I don't need to check PolarFinder and establish where Polaris should be on the circle - if it is moving around the circle without deviating from it, then it must be correctly aligned, right? If it wasn't it would drift inside and outside of the circle as it moved around?

So what's causing it? Any thoughts (other than the obvious - that I am a fool newbie doing something silly)

Thanks everyone.

Link to post
Share on other sites

PHD/guiding cant cope with everything

Things like flex could be causing it, how is it all setup, which dovetails and rings etc? I would remove the diagonal and use a extension tube instead, it will be better in the long term.

Also poor polar alignment causing drift in one direction in DEC

PHD isnt quite as easy as it wants to believe, sometimes just playing with the agressiveness settings etc can improve the guiding a lot.

Im sure someone who can analyse this better will be along soon :)

Edited by shaunster
Link to post
Share on other sites

The steady march off the plot in one direction is down to polar alignment. Or lack of it. If the mount base is not level you will get drift in both RA and Dec at the same time. It could be flexure but not balance.

You should not be using a diagonal for imaging. The image orientation problem is almost certainly down to the diagonal.

The field of view comparison between main and guide scope is irrelevant. It is the image scale you need to look at. Provided the main scope image scale is no more than ten times the guider image scale the software should cope. If you have MaxIm I cannot understand why you would want to use anything else for guiding. Some info here Astrophotography on guiding with MaxIm.

Is there a conflict between the main and guide cameras. With any camera with a shutter and if you guide through the main scope, you cannot guide when downloading the main image as the shutter is closed. Does the guide star do all its drifting when the main image is being downloaded. Are you using Artemis software? You can run the guide camera, main camera and do all your imaging and guiding as well as all the calibration, aligning and stacking using MaxIm. No software conflict possibilities at all.

Dennis

Link to post
Share on other sites

Thanks everyone for your suggestions. Sounds like alignment is maybe not as good as I had hoped.

The steady march off the plot in one direction is down to polar alignment. Or lack of it. If the mount base is not level you will get drift in both RA and Dec at the same time. It could be flexure but not balance...

And I only just re-attached my scopes! :)

OK, so I should check how level the base of the mount it. I checked the top of my pier with a bubble level, but interestingly that didn't agree with the bubble level in the EQ6. Which should I trust? How accurately are the EQ6 bubble levels installed?

...You should not be using a diagonal for imaging. The image orientation problem is almost certainly down to the diagonal...

You're right, and I am getting an extension tube this week. However, I should point out that the actual imaging isn't being done through a diagonal, just the guiding. The difference between guide cam and imaging cam certainly doesn't help me keep my bearings, though.

...The field of view comparison between main and guide scope is irrelevant. It is the image scale you need to look at. Provided the main scope image scale is no more than ten times the guider image scale the software should cope. If you have MaxIm I cannot understand why you would want to use anything else for guiding. Some info here Astrophotography on guiding with MaxIm...

Thanks for the link. That looks like a fantastic resource that I should definitely have read sooner. MaxIm has always intimidated me (it's on loan - I've borrowed someone's laptop that has it on, for me to see what it's like before I fork out the £600 on it), because it's such a huge programme and I didn't really know where to even start. That link looks like just what I need.

...Is there a conflict between the main and guide cameras. With any camera with a shutter and if you guide through the main scope, you cannot guide when downloading the main image as the shutter is closed. Does the guide star do all its drifting when the main image is being downloaded. Are you using Artemis software? You can run the guide camera, main camera and do all your imaging and guiding as well as all the calibration, aligning and stacking using MaxIm. No software conflict possibilities at all.

Thank you, Dennis, you've definitely sold me on MaxIm. The whole reason I borrowed a copy was because I liked the idea of being able to use one programme to do everything. My dome desktop, bless her, isn't the quickest tool in the box, and struggles to do more than one thing. But a conflict isn't the problem here, because that computer runs the guide camera, but the little laptop runs the main camera.

Thanks again for the link. That's my reading for the day.

And thanks everyone else for their thoughts.

(Nadeem, you're a harbinger of doom getting me worried about my mount!)

Link to post
Share on other sites

looking at the image it looks as if it is North to the left and then it is upside down! If that is right the drift is in RA.

Dennis

PS: I intend to put some info on using MaxIm for calibration/aligning/stacking on my web site soon.

Link to post
Share on other sites

Thanks again, Dennis. The headache I've had ever since the pressure dropped was getting worse trying to work that out!

I love the photo at the top of your homepage, by the way. My obsy is full of cat hairs too!

Link to post
Share on other sites

Hi,

I've been following this thread with interest as I have a MN190 and am looking forward to imaging with it. With regard to the bubble level in EQ mounts, they seem to be highly inaccurate. I have just replaced the altitude scale on my NEQ6 because the factory installed one was several degrees out, when reinstalling the new scale I levelled the mount using a decent builder's spirit level and noticed that the mount's bubble was well on the proverbial!

Best of luck solving your guiding problems

Regards, Herrman

Link to post
Share on other sites

Cheers Herrman. Do we all agree, then, that it's safe to assume the base of an EQ6 is flat enough to rely on? What I mean is, I set up my pier the same way Herrman has, by using a bubble level (not a builder's style one, but a round bubble level), so can I assume that this will mean my EQ6 is as level as I can make it?

Link to post
Share on other sites

Thanks, Ransu. Finland, eh? I work with a Finn. Nice lady.

Anyway, I did check the alignment of the polar scope back before the rig was permanent. It was easier then to move the mount around to find terrestrial objects to test the alignment.

Fortunately, I think I may be getting somewhere with this issue:

NGC281TrackTest2.jpg

Ignore all the faults with the image - for one thing I forgot to switch on the 314's cooling which is why the image looks so noisy! The point of the exercise, however, was to test the drift, and in that respect things are looking pretty good.

I am now using my new Atik OAG (still using PHD), so was able to remove the dual bar, and can now use just one scope. It's made the imaging assembly much heavier, but the overall weight on the mount is much reduced. The image represents a stack of x8 900sec images taken through my 80ED (hence the wider field than the previous MN190 images). However, I think if the problem was still there, it would be clear in these images. I chose to use this lighter scope for these tests so that I could make adjustments to the alt/az bolts on the mount without putting excessive strain on anything.

Now that it looks like the problem was down to the use of the dual bar and guidescope arrangement (either because of the bars themselves or because of the complicated balancing it introduced to the system), the next step will be to put the MN190 back on, and see if the tracking remains good.

Link to post
Share on other sites

I just read this thread and was going to say it is down to differential flexture between the 2 scopes but you have sorted that by using an OAG (Horrible things, I can never get them to work!)

Link to post
Share on other sites

Haha well, so far so good, but I am very aware that the love isn't unconditional. Whilst pointed at Pacman (hardly a sparse part of the sky), on 5sec exposures with PHD, there was one reliable star in the field of view to choose from! OK, so it's an f/7.5 scope, and the imaging will often be done with an f/5.3, so my chances of finding a star might be a little better. But if I arrive at a target and there is no guide star to choose from, I'm stuffed; I can adjust to find one. All I can do is keep increasing the exposure length on the guide camera and hope for the best.

Link to post
Share on other sites

I'm surprised you find it so difficult to find a guide star. I am currently guiding with an f8 scope and Atik16 and it finds loads of them. I generally use a 3-4s exposure.

Surely this is not down to PHD, either the camera can see stars or it cannot?

Dennis

Link to post
Share on other sites
I'm surprised you find it so difficult to find a guide star. I am currently guiding with an f8 scope and Atik16 and it finds loads of them. I generally use a 3-4s exposure...

That's more what I expected, rc. I think the baffle in the nosepiece might be the issue, but we'll find out soon enough (when I get the unbaffled nose)

...Surely this is not down to PHD, either the camera can see stars or it cannot?...

That's what I can't figure out. Although by reading further down it looks like I am not as familiar with PHD as I should be.

You're not using a Ha filter in front of the OAG are you? It might explain the lack of stars on the guide cam.

Good point, Tony, but no. The OAG sits in front of the filter wheel, so no filters are affecting it.

with phd and an oag make sure you take dark frames so that you will increase the sensitivity of the camera, also bin it 2x2

OK this bit is embarrassing, since I didn't realise you could bin in PHD. I'm assuming that's somewhere in the "brain" settings?

That would certainly help with the sensitivity, but would it not make the stars potentially less defined, and more difficult to use for accurate guiding?

Thanks everyone. ;)

Edited by fatwoul
Link to post
Share on other sites

I use MaxIm and set x2 binning on my guide camera pretty well all the time. Centroiding is not a problem and binning helps to keep the rapid fire corrections under control without going to a longer exposure. My guiding RMS is normally well under 0.1.

Dennis

Link to post
Share on other sites

with my oag i use bin 2x2 and i take darks at 1.5 to 2 secs sampling and i find guide stars with no problems. the stars are a bit flattened but that's due to the fact that these stars are at the edge of the mirrors of the scope. guiding is done with almost no problems, if i have really bad seeing i could loose 1 or 2 frames

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.