A problem with drift - Any ideas?

[[1ponders]]

I'm slowly coming to terms with my new imaging setup, but I came up against a problem last night that I'd like some feedback on.

After my session last night I found all my images has drift symptoms. I was accurately polar aligned, the autoguiding was behaving better than it has for the last week. Its been terrible seeing for a while. Its been difficult trying to guide with the stars flaring so much. I'm using the Losmandy GM-8, with a Meade 8" sct (with mirror lock to help reduce mirror flip/slip) for imaging and the Orion 80ED for guidescope. K3CCDtools V2 for autoguiding and ToUcam for the guide camera. Camera was 300D with 6.3 meade focal reducer.Total weith around 10Kg, about 70% carrying capacity of the mount.

One of the issues I've been having is the guide rings I bought are really poorly made and the guidescope has been known to slip. However when you look at the images the one of the Lagoon the drift is in dec (90 deg to what I would have expected slippage to have produced). One of the Dumbell is in dec and the other is in RA???? :confused: :confuse2:

All images were taken between 6:30 and 12:00 last night, with the Lagoon taken first and the Dumbell at around 10:00ish onwards.

Any ideas would be appreciated. I'd really like to get this sorted out before the Astrofest next week.

[Robby]

Hi Paul,
If the guidestar is locked on and it looks like it's tracking ok (+/- 2 error on K3) then they other things that can introduce drift error are...
1. Differential flexure. This is when the movement of the mount causes the guidescope the "flex" relative to the imaging scope. Guide guidescope mounting is essential.
2. SCT mirror flop. As the scope moves the SCT mirror can "flop" around a bit causing the imaging image to "move" relative to the guidescope. I wouldn't imagine this is you problem with the new Meade 8". You can get a kit that will lock the mirror. This is ok so long as you an external focusser (eg crayford JMI like you have).
3. Poor polar alignment. This will cause field rotation errors. It's easy to spot as you will notce each Raw is rotated by the same amount each time. Polar alignement really does need to be spot on for astrophotography even with auto-guiding.
4. Too short f/ratio on guide scope. You should be ok here though, but perhaps try a barlow in the guidescope & see if that helpds

You probably know all this anyway! Welcome the joyful world of astrophotography.

[[1ponders]]

Thanks Robby.

Differential flexure. Yes I'm not very happy with the guiderings I received. I've already rung the supplier and said I want another set. The adjustment screws are so loose in the threads that when tightening them up they will splay sideways and end up angled if I'm not careful with the tension. And when pointing towards the zenith the weight of the guidescope will still "bend" them. I ended up setting the guidescope up so that the front adjustment screw sit against the back of the lense cell to ensure the scope doesn't slide (has happened a couple of times). Having the GS so far back then makes balancing an issue.

The Meade comes with a mirror lock as standard so I was using that. It was one of the things that helped me decide to get the meade as I was aware of the mirror flop issue.

I thought that polar alignment was pretty good, but now that you mention the rotation of the Raw I did notice that when guickly flipping through them. I'll take more time next time to make sure its more accuratel

The other issue that I thought it might have been is a backlash issue. The autoguider seemed to struggle at times to get back to the zero mark when it did move off the center. The Dec line on the drift explorer window would move parallel to the zero line for quite a while before crossing back down. The same with the RA graph but nowhere near as much as the Dec graph line. I tried varying the K, Q, dead zone and interval parameters but when they started to make a difference the graph started zooming up and down all over the place. I'd then lower the settings and things would settle down for a while, then the dec would go up or down and then stay parallel again for a while. Does any of this make sense?

[Robby]

Sounds like you could have a flex problem.
Re Backlash... No problem on RA, as the motor is always moving in the same direction at guide speeds (ie 0.3x 0.5x & 2x).
Backlash can be a problem in DEC, but only initially. If you have to do any adjustment in DEC it will only be correcting in one direction. Also any DEC correction that it is doing is because of mis-polar-alignement. If you are perfectly polar aligned then you can turn DEC corrections off. I actually do quite often if I have poor seeing, as DEC corrections can often do more harm than good.
Cheers

[[1ponders]]

Yeah I've noticed that. At times I've turned the autoguide off and the dec has just hummed along nicely on its own :) Not last night though. So maybe its a combination of both flexure and polar alignment. If its fine again tonight I'll have another go. I'll spend more time than usual and make sure the polar alignment is running smoothly for at least 5 minutes. That will take one variable out of the equation. I might even have a read of the Losmandy manual and see how the backlash compensator works on the mount.

I'll see what I can do to strenthen the guiderings. If the new ones are no better when they get there I'll send them back and maybe get a set of Losmandy guiderings. I've heard they're pretty good :) What I'd really like is to get a "Side-by_Side" mount head so the guidescope is not attached to the imaging scope. Might have to work on that. couldn't be too difficult to make.

thanks Robby

[gbeal]

I/we both have the Losmandy Guidescope Rings, and they would have to rate as one of the better buys I made.
I can assist with the side by side as well, I made one up (albeit for the G11). I use this for the guidescope, but it does introduce more weight.

[[1ponders]]

Thanks for the info Gary, I'll definately look into the Losmandy option. I'm going to do a bit of research into the side by side option and see what design options are available.

[GrampianStars]

G'day Y'all
Paul
I've bolted then spot welded the rings together to make a cage
bars top n' bottom aligned perfectly
stops the flexture real good

[asimov]

VERY nice idea grampianstars. I did the same on one of my scopes...for reasons other than guiding. Structural logic.

[[1ponders]]

I've thought about doing that as well GS to remove the flexure. Unfortunately caging it doesnt' help the problem with the screw and that is I believe, part of the flexure problem. Besides after spending $140 I expect a product to work as advertise. I shouldn't have to start welding and retapping threads to use the rings as they are supposed to work.

[RAJAH235]

Just a thought Paul, There's definitely a prob. with that assy. The screws are almost fully in. Not to mention the loose,(overlarge & badly tapped), thread. It should be vertical to the tube. When the replacement arrives, I'd also be using something, (small, 1"? sq. curved plate with rubber pad), between the end of the bolt & the tube to increase the surface area to give more grip & save your paintwork & not damage the tube. :D L.

[asimov]

Not to mention, the less screw you have poking through, the less leverage on the screw.

[asimov]

I'm thinking now after looking at that pic...The rings are not the correct size for that OTA!? No doubt the OTA is close to be central in the rings Paul?

[[1ponders]]

That's what I thought as well John. But apparently its they are the right size rings. The other thing I noticed tonight is that the screws look like they are hard plastic. When the new ones arrive I'm going to do as Laurie suggested and place little feet on them and get longer screws. These ones are ridiculously short. Maybe even tap it out to a large gauge screw. Very frustrating.

[Striker]

Paul the ones I ordered have rubber tips on all screws.....and they are a much tighter fit ring for the orion 80Ed.

http://www.astro.premcom.com/ADM/ADM%20Products/MDS%20Description.htm

[asimov]

Yes. I would have suggested drilling & tapping to the next size up, but I didn't want to frustrate you further lol. Perhaps your pic is a bit deceiving (to me at least) It still looks not the right size rings to me!? As a temporary measure, perhaps plenty of thread tape round the screws to tighten the beggers up in the thread?

[[1ponders]]

They look heaps better than the ones I have Tony. I think I'll send mine back and get a refund.

[gbeal]

My sentiments exactly.
The choice is a juggle between getting a slightly oversized ring, and just having a little adjustment. The alternative is a set which are wildly larger (like your picture would indicate), and having stacks of adjustment, but at the expense of stability.
Given you now have a guidescope, perhaps look at the appropriately sized Losmandy rings.
I can help if you need.

[[1ponders]]

Yep they are wildy large Gary but the screws are so short I have almost no adjustment

[tornado33]

Have you considered a SCT off axis guider, http://www.meade.com/catalog/lx/lx_accy.html
They will stop flexure once and for all, added advantage, much less weight than a guide scope
disadvantage, cant guide on actual object being imaged, eg comets, and though I have little trouble finding guidestars with my Newtonian setup http://www.users.on.net/~josiah/focuser/IMG_9933.jpg
I havent used a sct with an off axis guider so cannot say how easy they are to use.
Scott

[Robby]

Main problem with OAG's (especially on SCT's) is the difficulty finding a bright enough guide star with the webcams low-ish light sensitivity.
But if you can do it, then yes, it's much better than a guidescope. I started out with an OAG, but got so frustrated with it, that it was a no brainer moving to a seperate guide scope.
Cheers

[[1ponders]]

Painfully frustrating Scott, really painful. I've perservered with the Meade OAG for a while and as Robby says its hard to find a bright enough star off axis to use the Toucam to guide with. I won't pitch it out the window yet, but it does make a great paper weight. I might give it another go further on down the track. I'll wait and see how the gudiescope goes. Especially if I can get a "Side by Side" adapter built.

[tornado33]

With the Meade OAG, does it allow you to search radially?, that is to move in or out from the center a ways to locate a guide star. With my Newtonian OAG setup I can move in or out about the field of a low power eyepiece to find a star, as well as rotate through a full 360 degrees. While in a few cases Ive had to settle for a fairly faint guide star, in most cases I come up with one thats not too bad. Also as the pick off prisim is built in to the focusser assembly but outside the focusser tube, I get unfiltered light from the guidestar
http://www.users.on.net/~josiah/focuser/IMG_9935.jpg
The pick off prisim is above the inlet to the focusser itself and is on the edge of the light cone and doesnt in any way intrude into the images.
The screw on bottom right of image allows me to move the arm in or out, tilting the prisim so I can search radially, and the whole thing can rotate through 360 deg.
Scott

[[1ponders]]

I can rotate it 360 deg by loosening the SCT threaed collar, but the whole unit includeing the camera then rotates which means I then need to loosen the small screws in the "T"-Mount Adapter to rotate the camera back to the best position. Unfortunately the Meade OAG doesn't allow for radial movement of the pick off prism.

[tornado33]

Bummer re the no radial adjustment,
This Lumicon OAG may be better
http://www.lumicon.com/eg7.htm
Cannot say for sure but they do claim it has radial adjustment too.
List of dealers here http://www.lumicon.com/2dealer.htm
Scott

[[1ponders]]

I've thought about the Lumicon, Scott. I'm a bit gun shy of getting another OAG after the frustration of my existing one and I've been in contact with a number of people over the net who have had one and have ended up going for guidescopes.

I'm currently tossing up between getting the Losmandy guiderings and mounting rail, which I've heard nothing but praise for, or get just the rings and get a "side by side" mounting plate made. I'm definately swinging towards the "side by side" though. The idea of keeping the center of gravity as close to the mount head as possible is very appealling to me. I was quite amazed at how much addition weight I had to add to the counterweight arm to balance the setup with the Orion mounted on the top of the Meade. Lowering the amount of counterweight and keeping everything as close to the mounthead as possible has got to be a benefit imaging.

The one thing that does sort of swing me to the OAG is the complete removal of all the additional weight of guidescope, rings, rail etc and the corresponding reduction of "lever Torque" (don't know the proper name for it, but that's how I think of it :P )

Hmmm....OAG, less weight, more hair pulling finding a guidestar........Over/Under guidescope and imaging scope, easier to find a guidestar, more flexure and "lever torque" and more counterweight......"Side by Side" guidescope/imaging scope, easy to find guidestar, less flexure than Over/Under, less "lever torque" less counterweight, maybe more total weight to the mount due to increased amount of material required to construct as compared to Over/under rail........ :confuse3: :confuse3: :confuse3:

OAG winds hands down for weight........but I think the "Side by Side" wins when looking at the overall picture. That's how I see it anyway. Would love to hear some comments on my beliefs and deductions. would love to hear of some alternatives. I've even thought of the Tau Ceti XY finder http://www.sciencecenter.net/hutech/gd-acc/xyfinder/ so I can use normal mounting rings and do away with the need for a guiding OTA balanced on screws. Decisions, decisions. If this keeps up how am I ever going to afford the buy the Argo Narvis I want :lol:

[gbeal]

Paul,
the Lumicon does offer a sort of radial adjustment, but I still believe the frustration factor is way up there.
In my case the difference between weight away from the pivot (separate guidescope on top) is similar to the side by side, as the side by side is HEAVY. Maybe I could skinny it down a tad, but not that much. I did have a picture of a Tak accessory, and this made an effective side by side system, with what appeared to be WAY less weight than my homebrew system.

[[1ponders]]

My thinking Gary was that with the guidescope in a side by side position the amount of leverage, say with the mount in a horizontal position which would be the worst case scenario, would be much less than with the guidescope in the over and under situation. Like carrying a heavy weight with outstretched arms or tucked in close to the body. The amount of mount flexure and sensitivity to vibration must surely be less. Plus I believe it would be much easier to balance, especially if the side by side was on a sliding dovetail that could slide "sideways" to balance the disparate weights of the scopes over the mount head.