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????
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.
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.
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?
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
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.
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.
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.
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.
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. L.
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?
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.
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?
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.
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
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A problem with drift - Any ideas?
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.
L.
