PHD: Flat graph, still trails?

[Uranium235]

Exactly as the title suggests. I was out shooting M45 the other night and even though the PHD graph was perfect, I was still getting slight star trails, in the the same direction on all parts of the image (and the image seemed to "creep" slightly).

After about 20 min of this I went down to have fiddle with the finderguider and choose a couple of new alignment stars for the mount. This seemed to clear it up a bit, but im sure it cant be the PA otherwise PHD would have given me loads of grief.

Is this flexure?

[Komet]

It may well be that your finder and your scope are looking at different things and you are seeing rotation. It also is likely to be some form of flexure. The guiding graph is only a reflection of what the guide scope is doing. Your main scope might be banging about all over the place.

[Uranium235]

Thanks Mick, Ive been hoping somebody might know something about it. Its only recently that this has started happening. Theres no rotation in the fov, the movement is all in the same direction - even on the stars in the middle of the frame. I will attach a rejected sub from the propellor I was working on. If you look closely you will see the stars arent quite right.

Ive since stiffened up the finderguider and tightened everything up (tube rings etc..). But I noticed another problem, sometimes the flattener does not sit square in the ED80 focuser. In fact I was quite alarmed that it was rattling about slightly (it looked like it was going to fall out!) until I really tightened up the screws with a pair of pliers, finger tight isnt tight enough it seems.

To be honest, I think the whole push to fit arragement on the ED80 needs to be replaced with something that uses a self centering compression ring (the Baader ones look quite nice... nearly 100 quid though!).

Cheers

Rob

[swag72]

I have come to hate the push fit and screws that the telescope world seems to use. I much prefered the screw in arrangement with the redicer and 120ED focuser, but the focuser was ****. The push fit leaves so much margin for error I think.

I hope you get it sorted Rob.

[ollypenrice]

The original problem can only be flexure or polar alignment, really. The test for Pa is twofold. You pass the first one (all the same direction in the distortions) and the other is, does the the last sub show roation with regard to the first? Overlay them in Ps to find out, maybe?

Flexure could be coming from rocking of the push fit into the reducer. Seems likely to me. I always ditch brass compression rings for starters. The devil's confetti, each spacing devoted to it's own brand (and even Tak into Tak got stuck and had to go through the usual Olly amputation!)

Olly

[blinky]

I have exactly the same issue and have never really got it sorted - I suppose the best thing is to use an OAG but I just done get on with them and prefer the side by side arrangement.

[Uranium235]

Thanks Olly, I had a litte look at a few of last weeks images and compared the first and last image from that particular run. The results were... odd to say the least. No rotation as such but the misalignment increased from one side of the frame to the other. I did a little screen grab to illustrate.

Im starting to think it might be the push to fit arrangement now and the camera chip isnt square with the light cone.

Sara, at first I just used the screw-in method of attaching the reducer, but I found it really difficult to rotate the camera to suit the target - hence getting the FLO push to fit adapter. Whilst theres nothing wrong with the FLO adapter, its the actualy drawtube on the ED80 that is the weak link - those two screws do a poor job of holding everything securely. Fine for an eyepiece, but rubbish for holding a reducer, filterwheel and camera.

As blinky mentioned, OAG might be the best way to go to eliminate flex but the only problem with that is ive got limited travel left on the focuser - perhaps 30mm at most.

[swag72]

I always ditch the brass compression rings for starters. The devil's confetti, each spacing devoted to it's own brand

Olly, by brass compression rings, do you mean the rings inside of the focuser that compress as you tighten the screws? If so, I'd love to know what you replace them with and what differences you see.

Sorry Rob, hope you don't mind me asking in your thread!

[steppenwolf]

Differential flexure is a major problem for me even with everything tightened up fully which is why I am about to flirt with an OAG for my mono + filters imaging although I will still be using a guide 'scope for my larger sensor colour CCD.

Slippage of push fit components is a real contributor to this kind of issue and even my Feathertouch focuser is a push fit connection!

Olly, please don't get me started on non-matching compression rings!! I actually like compression rings (with two screws), they do a fine and solid job but only with some fittings, others literally pop out as the ring in right on the edge of the undercut - madness not having an industry standard!

[martin_h]

I had exactly the same problem with push fit into a SW focuser, my solution was to drill and tap a 3rd hole opposite the original 2, and to tap it out to 5mm so I could get a decent size screw in there.

[Sp@ce_d]

I've been through the same, the weak link is definitely the screws. I searched high & low for an adapter that screwed between the flattener/reducer & the SW tube thread but it looks like it would have to be custom made. FLO's push fit would be perfect if it had a shoulder & ring to tighten it to the tube.

Something else that made a big difference was collimating the lens cell & focuser. I discussed this (with piccys) over on Sara's thread here: http://stargazerslounge.com/imaging-image-processing-help-techniques/159724-one-good-corner-when-not-using-reducer-how-come-2.html

[kookoo_gr]

If the problem is with the compression ring of the focuser i would suggest the hotech sca adaptor First Light Optics - Hotech 2" SCA T-Adapter the imaging system is solid as a rock at the focuser and centered at the optical axis. I had a similar problem like you describe and with the addition of this adaptor i will never use compresion rings or screws at my imaging system

[swag72]

That looks good - Clearly the left hand side of the FLO image goes into the focuser, the other end is what? A 2" tube? Also, how would this affect focus with a CCD?

I have a 0.85x reducer screwed into a FLO adaptor First Light Optics - FLO Adapter for Skywatcher Focal Reducers that is then able to fit into the focuser - Would the Hotech accept my 2" nosepeice and would it affect focussing distance?

[Uranium235]

If the problem is with the compression ring of the focuser i would suggest the hotech sca adaptor First Light Optics - Hotech 2" SCA T-Adapter the imaging system is solid as a rock at the focuser and centered at the optical axis. I had a similar problem like you describe and with the addition of this adaptor i will never use compresion rings or screws at my imaging system

The problem is the lack of a compression ring on the focuser tube, the FLO adapter works perfectly - it might be that the safety undercut isnt meant for screws. It was the same situation with the coma corrector on the newt, unless the screws are mega-tight it has a habit of going skew-iff when moderate sideways pressure is applied.

Ive found the Baader adapter on TS - and it looks real nice. Self centering, and it can handle a massive load (up to 24kg) so you could pick the telescope up by it, wave it around and it wont budge (not that youd want to!).

Baader 2' KlickLock Klemme M56 für Skywatcher und Celestron

Just a thought, does anyone know of a cheaper version of this? Im surprised that AE dont have something similar, anyone who could knock a load of these together for a reasonable price would do quite well.

[martin_h]

I looked at that adaptor...then I looked at the price!...err no thanks.

I only went through all that faff just to get an PL filter in the train, I might just take it out and go back to how it was before...screwd direct to the focus tube

[ollypenrice]

Sara, yes, it is the lack of an industry standard on compression rings that makes them a menace. Adding a third screw is a good idea.

When I tear out brass compression rings and pelt passers-by with them* in a steaming temper I don't replace them with anything. I couldn't care less if my EPs get marks on the barrel. I look through them, not at them (not after the first week of caressing new arrivals anyway...) I never get any issues with push fit on my imaging scopes but screw fit would be better provided you have a rotator.

Olly

*No real passers-by have been harmed in using this piece of hyperbole.

[Uranium235]

Im out imaging at the moment and after a few adjustments by mounting the FF/FW/Cam under gravity (pointing the ota straight down) and using pliers to tighten the thumbscrews so I think its going to stay put tonight

Initial signs are promising, just need to smooth out the guiding a little more before I can definitely say its fixed.

[Starflyer]

I use cap allen grub screws as replacements for thumb screws, not as convenient, but you can get them tight

If you have a piggyback arrangement (finder guider etc.) and are getting flex then a block of polystyrene under the guide cam, between it and the tube, and a wrap or two of insulating tape can help hold it steady.

[Uranium235]

I use cap allen grub screws as replacements for thumb screws, not as convenient, but you can get them tight

If you have a piggyback arrangement (finder guider etc.) and are getting flex then a block of polystyrene under the guide cam, between it and the tube, and a wrap or two of insulating tape can help hold it steady.

Thanks starflyer, Im already on the case as last nights imaging run eliminated PA as the cause of the problems, so it must be flex.

Ive just stiffened the finder up even further with lashings of extra masking tape, the polystyrene block is a good idea - think I might have some lying around somewhere.

Its going to be cloudy for the next few days (probably because I brought a 30A power supply today..... powwwer!!), so it will be later in the week before I get to try it out again

EDIT:

To confirm the flex issue, Ive just compared a couple last nights luminence frames from M45, one hour apart - and it definitely seems that the image is creeping all in the same direction, but at least the camera is now square to the light cone as the drift gave the same star spacing all over the frame.

[RobH]

It'll definately be differential flexure, especially if you're seeing a gradual march across the field over a series of subs.

BTW....your guidescope doesn't need to be pointing at exactly the same target your imaging scope's pointing at (obviously if it's tens of degrees out there will be issues!!)....close is perfectly alright. I never bother wasting time trying to point my guidescope at the same object...I just adjust it until I've got some good guiding stars.

Cheers

Rob

[Uranium235]

Thanks Rob, I guess Ive got room to tighten the screws on the finder then - just a wee touch but not too much becuase PHD is what I use to assist me in calibrating the goto.

Rob

[Uranium235]

I can now (touch wood) say that the flexure problem has now gone. When I was messing with the worm gear a couple of days ago I noticed that the saddle hex bolts were a bit loose, so I tightened it all up and added extra superlube. I think this, combined with all the extra tape on the finderguider has cleared things up nicely. Even the guiding was behaving the other night - managing to knock out 35x360s subs without dec backlash.... I love it when a plan comes together