Newt Alignment Issue.

[Tandum]

I'm trying to setup a new newt but the laser is showing movement and maybe someone has some ideas on what to look at next.

What is happening is I have the newt on the mount, counterweights down, newt pointing west and center the dot on the primary. If I rotate the newt east, the dot moves to the inner edge of the primary dohnut. The focuser is now reinforced with Galv Plate, see pic, and I can hang alll the camera gear off it, in a bag with string, 2.6Kgs worth, and the dot remains dead centered.

I have unscrewed the 3 collmination screws right out and screwed the secondary center bolt tight to fully tension the spring, recentered but with the same result. I've added washers to the spider to get it as tight as I can, same issue. I've moved the tube so the rings where close to the focuser in case the tube was deforming somehow, same issue, but the bag of camera bits would have shown this as a problem I guess.

All I can think of is to remove the primary and stick paper there to see if the dot moves with the primary out of the picture. Apart from that I'm out of ideas. Also note the focuser may not be square to the scope but I doubt that matters in this instance.

I thought I'd fixed this with the galv plate, but apparently not. Does anyone else have any ideas on what to look at?

[MrB]

Could it be the tube flexing?
I know you've tried moving it already to support it closer to the focuser end, but with the weight of the primary at one end and all the other gear at the focuser end, my guess is the tube is bending like a banana.

[Tandum]

This happens with just the laser out front. It's only 1150 long with 500mm out front and 300mm out back with just the laser in the focuser. The rings are 330mm apart, I don't have a plate longer than that here. I don't see the difference between level west and level east? You'd think others would have seen this before if it where a tube problem.

[MrB]

Hmmmm :confuse3:
Perhaps there is an issue with the rigidity of the spider and the weight of the secondary?

[Tandum]

It is a pretty big secondary, 3.5" I think and I've screwed it down as far as I dare. I wonder if I could clamp something to a spider leg to make a movement gauge. I guess the gauge pointer might introduce bending :(

[gbeal]

Wild guess. Not the laser "rocking" slightly within the eyepiece adaptor is it? I know the set-screw in my solar scope diagonal allows the likes of a barlow to "rock" slightly.
Gary

[pmrid]

Robin, as a matter of interest, is the movement of the laser consistently in the same plane or does it tend to be all over the place? If it is in a consistent plane, then the question is, does this correspond in any way with the weight distribution on the tube. If, for example, the movement tends to be left to right, then flexure in the tube probably isn't the culprit. If it is up/down, perhaps it is.
Peter

[Tandum]

I can't see that happening here Gary, it's in pretty solid. It's a 2" laser and a very tight fit. One of those Howie Glatter lasers as seen in the pic above.

Pete, I can get it back on target by finger moving the tip of the secondary toward or away from the focuser. The focuser on the other hand needs quite a bit of push to make it move it in that axis.

I pulled the primary and stuck paper over the end. I'm getting about 2.6mm movement from the dot by moving the scope level west to level east. I also note, with the primary out, that if I loosen the rings and rotate the tube the dot comes back on target, so something is moving up front. Back to fiddling with it I guess.

[Tandum]

I thought I had this licked today. I found the holes in the tube for the nuts on each spider leg was too big and the whole spider could move in/out, not by much, but enough to cause issues. It was looking good this arvo but tonight the movement is back. I'm lost, It looks like a spider issue but I can't find it.

[Shiraz]

make sure the secondary is firmly attached in the shroud. The secondary in my 8 inch f4 came adrift from one of the three double sided adhesive tape pads that held it in place. It looked fine and even felt fine until one pushed hard against the side of the mirror - then it moved slightly. Fixed it by soaking in water to soften the tape and then very gingerly prying the mirror out - reglued with silastic and it hasn't moved since. Recently tried to remove it to replace with a higher reflectance secondary, but couldn't get it out -- ah well.

the other thing to consider is, if that is the standard black plastic GSO secondary holder, it can flex quite a bit under load anyway. When I couldn't get my secondary out of its holder, I made up a new one out of PVC pipe fittings and that is rock solid compared to the original.

just a couple of possibilities. good luck.

regards Ray

[Tandum]

Cheers Ray but I revisited it this evening and the nuts on the spider legs are still moving within the tube holes. My fix for that was putting tape around the nuts to remove the gap but that has failed. It looks like the tube just cut through the tape. I have put washers under the nuts and did em up tight and almost all movement is gone. I think I might replace the spider and holder, I don't trust this one at all.

[MrB]

Might be a bit rough on the tube, but you could try a star washer under each nut until the new spider arrives:

[Tandum]

I thought I might just make it sit in the dunces corner and get the tak into the dome.
Still waiting for adapters and an sx wheel before the big newt can take an image anyway :)

[Shiraz]

good to hear and thanks for posting your progress.

must have a look at my spider then - I gave up trying to get rid of the variable flexure and now just re-collimate for the part of the sky I will image in. But I don't use a coma corrector on my 12 inch - guess you will use one and need to get the alignment right for any viewing angles. regards Ray

[Tandum]

We shall see, I have a 3" asa corrector here for it.

[MrB]

Niiiice!
Did you go for the 0.95x or the 0.73x ?
I want one of each! haha

Would be interested to hear how much it cost landed (duty etc)

[Tandum]

I had to have the 0.95x (3 KORRW) to get the image circle I need.
The .73x has a 28mm image circle this one is 50mm. Backfocus is the challenge.
I got it 2nd hand from the US for just under a grand.

[RobF]

Sounds frustrating Robin. I know Brendan replaced his spider and 2ndry assembly and swears the whole thing is much more robust now.

My only suggestion to pin it right down to this assembly would be to do your laser on to paper test again (primary removed), but with tube on a table rather than mount. Then roll the tube 90 degrees at a time and see if you still get the movement. With table supporting tube that would pretty well seal the case as spider+secondary problem surely? Sounds like the focuser is plenty rigid for holding a laser steady, assuming definitely no movement in focuser barrel when locked.


Heck, I only have to look at my focuser and alignment moves. Once you're tracking an object these errors (although frustrating) aren't really going to matter too much. I'm pretty sure I see a tiny amount of mirror movement from one side of meridian to next however such that you don't get objects in FOV exactly. In EQMOD you can do alignment stars for each side of the pier and that fixed my probs in that regard nicely, and with plate solving its show stopper. Small price for light sucking power of a Newt?

[RobF]

Can I drool on it sometime? :D

[Tandum]

Here's a pic Rob, it's big. I also got a 10mm thick OAG from Germany that is made to bolt to it :)

http://www.iceinspace.com.au/forum/attachment_browse.php?a=128841

[RobF]

Mmmm - Teleskop service short lightpath OAG - have been doing the sums on one of those for years, but can't quite make it work without risk of 1-2mm short. Great choice! :evil: