It was the OTA - not the mount!

[g__day]

Well I have been pondering the tracking accuracy of my Vixen Atlux mount for quite some time now, so I finally tried something different, polar aligning using a Canon 400D attached to a 127mm (1500mm focal length) MAK rather than the C9.25 SCT.

PEMPro said I was West of the SCP and I eventually got it to less than 0.3 arc minutes of error. Then I tracked Jupiter for 16 minutes before clouds rolled in. I put the Canon's central focusing dot dead centre on Jupiter - it was about 1/4 Jupiters diameter. After 15 minutes the dot wandered South in DEC maybe 1/5 of Jupiters diameter but RA was dead centre.

PEMPro doing 10 Worm cycles said PEC was less than +/- 2 arc seconds - so that is pretty fine (if I didn't screw up a reading somewhere) and RA and DEC drift per worm was like 0.00014 something?

So I tried imaging Eta Carina, unguided for a bit over 16 minutes (969 seconds) before the clouds came in - attached is what I got (Photoshop CS2 levels and curves - no sharpening or deconvolution of stars). I also include a 512 second shot of Eta Carina.

* * *

So this indicates I have have mirror flop on my SCT if I keep seeing star trails (as a 2350mm focal length SCT thru a Lumincon OAG gives about 1570 mm focal length - almost the same as the MAK). Strange mirror flop as it has always appeared as a constant star trail about 1-2 arc seconds per minute...

Question good folk - how to best eliminate this? I seem to remember someone racked their mirror all the way back then added a 10:1 SCT focuser at the rear of the OTA to achieve focus. Racking the mirror all the way back I supposed stiffened it considerably to eliminate nearly all flop, then you just have to play with focus tube extenders at the rear of the focuser to achieve focus I guess.

Alternatively you could remove the mirror, fit a spring around the baffle tube and replace the mirror - so that the mirror cell is always under expansion tension to minimise movement. Anyone else got clever ideas? I have a feather touch 10:1 fine focuser, so I can't add a Hutech mirror lock. What might folk here suggest?

[avandonk]

Use the Goddess of sticky situations ARALDITE!

Bert

[Ian Robinson]

One of the reasons I've have avoided SCTs.

Visually you would probably never see this problem , I think someone did a write up about the problem you describe in one of those British astronomy magazines several years ago and came up with a fix that solved it permanently (involved disassembling the SCT OTA).
Sorry - you'll have to find the piece as can not remember when it was.

[g__day]

Interesting google searching this, it appears the problem is widespread and I ponder whether Celestron or Meade warranty tolerances of mirror flop or shift? One poor guy had to re-focuse a C11 when he slewed more than 30 degrees!

http://astroshed.com/observatory/c11saga/c11saga.htm

Was the best article I read, where the guy dissambled everything twice using advice from BinTel tightened collar and finally tried a violin wax / vaseline grease on the main focus tube to reduce mirror wiggle!

Amazing that this isn't just machined far more precisely, or they use PTFE slides or tiny ball bearings between focus tube and mirror cell to eliminate all wiggle.

Surprised there isn't a buyer beware or notice of tolerance that should be achieved else you send the OTA back for a service!

This is also a good read:

http://www.astronomiainumbria.org/advanced_internet_files/meccanica/easyweb.easynet.co.uk/_chrish/lx_flop.htm

[Ian Robinson]

I was considering buying a C11 on a CGE when they were introduced and was turned off big time by this issue, and Meade are apparently worse offenders.
Decided to go to a 10" Newtonian instead.

[g__day]

Well I tracked Jupiter for an hour unguided, and the central finder dot of the Canon 400D barely moved off the centre of the red giant - and the two times I checked (mid-way and at the end). So I would say that is under 15 arc seconds drift in an hour - good enough to image off.

I still could be seeing some periodic error as shots at 600 and 1000 seconds had slightly elongated stars - but shots as 1000 seconds weren't any more elongated than shots at 600 or less seconds - indicating the start could be drifting forwards and backward in RA according to slight periodic error. I'll look at this soon.

Ring now I'm testing the C9.25 again on the Helix Nebulae - unguided for 1,000 seconds. I just did a shot on the MAK so I want to compare it to the SCT.

I still have to coliminated the SCT properly - I think Bob's knobs are now in order! Only once all other factors are ruled out (collimination and PE) can I get a feel for how bad mirror shift is. It could be that it accumulated to throw my drift alignment out - meaning tracking was impaired. I'd like to see if I can centre and track Jupiter in the C9.25 now for an hour! Then and only then will I have a better feel for the mechanical errors I have to tune out.

Ian - at that weight and focal length I reckon get your mount settled first before you choose and imaging platform. Guys here get great results - so it must be achieavable with enough care and skill I hope!

Matthew

[Ian Robinson]

I've assembled the Atlux , haven't powered it up yet.

That can wait until I have my 10" newt OTA reassembled and I get hold of the 40D I'm buying.

[g__day]

It ended up at 4am before I got too tired, but by then I tracked Jupiter for 45 minutes on the C9.25 and it drifted from the dot being centred on Jupiter - to the dot approaching its edge - drift mainly in DEC - so that is very good news, a drift of say 10 - 12 arc seconds going off Juptier's angular diameter versus the diameter of the Canon's central finder dot. Also I spend most of my time tracking on the MAK when Jupiter was low in the horizon and the SCT when it was high - so I need to swap scopes and observations around to directly compare results.

More good news for the C9.25 - shots of the Helix nebulae unguided at 600 and 1,000 seconds also showed only consistent star elongation - not drift.

So I am begining to put this down to mirror shift in the SCT during polar alignment, not constant mirror shift with elevation (a far stranger happening) that caused a large part of my problem.

Detection and diagnosis last night was slowed wayed down when shots from the Canon kept dissappearing (not going from DSLR Shutter -> Canon driver -> Canon Zoom Browser. I kept losing long shots after two or more shots! I eventually had to shut ZoomBrowser down to get it to receive just a single shot. I finally realised at the end of the night / morning that all my RAW files had somehow changed association to Maxim DL or DSLR - not ZoomBrowser - so I think running PEMPro or the wrong Maxim product at some stage in the evening changed the Windows files associations of RAW files so ZoomBrowser never got to see the shots - annoying gremlin!

My biggest take aways from all this is never give up, and always be cautious drift aligning off a SCT where mirror movement might throw off your results. Had I drift aligned off a smaller MAK or refractor I might have just saved 8 months of effort and frustration!

So I'll keep folks posted - and as I said next step - colliminate the SCT, then work on improving PE as best I can (I think I'll train PE off the MAK, not the SCT after this experience).

The effort, skills and experience that is required to have your gear set up optimally for long duration, long focal length imaging really is something!

[madtuna]

Matthew.. I ordered my Bobs knobs for my C11 dirrectly from Bobs site, $16.00 via paypal and they were here in under a week

[marki]

Bob's knobs are the go for sure. I don't know if the Celestron is anything like the meade but you can only adjust the secondary using allen keys. This is not recommended in the dark and the first time I tried I must have slipped onto the corrector a dozen times before I gave it up as a bad joke (ordered Bob's knobs that same night). I also had the secondary moving as I slewed around when I first bought the scope (not good for collimation!!!). After much reading on several forums the solution seemed to be locking one of the collimation screws down fully and using the other two to adjust. This has worked well and the scope holds collimation and focus well now. I wish mass produced scopes were perfect out of the box but half the fun of astronomy for me is tinkering and I wouldn't have it any other way :D.

[g__day]

I just wish there were seriously good do's and don't documented somewhere. Seriously - I wasted many months because mirror flop with can deliver incorrect results when drift aligning. So you assume alignment is near perfect when its clearly out. I reckon this warning should be posted on every how to drift align guide!

[Zuts]

I dont see how mirror flop can effect drift aligning. You are aligning the mount and not the scope. Once you have chosen the drift star you just keep going until it stops drifting. Usually you slew at two or four by to recenter the star in the eyepiece after adjusting the mount knobs. In drift aligning, mirror flop would at most be equivalent to cone error which doesnt affect drift aligning at all.

The problem with mirror flop is that it affects focus.

Cheers
Paul

[g__day]

Well two side by side scopes - one says the star is not drift - the other says it is. Focus looks pretty good on both - so I wonder what can cause the pointing error if its not a small - say 7 arc minute mirror shift?

The fact two side by side scopes reported different results under full automate drift aligning - and believing the small scope produced such dramatic benefits for both scopes makes me think it has to be the mechanical flexure in the larger scopes optics causing this error.

[Zuts]

If your mirror is shifting then why isnt the focus changing?

Paul

[g__day]

Puzzling - I agree, but I can't find anything else that readily explains observed behaviour. Could a slight mirror shift be small enough to affect drift alignment but not great enough to be readily discernible when a newbie astrophotographer looks at shots - or should the degree of mirror shift to account for what I am seeing be hugely discernible in photos?

[Bassnut]

16min unguided???, 2arc secs PE????, you are KIDDING right???, your as about as close to nirvana as it gets, start imaging dude, with guiding, youve got it way better than most would dream of, no problem with your rig thats for sure, sheesh, get on with it :P

[g__day]

Fred,

I can't believe the PE trained result myself - but I can't see where I stuffed up the procedure. I'll re-run it later and see if results are consistent, I would be delighted with +/- 5 arc seconds, 2 seems way too good to be true. PEMPro reported ridiculously low PE - I'm keen to test this once the skies clear! Perhaps my image scales was out - but it was PEMpro that calculated it by looking at sky trails on a snap with the mount paused for 10 seconds.

The two shots in the first post are a 10 and a 16 minute unguided snaps - stars are slightly elongated meaning there was some PE or a bit of DEC drift.

Frankly I am dying to get the scope properly 3 stage colliminated - I guess its basically at stage 1 collimination, I'd love to see what can be achieved when the gear is all working together well.

Fred - it my shots were even half as good as yours I'd swoon! In a few years maybe I'll be close. I'd love to see what I could do with a serious S-BIG camera - many moons away for now!

[Ian Robinson]

Shifting sounds from his description to be side to side , not in - out , so you'd expect focus not to be detectably altered, and the shift is very small too cf the sensitivity of his focusing.

If he's able to get +/- 3 PE unguided with his Atlux , then I am pleased I bought one too , I've not powered up or used mine yet. Other issues to resolve first .

Unless I can get a change in the behaviour of my neighbour , I am not likely to be able use my back yard for observing of imaging anytime in the forseeable futures and will have transport my gear to do anything with it.:mad2:

[g__day]

I'll keep experimenting once the skies clear, but for now I'm pondering I should do all polar alignment on the MAK.

The rotten thing is now how much do I trust the SCT if I see any drift in it?

Ian - nick your neighbours light bulb - varnish the electrical contacts and but it back! Do that a few times and he'll realise his backyard is supposed to be dark at night! Or tell him the neighbourhood is tring to go green and leave him a brochure on those solar power night lights that you plant into the ground :)