SCT Collimation Failure: Can I call out the AA

[Joves]

I think that's exactly what has happened, I think perhaps the first two knobs may have been a little tighter than they should have been which has pulled the tilt of the secondary when I've removed the final screw for replacement. Wonder what I should do now?

Thanks for your reply, by the way.

[Joves]

Cheers mate, I'll give that a go.

[CodnorPaul]

Tilt tube directly down on its mount to let gravity help, slacken off the knobs and remove them one at a time to replace the spacers and put that one back in before removing next one - do not tighten them up.  When replacing them gently pull on the installed ones (or one of) to make sure the secondary is not moving away.  Hopefully you will then get them all back in with spacers - only then tight them up evenly to remove the slack.  Then the collimating fun begins.

I would suggest removing the problem one as the first one

[Joves]

Thanks for the advice guys, very much appreciated.

Unfortunately, no luck at this point. The problem one is an absolute problem child. I can get the other two on with spacers but no chance of getting on the third, regardless of how much I "play" with the other two whilst trying to grip some thread.

Course collimation isn't a problem without the spacers, however, I do understand that it's probably wiser to have them fitted. Is it fairly typical for one knob to be a fair bit tighter than the other two when the scope is collimated?

[CodnorPaul]

No its not - well not in my case anyway.

Maybe double check with Bob's Knobs that you have the correct ones just in case?

[Tinker1947]

You do need some depth to the threaded hole or you will end up stripping the thread, when i done my Secondary Knobs i brought them to long then spaced out the thread with 3mm washers to give a good depth to the thread and also make the Knobs stick out a bit, there not "Bob's Knob's" but only cost £5 and are stainless steel...i could replace the washers with spacers maybe will one day...

[Joves]

I think my issue is actually the tilt of the secondary on the pivot. The knob at the 7.00 position is quite a bit tighter than the other two when in course collimation. It would appear that, even with the other two loosened (or even with one removed) the mirror can't be as manipulated by pulling on the 7.00 knob as it can when doing the same with the others.

Anyhow, I seem to have hijacked this thread a little. Sorry Sp@ce_d!!

[ChrisLX200]

When I made these the 'washers' were actually bronze bearing seats, the SS shafts turned on these rather than against the plastic secondary housing. If you are having difficult getting the third screw in place then carefully tighten the other two fully to pull the secondary back aginst the spring, when all the way back it should straighten up within the housing and allow the third screw to be inserted. Once they are all in and fully screwed in then wind them all out about 3-4 turns each. Be careful that if one 'feels tight' you may have cross-threaded it - not too difficult because the threads are fairly fine (#7-32 for Meades).

ChrisH

[Joves]

Thanks Chris and thanks to everyone else also.

Good news, I just got it sorted. Sheepishly took a punt and removed the secondary mirror to see what was going on. Was a gamble, as was likely to be a do or die moment for the scope, but curiosity and frustration got the better of me. An even bigger gamble considering I had no idea how the thing was configured behind the three screws fastening the casing. Let's just say I felt like a quivering MacGuyver as I loosened that last screw!

As it turns out, one of the collimation screw springs had become dislodged from where it was supposed to be, so removing the secondary was the only option for possibly fixing it. I figured it was easier to simply attach Bobs Knobs with the secondary mirror removed, which it was. According to ol' Bob's instructions, this model LX90 uses only a pivot and no springs... Apparently not the case!

Anyhow, Secondary is now back in place, knobs are attached, course collimation is done and all three knobs have similar tension with plenty of play each side of rotation for finer adjustment. All's well that ends well, so I'm quite happy now.

Thanks again folks. I was almost resigned to being satisfied with not having the spacers included in the configuration, which would have no doubt led to countless nights of miscollimation frustration (and possible damage to the secondary mirror) had it not have been for your advice. Really appreciate your help!!

Cheers,

Aaron

[PeterCPC]

I made an artificial star using a torch with a 35mm film container over the end with just a small hole in it to produce a pinpoint light source. I set the torch up about 30 feet away and was able to get an approx collimation that I could then adjust at night if required. It worked. (best done on a dull day of course)

Peter

[Sp@ce_d]

Well, I found my laser I'd forgotten I had.. and Michael is dead right.. Laser collimator's don't help. It's just a big splat on the target grid

Although at least I've managed to tweak it enough to actually show up on the target grid now.  Fingers crossed the forecast is right for clear sky tonight although it's been wrong for most of today so far.  I've been trying to avoid the pain of having to take the thing off the mount & out of the obsy so I can get far enough back from it to follow Michaels instructions. A star test tonight would be welcome.

[earth titan]

The only use for the laser, as far as I could see, was to get the secondary back in the same place after fitting the 'knobs'.

Have to say I got more of a point source than your photo shows!

[Sp@ce_d]

Well, I've given up for the second time... surely I cant be that thick at getting to grips with this!

I was quite pleasantly surprised to find that my rough alignment with the laser looked like this before I touched anything on a star test.

but that was the best I had it to all night.. it was down hill from there. Try as I might there is just no way I can get the central obstruction into the centre!

I also had the secondary out this afternoon before I did the laser align to check & everything looks ok. I'm now wondering if this has ever been right as I was trying to get the concentric rings parallel before I fitted Bobs Knobs & not taking notice of getting the central obstruction into the centre first.

[rowan46]

Google Duncan's collimation mask I send to use one on my c5 and found it a great help

[Willarnold]

What about an artificial star test indoors? Has anyone used this http://www.hubbleoptics.com/artificial-stars.html 

[Sp@ce_d]

The saga continues... Enter the Advanced CT Laser Collimator

Having been through the whole alignment setup including the focuser tonight, I still think I'm seeing the same issue. I don't know if this is good or bad but I guess its consistent using a different method! Although due to cloud I haven't been able to do a star test to prove it yet.

I think the issue I have is indicated here by the bottom laser dot being "off track" compared to the other two. Try as I might I cant get it to move that last bit as the adjuster is fully tight. Loosening the opposite two doesn't seem to have the opposite effect. Which was the same problem when doing a star test. At the focuser end it is slightly off centre too. Now this may mean the secondary is not central however I've spend some time getting the corrector plate adjusted tonight & you can see the shadow is central in the picture below.

I'm tempted to remove Bobs Knobs at this point & put the original screws back in before I go through the whole set up again tomorrow. I don't really expect a difference but probably worth a good inspection of the threads & spacers at least.

Edit: Hmm the piccys flipped sideways on here.. looks fine on the desktop.. ok so the offending dot is on the right then.

[deepspacehunter]

I would check to make sure that the SECONDARY is lined up perfectly with the FOCUSER... looking at your shadow (not perfectly round)... this might be your issue....I had a PARKS OPTICAL 10" Dob... and I had the same issues... I checked the way that the SECONDARY was LINED UP with the FOCUSER . it was not lined up well..I moved the SECONDARY until it was DEAD CENTER with the FOCUSER.. and that FIXED EVERYTHING ! Good Luck...

Mark

[Sp@ce_d]

I would check to make sure that the SECONDARY is lined up perfectly with the FOCUSER... looking at your shadow (not perfectly round)... this might be your issue....I had a PARKS OPTICAL 10" Dob... and I had the same issues... I checked the way that the SECONDARY was LINED UP with the FOCUSER . it was not lined up well..I moved the SECONDARY until it was DEAD CENTER with the FOCUSER.. and that FIXED EVERYTHING ! Good Luck...

Mark

Cheers Mark I'll check that out again tonight. I must admit its a pretty hefty focuser. Moonlite with rotator & focus motors. So I've been looking out for flex. I was thinking of putting the scope back to minimal configuration by getting the visual back back on & see how that looks.

Sent from my iPhone using Tapatalk

[Tinker1947]

I think having just the visual back and the Hotech adapter placed in it will be more accurate.....

[skywatcher58gb]

Hi I bought a set of Bobs Knobs for my LX90 and when fitted they did'nt seem to collimate properly, I bought a second set and when fitted they did collimate properly. I suspect that I did not fully engage the threads of the first set. One suggestion is to refit the old screws, achieve collimation as per your first photo, then fit one Bobs Knobs at a time ensuring collimation after each change, hopefully after the three have been inserted/collimated the problem will be overcome.

Dave

[Sp@ce_d]

Yes, probably going back to basics and proving I can get that collimated, is a good idea. Then I can work on from there. But as this is for imaging I'll need to get the focuser alignment spot on. Something else I'd forgotten to consider. I have a reducer in the light train. I'd forgotten about that as it's fitted inside the Moonlite... Doh.

I have to say the Hotech CT seems quite straight forward once you get the hang of it. You do need a steady floor. Video 2 on the link from FLO's site is the one I've followed.

Sent from my iPhone using Tapatalk

[Sp@ce_d]

Hi I bought a set of Bobs Knobs for my LX90 and when fitted they did'nt seem to collimate properly, I bought a second set and when fitted they did collimate properly. I suspect that I did not fully engage the threads of the first set. One suggestion is to refit the old screws, achieve collimation as per your first photo, then fit one Bobs Knobs at a time ensuring collimation after each change, hopefully after the three have been inserted/collimated the problem will be overcome.

Dave

Yes that's worth checking again

Sent from my iPhone using Tapatalk

[billhinge]

The saga continues... Enter the Advanced CT Laser Collimator

Having been through the whole alignment setup including the focuser tonight, I still think I'm seeing the same issue. I don't know if this is good or bad but I guess its consistent using a different method! Although due to cloud I haven't been able to do a star test to prove it yet.

I think the issue I have is indicated here by the bottom laser dot being "off track" compared to the other two. Try as I might I cant get it to move that last bit as the adjuster is fully tight. Loosening the opposite two doesn't seem to have the opposite effect. Which was the same problem when doing a star test. At the focuser end it is slightly off centre too. Now this may mean the secondary is not central however I've spend some time getting the corrector plate adjusted tonight & you can see the shadow is central in the picture below.

I'm tempted to remove Bobs Knobs at this point & put the original screws back in before I go through the whole set up again tomorrow. I don't really expect a difference but probably worth a good inspection of the threads & spacers at least.

Advanced_CT_Hotech_01.JPG

Edit: Hmm the piccys flipped sideways on here.. looks fine on the desktop.. ok so the offending dot is on the right then.

Hi

When I contacted Hotech some time back about error margins I got the following info ... Hope it helps, I can however understand you wanting to get 'dead on' accuracy, it can be done

Q1: Is this reasonable accuracy?

Yes.  Within half track is acceptable. 

Q2: Large diffraction rings around the dots in the dark, normal?

The image has been expanded by the secondary mirror twice (in & out of the optical train) so you are referencing the far-field image of a laser dot.  As long as you use the center of the dot as the collimation reference on the track will be fine.

Q3: Three fine focused laser dots?

Yes, they are from the reflection from the corrector plate, and please ignore them.

Q4: Ideal collimation distance:

a). Three collimation dots far apart:

- If the dots are far apart outside the target, please adjust the telescope focus to bring all three closer.  When the three dots is on the same track (4.5) as the laser exit dots, it means the reflector mirror is positioned on the focal point.  This is the ideal collimation distance.

. Co-alignment distance:

- The co-alignment distance is when you use the crosshair projection and the diffuser technique to line up both the collimator and the telescope in the first stage (like aiming & tracking a star during star collimation).

- The farther away from the scope the higher the accuracy you can achieve but the co-alignment became too difficult to aim square and the collimation became too sensitive.

- The ideal co-alignment distance is a little shorter than one tube length in front of the scope where the crosshair or the diffuser donut on the collimator target is about the size of ring 1 or 2.

[Sp@ce_d]

Went through the whole set up routine again with the Advanced Laser. Corrector plate & Focuser alignment seemed to need the most adjustment. Anyway,  a star test looked good in the eyepiece so I've put the camera back on. Not 100% but close enough for now after all that, thought I'd never get it in. I think it was the corrector alignment that's the main issue... Quite impressed with the Advanced Hutech Laser I must say.

This is how close the laser got it... I'll try and centre it a wee bit more when I've recovered...

[Sp@ce_d]

Well, after all that it doesn't seem to have made any difference to the triangular stars and "soft" overall focus. Back to the drawing board..   I should have stuck with the fracs..

Focus Check

30s Guided

300s Guided