What did I bugger up?

cjpops (Craig) · #1 03-09-2018, 10:28 PM

I did a mirror clean and collimated my 8” skywatcher dob.

Went outside tonight and both Saturn and mars are a blurry disc with no surface detail and I cant get a good focus on any eyepieces..

The Orion laser collimation is right in the middle for both mirrors.
(Fair Bit if god rays too) - I had better viewing before I did the clean.

Have I managed to damage the mirror? 😞

Lee · #2 04-09-2018, 06:59 AM

You'd have to be pretty rough to damage the mirrors, and then it should be obvious to you that you had done so....
Can you focus stars to pinpoints?
Did you make sure to not put the primary mirror back in too tightly?

What you describe could be from bad seeing conditions - could you focus with a low power eyepiece?

mental4astro (Alexander) · #3 04-09-2018, 07:20 AM

Over tightened primary can be one factor as Lee mentioned.

Did you also remove and clean the secondary mirror? If you did, and did not replace it with the correct orientation to the focuser, the laser can actually give you a false reading that everything is "good", when they are not. A laser alone will not help to collimate the secondary. If the secondary is not square with the focuser, it is still possible to get the laser to point to the center spot on the primary and tweak it, but when you look through the scope the image will be stuffed. To deal with the secondary mirror you need to use a Cheshire eyepiece, and use it to get the secondary mirror set correctly and only then ook at the primary.

Are you finding that you cannot get the focuser to ruck in or out sufficiently to focus? Like Lee asked, do the stars come to pinpoints?

Is your dob a solid tube or collapsible one? If the latter, one of the strutts may not have been fully extended or tightened - I've done this too and it will affect focusing performance.

Might seem like a dumb question, but have you looked down the draw tube of the focuser?

Alex.

Allan_L (Allan) · #4 04-09-2018, 07:27 AM

Yeah, I tend o agree with Lee.
My first thoughts were that seeing conditions were pretty bad.
I also agree that there is not too much that you can bugger up in the process.
The one exception (as Lee also mentioned) is to over tighten the primary mirror clamps. But your description did not support that type of symptom.

Depending on your mirror cleaning process and rinsing off process of course.
Shine a torch on the mirror surface to make sure there is no residue on the surface. But remember, even a clean mirror may find it tough to stand up to the torch test and look pristine. Don't be too critical.

Give it another try when conditions are stable to re-assess the situation.

cjpops (Craig) · #5 04-09-2018, 07:41 AM

Thanks guys.

I have the solid tube 8" skywatcher, the mirror is much cleaner than it was but I have to admit I did tighten the screws on the mirror (not the collimation screws) because first time I did the collimation the main mirror 'clunked' whenever I move the dob and this made the collimation out again. So I 'thought' the screws holding the mirror needed to be tightened which has now stopped this mirror movement.

I didn't touch the secondary mirror.

I can see the stars better on a 32mm Plossl Eyepiece - not completely sharp though, but when using a Barlow or simply the 17mm and 8mm Hyperion its worst and blurry and no focus is possible.

I vaguely remembered a while ago on here that somebody had a skywatcher dob and they had some adapter still on the focuser that needed to be removed or something which caused their blurry images.. but anyway I was able to see Saturn well enough before I messed this up so I guess that can't be it.

doppler (Rick) · #6 04-09-2018, 08:47 AM

With Skywatcher scopes you need an extension piece on the focuser to use it for visual and this needs to be removed for photography. Try sliding the eyepiece out of the focuser tube a bit to see if that helps.

As for cleaning mirrors I leave mine in the cell when washing, very little collimation needed when I put the cell back in the tube.

Rick

Lee · #7 04-09-2018, 08:57 AM

Quote:

Originally Posted by cjpops

Thanks guys.

I have the solid tube 8" skywatcher, the mirror is much cleaner than it was but I have to admit I did tighten the screws on the mirror (not the collimation screws) because first time I did the collimation the main mirror 'clunked' whenever I move the dob and this made the collimation out again. So I 'thought' the screws holding the mirror needed to be tightened which has now stopped this mirror movement.

I didn't touch the secondary mirror.

I can see the stars better on a 32mm Plossl Eyepiece - not completely sharp though, but when using a Barlow or simply the 17mm and 8mm Hyperion its worst and blurry and no focus is possible.

I vaguely remembered a while ago on here that somebody had a skywatcher dob and they had some adapter still on the focuser that needed to be removed or something which caused their blurry images.. but anyway I was able to see Saturn well enough before I messed this up so I guess that can't be it.

The clips that hold the mirror are mongrels - too tight and you pinch the mirror, too loose and it flops around. Shouldn't affect you view or focusing ability too much I wouldn't think.
If you get a minute in daylight try and focus on something very far off - that way you'll know you can at least get close to stellar focus.

I recently couldn't get my imaging newt to focus, I racked the focuser in and out with nothing, I thought my CCD was toast, then realised I had used an extension tube as a dust cap after removing it from the scope, and had left it on the CCD....

Allan_L (Allan) · #8 04-09-2018, 10:16 AM

Skywatchers do not need an extension tube for visual.
The "extension" you may be referring to (Rick) is actually just the eyepiece adaptor.
Admittedly, the 1.25 adapter does unscrew, to allow a T-Ring to be attached for photography, but it is not an additional extension tube as such.

I have found the clamp screws need to be only just touching the mirror surface, very lightly.
I don't know exactly with the 8" but I know on some sizes of SW Dobs there is an additional set of thumb screws in the side wall of the cradle. (they were on my 12" SW but not on either of my 10" SWs.) I think they try to make one mirror cell cradle size fit two mirror sizes. (while my 10 was snug, my 12 could also take a 14, hence the need for the extra thumb screws. I think)

I have found that mirror cell movement (in my case at least) had nothing to do with mirror clamp screws, but was caused by the locking screws (next to collimation screws on base) coming loose.

SkyWatch (Dean) · #9 04-09-2018, 12:09 PM

Hi Craig,

It might have just been a combination of bad seeing and a warm mirror. Did you allow an hour or so for cool-down?

If the mirror retaining screws are too tight you would normally see odd-shaped stars because of the pinching effect of the screws.

WRT the seeing: I have had nights when Saturn was just a blur even at 40x...

Good luck,

Dean

cjpops (Craig) · #10 04-09-2018, 06:18 PM

Didn’t give it a chance to cool down and perhaps it was a mixture of the two with bad seeing. I did notice a fair bit of movement before focusing on a star/planet - is this an indication of some poor weather up there affecting the viewing?

Anyway, I un-tightened the mirror slightly and then did some day test pointing the dob at a distance tower and it’s clear on all eyepieces and good detail. The 8mm Hyperion was a tad dull is this due to ‘lack of light’ from the 8” size? Say, if I had a 12” dob this eyepiece would improve. Sorry don’t know the exact terminology wording to describe it.

It's been a long while since I had my 12" Lightbridge and its all slowly coming back to me.

So will try Saturn again tonight hopefully it’s better.

SkyWatch (Dean) · #11 05-09-2018, 10:30 AM

Good luck with that Craig! I hope last night went well.
The movement you noticed before focussing would most likely be air currents in the tube as it cools down, atmospheric turbulence, or a combination of both. Tube currents are normally slower and more regular than atmospheric turbulence.
The "tad dull" image in the 8mm eyepiece is probably due to the small exit pupil/high magnification (I assume the eyepiece is clean?). The view should be brighter in a 12" scope at the same magnification (that would need a different eyepiece though), as well as slightly brighter with that same eyepiece (but at a higher power) because the 12" has f5 optics, and therefore a larger exit pupil to your f5.9 Skywatcher.

All the best,

Dean

cjpops (Craig) · #12 05-09-2018, 07:18 PM

Thanks for that information Dean makes sense now. I was thinking of saving up for a Televue eyepiece which is about $800ish at Bintel but i might be better off getting a larger dobsonian first before the eyepiece?

Anyway, the view is all good now - i can see Saturn clearly and the details/ring - beautiful!

Appreciate your help everyone. Cheers!

Lee · #13 05-09-2018, 08:05 PM

You should check out something like a nice Televue Plossl before you drop that much cash on an EP.... you might be pleasantly surprised....

SkyWatch (Dean) · #14 05-09-2018, 11:09 PM

Quote:

Originally Posted by cjpops

Thanks for that information Dean makes sense now. I was thinking of saving up for a Televue eyepiece which is about $800ish at Bintel but i might be better off getting a larger dobsonian first before the eyepiece?

Anyway, the view is all good now - i can see Saturn clearly and the details/ring - beautiful!

Appreciate your help everyone. Cheers!

Great that all is working now! Well done! If I were you I would spend time getting to know/use your scope and eyepieces before committing to any extra expenditure. A $800 eyepiece will give you a wider, sharper (maybe) field of view than your current one (at the same focal length), but it wont gather any more light: that is the function of the telescope aperture. A nice TV Plossl will give you a lovely view as Lee has suggested, and at around 1/6th of the price of an Ethos... it just wont give you as wide a field.

Best advice is to join a local club, go along to members' nights and check out their equipment: maybe borrow an eyepiece at these events and see how it goes in your scope. Most astronomers love showing off their equipment!

All the best,

Dean

cjpops (Craig) · #15 06-09-2018, 09:26 AM

Thanks Lee - I do have a skywatchers plossi kit but been using the hyperons more as more clearer - would I expect far better detail on a televue plossi? any suggestion which eyepiece to start with?

Great idea Dean, will look for a local club - might attend upcoming snake valley astro camp.

Lee · #16 06-09-2018, 09:36 AM

Quote:

Originally Posted by cjpops

Thanks Lee - I do have a skywatchers plossi kit but been using the hyperons more as more clearer - would I expect far better detail on a televue plossi? any suggestion which eyepiece to start with?

Great idea Dean, will look for a local club - might attend upcoming snake valley astro camp.

I can't comment on the Hyperions, but I'd expect the Televue to sink anything Skywatcher can put out....

I have the 10, 20 and 32mm Plossls, I really like the view through the 32 to be honest.....