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Focusing bother with 6mm on SW200p


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Hi all,

Somewhat optimistaclly I set my scope out earlier banking on clear skies. Ithas since clouded over (boooo) but I was able to get a quick Saturn view in.

Trouble is, my 6mm has suddenly decided it won't focus. I've had focus out of it before (I sometimes go mad and stick on a barlow too, not so good focus) and it was fine. All of a sudden I get the light from saturn but no where near focus?!?

It resembles a star when you defocus for collimation. And for the record, I haven't collimated. Ever. :) I have no means for collimation. Could that be the problem?

Other EPs seem fine, albeit and much lower mags - 32mm and 13mm seemed to cope fine.

To achieve anything like focus I had to pull the 6mm from the scope about 0.5 inch - I got what could be described as focus, but nothing like accurate focus.

Any ideas? I'm sure I'm missing something obvious here.

Many thanks.

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The collimation of my SW200P was worse than shocking out of the box, rendering the 10mm EP next to useless. I'd suggest that collimation should be high on your list of priorities as the difference it makes is not small.

Russell

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Could be poor seeing or poor collimation. You really should sort something out for that, it really makes a big difference to the detail you can see. Especially important when you are trying to view small bright objects like...say... Saturn at high mag...

Have you checked that all the lens elements are still securely fastened? I had an EP once where the bottom set of elements had come loose up inside the chrome barrel. I did them back up again firm finger tight and had no bother after that.

Edited by RikM
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Well with a 6mm in 1000mm focal length and well collimated scope you're at 166x magnification. In poor seeing the views will be fleeting glimpses of clarity, and even then a tad blurry. Anything under 12mm needs a good, clear, dark sky for the best views. If the seeing is poor, try a lower magnification and watch for high level thin clouds - they conspire against you lol. The 6mm with a 2x barlow magnifies the object 333 times. In UK seeing any scope will struggle over 250x. :)

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Appreciate everything you have all said, but when I mean 'focus bother' I mean a complete inability to get any kind of focus at all. To demonstrate, here's a quick example of what I mean (through the 6mm):

focus.jpg

The seeing was awful last night and I wasn't expecting much at all. The above illustration is a when the 6mm is correctly placed in to the scope, only by pulling it p out of the scope a bit could I resolve the shape of Saturn. So, while I expected the view to be rough, I'm concerned that the EP is no longer able to sit properly in the scope. It needs to pulled this way and that to be able to resolve (to the best of it's ability given the conditions).

This is just in case I haven't explained myself very well with my initial post - it may be that all your well received advice above stands. But just in case.

Many thanks! :)

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First check the focus on the Moon, or distant terretrial objects in daylight. Saturn might have been a seeing problem.

If you can reach focus by pulling the EP up a bit in the focuser, such that you can still screw it tight, then you've got no problem - just use it that way.

To get focus with an EP it needs to be a particular distance from the primary (measured from primary to secondary and from secondary to EP). You seem to have found that your EP suddenly needs to be moved further away than it used to - the only way that could happen would be if the primary had got moved up inside the tube, or the secondary had moved, or your eye has changed.

Since you say you've never collimated, I don't see how the primary or secondary could have moved, unless perhaps ypou dropped the scope at some point. And I tihnk we can also discount a drastic change in your eyesight.

If your scope were a flextube then I'd put it down to the length of the extension having changed a bit, but it looks like your scope isn't one of those.

I have a flextube, and I've had exactly the sort of problem you're describing, but in my case it was due to moving the primary too far during collimation, or not extending the struts properly. And incidentally, miscollimation in itself will not produce out-of-focus images.

But as long as you can bring the EP to focus then you're fine. If you find it awkward getting the right position then a parfocalising ring would hold it up to the correct height in the focuser.

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Odd indeed! I'll try it in another scope as soon as I can. I'll also look into a parafoc-im-a-bob ring too :)

No change in my eyesight, no dropping either (unless somebody else did, if so they're in a bit of bother themselves :icon_eek: )

Thanks for your help!

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It looks like you have lost an element of that eyepiece. Does it rattle? Can you use it as a magnifying glass? Its usually easier to try this with the eyepiece backwards, even so it can be tricky as 6mm means focus will be very close to the eyepiece surface, less than 6mm.

Or, you are being a complete muppet and focus with that eyepiece is with the focuser INWARDS. I'm just saying! :)

Regards

Barry

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Being a muppet hasn't been ruled out! There can't b any elements missing from it as it's quite a simple design (Celestron) and I've never had it apart. I tested the magnifying glass technique on your smiley face above and it works to resolve the RGB elements of a pixel - surprising clarity actually.

I tried the focus every which way. I've ordered a parafocalising ring from AstroBoot and that should be here early next week. I'll just have to use it something like the moon to achieve the correct focal distance. While that should solve the problem - it's still a mystery to me why all of a sudden I can't get any kind of focus out of it. Is it normal for the position of the lens in the holder to vary from one scope to another? If so, then I'll chalk it up to being a noob and move on.

As always, I appreciate all of your responses. :)

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Hi all,

Somewhat optimistaclly I set my scope out earlier banking on clear skies. Ithas since clouded over (boooo) but I was able to get a quick Saturn view in.

Trouble is, my 6mm has suddenly decided it won't focus. I've had focus out of it before (I sometimes go mad and stick on a barlow too, not so good focus) and it was fine. All of a sudden I get the light from saturn but no where near focus?!?

It resembles a star when you defocus for collimation. And for the record, I haven't collimated. Ever. :) I have no means for collimation. Could that be the problem?

Other EPs seem fine, albeit and much lower mags - 32mm and 13mm seemed to cope fine.

To achieve anything like focus I had to pull the 6mm from the scope about 0.5 inch - I got what could be described as focus, but nothing like accurate focus.

Any ideas? I'm sure I'm missing something obvious here.

Many thanks.

If I am up your way in the next few weeks more than happy to pop in with my cheshire collimator and give you a rough tutorial on collimation.

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Quick update. I tried the 6mm in a 70mm Meade refractor I had lying around. The EP would give me focus once I had adjusted it's position in the holder. The amount I had to pull the EP back was very different to that on the SW200P - giving me reason to accept that the position in the holder is dependant on the scope.

I then took the optics out of an old 2x Barlow and used it to extend the distance from EP to scope. This worked, taking the focus from the outer most extreme on the refractor to about half out.

Next time I'm out with the newt I'll test the arrangement there too - if it's a match then I'll leave he old Barlow mounted to the EP.

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If I am up your way in the next few weeks more than happy to pop in with my cheshire collimator and give you a rough tutorial on collimation.

@Simms - I'd really appreciate that! PM me nearer the time if you think it's possible and we'll try to arrange something. Genuinely and pleasantly surprised by the level of support and character of the SGL members!

:)

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Hmmmm.... that pic looks as if you're looking at a street light in fog and the only thing visible is the central obstruction. I'm tempted to ask if the tube had cooled down adequately and if there was any dew - maybe the secondary was misted up a bit? Do you use a dew shield? Did your nearest ep (8mm) work ok on the night in question? Was the eyepiece out in the cold before you used it or was it warmed up in a pocket? Any of these could give a clue as to what's going on here.

Stabbing in the dark - do you have any kids with sticky fingers who may have been handling the lens? :)

Edited by brantuk
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Hmm, is the problem the focuser itself? Not achieving the same extension it should. That would explain why it suddenly wont focus. Loss of tension in the focuser? Jamming on something? Got a faint recollection of an issue with focusers. Needing a flat filing down, anyone?

Barry

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It probably seems like a silly question, but you talk about focusing by pulling the eyepiece in and out of the holder - you do know the focuser moves in and out and the eyepiece remains stationary in the holder?

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Hi All, sorry for not getting back sooner.

All other EPs work fine and the focuser itself is a RnP type (although it does have a lock screw underneath). Checked that, but not the problem.

I have now tried the 6mm in a couple of scopes and it seems the EP itself is must be set in the holder at a certain distance (by which I mean not fully inserted, pull up a bit and then screw the EP locks down).

As previously said, I dismantled a 2x Barlow and used it as an extension and that pretty much enables adequate focusing in a refractor - due to the clouds I haven't yet tested this method on the newt.

If it works out, I'll leave this makeshift extension attached to the 6mm. The EP itself didn't come with an extension or any other accessories to assist with this but I bought some parafoculising rings from astroboot which, when correctly adjusted, should be a permanent fix to the problem.

The only anomaly left is why this has only started to happen - perhaps I never inserted the 6mm properly before?! I don't use that EP much so maybe I never noticed a problem.

The simplest answer is usually the right one I suppose. Until I can see some skie at night then I won't have any further information. A pain as I should take delivery of my ETX this week too :)

Note to self: Email the Met Office and tell them how dissatisfied I am with their weather. Tell them they must try harder. :)

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