C11 scope...how do i focus?

[ncjunk]

Now, i know its the knob and i have a eye piece in and a diagonal but all i am getting is blumming big doughnuts and big ones.

It is my first sct but i am used to refractors....i remember my problem with the refractor being a missing diagonal and not having enough distance to obtain focus.

So any clues as to what idiotic thing i am doing with the sct?

Thanks!

[RobH]

Hello Neil.

Have you tried pulling the eyepiece out of the diagonal and moving it away while still keeping some stars or a distant object in view to see if you are getting closer to focus?

Rob

[John]

SCT's have a very long focus travel - when you are going in the right direction the doughnuts should start to get smaller until you eventually get a focused star image.

[brantuk]

Errmmm... you do have a diagonal in it right?

[brianb]

Errmmm... you do have a diagonal in it right?

Well it shouldn't matter, mine will focus without one.

You might be confused by the amount of travel there is in the focuser - it takes about 60 turns to go from one end of the travel to the other.

[brantuk]

"Well it shouldn't matter, mine will focus without one"

Fair comment - are the doughnuts perfectly concentric - might it be a collimation issue? Or it might be a question of going super slow accross the entire focus range.

[ncjunk]

Thanks for the replies,

I tried moving the eye piece and it only made a slight difference. The confusing thing was that the focus seemed better with no diagonal in.

I will have a look during the day as i dont think the knob could turn 60 times, i thought it had stopped (it seemed at its limit).

I will let you all know what i was doing wrong when i find out!

[ncjunk]

All doughnuts were round, only at one end of (what i thought is the focus range) did it go slightly flat on one side.

Hhhmmm doughnuts...

[Merlin66]

Neil,

The C11 and all SCT's will definately focus with or without a diagonal. If the "hole" in the doughnut looks pretty central (adjust by collimating the secondary) then SOMEWHERE in the focuser travel you WILL find the best focus position. You could use a Badhinov mask to help.

The only time it has never focused (for me) was when the target object was only 30 mtr away!!

Practise.....

[ncjunk]

Thanks, that means i am not turning the focuser knob enough then. Probably what i thought was the end of the focus range was just a sticking point.

[RobH]

Whatever you do, don't force it!

[ncjunk]

Well put Rob!

I found the problem, no one to blame just a simple error, the guy i bought it from had removed something which was attached to the scope and tightened the screws back up. These screws were then impeading the mirror which is why the doughnuts were deforming due to contact with the mirror.

I don't think there is any lasting damage to the mirror just a slight mark due to contact with the screw. Hopefully the mirror hasnt deformed.

Thanks for the comments as it told me what i should be expecting with number of turns etc...and i am glad i stopped last night and checked it during the day!

It has been in a garage since August i hope i havent buggered it on the first light!

[Merlin66]

Neil,

That is VERY SCARY!!!!

it's not a problem you encounter everyday, and certainly not high on my check list. It just goes to show how difficult it is to try and "problem solve" remotely without knowing the instrument or seeing it in detail.....

I'm glad things worked out - but I would be having words with the seller!!

Ken

[brantuk]

Sounds like he may have had an electronic focuser screwed to it (which takes longer screws) then just screwed the longer screws back in after removing it. Glad you found out and got sorted

[ncjunk]

In a word....[removed word].

It bent the mirror so i have what looks like a flat tyre when defocused.....slightly depressed now, i can safely say thats the worst first light i have ever had!

I cant even be bothered to set up the ed80....gonna open a beer instead.

[davew]

Neil,

What can I say ?? Be assured there are at least two depressed now and as more view this thread I'm sure the number will increase.

How does a glass mirror bend like that ?

[removed word] indeed.

Dave.

[ncjunk]

Cheers Dave,

Worst night <Sob>....and the beers wwwaaaarrrrmmmmm! aaarrrrrggggghhhhh!

Off to bed now......hope the wife doesn´t ask if the telescope was ok....I´ll probably blubber

[Merlin66]

OUCH!

OK the good news is that if the edge of the mirror is undamaged.... then I'd say maybe the mirror is tilted/ off set on the baffle... now that you've removed the offending screw (!!) run the mirror up and down the full travel of the baffle.

if the mirror is physically OK then this shoulsd "re-set" it....

Worth a try....

ken

[ncjunk]

Nope, you can see where the mirror has bent so its not just twisted.

[Merlin66]

I assume you mean the baffle tube has been bent?

[ncjunk]

Sorry i am not familiar with the construction so all i can say for sure is that:-

1 when defocused the doughnut is showing a flat side like a flat tyre.

2 when looking at the mirror the side where the screw hit appears to be leaning which agrees with point 1 as light is being reflected at a different angle.

Imagine a straight line going across the mirror and affecting 1/4 of the mirror. This 1/4 is now at a slightly different angle to the rest of the mirror.

As i say, i know nothing about how the mirror is mounted which is why i initially stated it was bent.

I may fire an email off to celestron and see what they say.

[Merlin66]

OK

The mirror thickness is such that it can't physically bend...

I'm therefore assuming that its not sitting straight on the support baffle.

The Black edge you seen in the doughnut is the mis-collimation effect....

If the surface of the mirror looks bright and unchipped etc then I think it can only be a support problem with the baffle.

Have you checked all the screws now clear(!!) and have you tried to focus - moving the mirror up and down the baffle???

It seems strange that pushing against a screw has moved the mirror so much...

Ken

[ncjunk]

Hi Ken,

I have removed the screw and it now focuses fine. I have tried moving the focus the full length of travel and will see later if it re-aligns the mirror...i dont think it has.

It is showing a large coma effect when focused, which is understandable considering the defect. The views of Jupitor were still impressive but you could tell something wasn't right.

I will shine a light through one screw hole and look through the other to see if i can see anything (when i get back home)

The mirror finish is still perfect apart from the distortion.

I appreciate the help, i may see if i can find some manual for taking it apart later.

[Mike_F]

The mirror finish is still perfect apart from the distortion.

Hi - before you consider disassembling the scope, why not take a pic of the distortion you can see on the mirror and post it here? You'll get some more accurate opinions and advice that way.

It can be a pain in the bum to take photos of mirrors, especially with a flash. I usually put the camera off to one side and take the shot at an angle. YMMV.

It does actually state in the manual that the accessory mount screws should not be tightened up if they're not securing a shoe or bracket - as they may be long enough to impede the mirror in exactly they manner you describe here.

However, I'm also really surprised that it's managed to distort the mirror though. They're usually pretty hefty.

HTH.

Mike.

[ncjunk]

I think the mirrors fine. I have double checked it this morning and the mirror looks ok. I have downloaded some photos of how the sct is taken apart so i now understand what Ken was saying. Its a thick mirror so is unlikely to be damaged.

It sounds like i have knocked it out of alignment slightly so will start looking at collimation and take it from there.

I could be doing any number of stupid things here!