C9.25 focus problems

[markyt1]

Looking for a bit of advice.  I have a celestron C9.25 which I took out the other night for the first time in a wee while.  I was unable to achieve focus on anything including Jupiter.  I went right from one end of the focus range to the other without success.  It's not atmospherics, dew, eyepiece or diagonal issues as I've bottomed these out and the mirror is moving when I turn the focus knob. When trying to focus on Jupiter all I got was a black central circle with a wide white band round it.  It was fine a few weeks ago when I had it out last.   I haven't got a clue other than collimation?  Does anyone have any ideas?  Thanks in advance. 

[PeterCPC]

Have you tried collimating it with a star test?

Peter

[si@nite]

I would say a focus problem, if the mirrors moving you should be able to achieve focus, as Peter says a star test would be the best way to prove it, see if you can bring the star into a nice tight focus both inside & out!

[Dave In Vermont]

With a black center/circle and surrounded by a light circle, it sounds like it's giving itself a star-test, using Jupiter! That is bizarre. Have you checked for anything blocking the target (Jupiter)? Both inside the OTA, as well as outside anywhere? Is the finder on Jupiter? Is the cover off the front of the scope (I had to ask)? What eyepiece are you using? Anything wrong with the diagonal?

In other words - have you gone over every square mm? Tried a different eyepiece?

I'll be watching this thread. I'm getting very interested!

Dave

<edit> PS: Welcome to SGL, Mark, it's nice to have you join us!

[JG777]

This does sound strange, you can set it up in daytime and attempt to focus on a distant object and go from one end of focus to the other very slowly. It may be that you turned the dial a tad too quickly and missed it in addition to what Dave mentions. Also it might be easier to spot any other possible malfunction if you test during the day.

[Mr Spock]

The doughnut shape ring is an image out of focus. If that ring doesn't either contract or expand when you focus then the mirror is not moving. If it does change in size but doesn't come to focus then the length of your optical train has shifted somehow.

[knobby]

Hi Mark, when you see this donut shape , if you try focusing does it get smaller or larger ? If it starts getting smaller then carry on turning the focuser in that direction, it should eventually focus.

I have a C8 ( your scopes little brother) and the focuser has about 30 turns from one end to the other , go slow and don't force it .

Good  luck 

[dweller25]

I would try the following in daylight....

1. Look down the optical path from the rear of the scope - any obstructions or equipment like focal reducers in the way ?

2. Place the rear of the scope on the floor and look at the position of the mirror in relation to the baffle that it slides along - now put the scope above your head and look up at the mirror - it should not have moved - if it has the focus mechanism has become detached.

3. If all is well so far from about 2mts away look at the front of the OTA - you should see lots of circles which should all be concentric if the scope is collimated - if not follow this procedure...

http://www.robincasady.com/Astro/collimation/

4. Then aim at a distant object - not less than a mile or more away and try to focus on it - try a low power eyepiece (20 - 30mm) it should be possible.

Let us know how you get on

 

[markyt1]

Thanks for the suggestions.  I'll let you know how I get on.  It's just stumped me as the ota hasn't been knocked, no obstructions etc and I can't see a thing out in.   Ordered some bobs Knobs to help if it's a collimation issue.  To be honest I don't think there is much that can go wrong with the scope there is only one moving part.  I take it when I reach the end of the focus range the focuser won't move??  I'm sure in my older CPT scope it didn't.  But I might be wrong as quite often I am. ? 

[markyt1]

Just been out the back with the scope and still it won't focus on anything.  At one on of the focus range it appears as it ifs starting to come into focus and the other end of the range it's a blurry mess.  The mirror isn't moving on its own, all covers etc are off.  I've tried several eyepieces with and without the diagonal and still the problem persists.  I've attached a pic of the scope showing the view down the ota.  Im a tad frustrated as I have a rather expensive ornament!!  

[Steve Ward]

Doesn't by any chance have a mirror lock fitted that has been forgotten about ... ?

[JG777]

Just wondering if you have a nearby astronomy society you can contact, maybe someone can have a look for you? Or is it under warranty? You pic has a lot of haze and very backlit from the sun, maybe some close in shots might help folks identify something for you?

[markyt1]

I read that there is no mirror lock on this scope.  It was working fine a few weeks ago and sat on its mount in the kitchen since.   It was bought 2nd hand last year and has worked flawlessly till the other night.  

[Owmuchonomy]

I own one of these and from that photo it looks like the primary has come way too far forward? I hope not and that its just the way the photo has rendered the situation.

[ronin]

What you describe sounds exactly like a focus problem, have seen it on a few SCT's when they are way off focus. Assuming the mirror's have not jumped then is there something else that you used to have in the optical train, thinking of a small extension tube or something. Perhaps the other way round have you a small extnsion tube that you dropped into the eyepiece unit for safe keeping and have forgotten to remove it.

That is about all I can suggest without suggesting the mirror has jumped out of its screw adjustment and you turning the exterior bit is doing nothing.

I seem to have the impression that it is not a great job to put right but there are so many SCT's and Maks around it may be easier on some then others. If you are a member of a club I suggest you enquire if anyone has had the same thing occur.

[Phil Fargaze]

Don`t know if this helps but here is a picture of my C9.25 from about no more than a metre away. I am wondering as well if the mirror looks too far forward.

 

 

[michael8554]

You mention you were getting near focus during the day. You may not get to focus unless you were pointing at something a mile away. Try it indoors in a dark room through the curtains, maybe upstairs if you can see a really distant object, and find the focal plane with a piece of cardboard held behind the scope ( no eyepiece). Then turn the focus 10 turns and confirm the focal plane has moved. If it has moved then turn the focus knob again to bring the focal plane to roughly where the eyepiece would be. Then test with an eyepiece.

Don't think it can be collimation, unless the secondary has fallen off, I don't know if the Farstar bits can come adrift.

How was the scope stored, was it nose down, maybe the primary had come off its focus rod?

Michael

[dweller25]

Hello Marki,

Take the diagonal out and try to focus on a distant object again with various eyepieces......

[Peter Drew]

There is a circlip at the front end of the shade tube that acts as a stop to the forward motion of the primary mirror. In the event of an accident or the focusing being forced, the tube that the mirror carriage rides on can get jammed on this circlip. A rare occurence but it has happened. I'm certain that the problem is a focusing action issue as the collimation in the photo looks good.

[nddeyne]

Hi,

I've just checked mine. With the primary mirror all the way back at the rear end of the OTA, the screws around the circumference of the OTA securing the back of the scope are about 0,5cm in front of the surface of the mirror. When moving the mirror all the way forward, it stops at a retaining clip that is mounted on the baffle tube.

Have you checked if the secondary mirror is mounted and fastened properly? It can be removed by unscrewing the knurled ring on the corrector plate.

Some additional instructions can be found over here: http://www2.astro.puc.cl/ObsUC/images/3/32/Manual_C11.pdf

Hope you can get this sorted...

Nico

[markyt1]

Evening guys.  Well just popped out with the scope for a test.  Still the same problem,  but noticed  as I near focus the image jumps to the outer edge of the view field. Im convinced it's not collimation and user error and I'm hedging bets at something to do with the primary mirror baffle.  The secondary seems OK as inspected it earlier and I checked the position of the mirror in relation to the screws round the circumference and  seem to be the same as yours Nico.??  Thanks to everyone for the input again.

[nddeyne]

I sure hope the baffle tube didn't come undone... :-/

Maybe check this: http://www.cloudynights.com/topic/402376-celestron-925-sct-rotating-visual-back/#Post5603269

[markyt1]

The tube seems ok it dosent rotate when screwing in the visual back.  

[PeterCPC]

You haven't put any new screws in have you? They can impede the mirror.

Peter

[knobby]

Just a thought are you turning the knob on the back of the scope or do you have a crayford add on focuser attached ... show us a picture of the back of your scope .