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Telescope won't focus anymore

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#26 ButterFly

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Posted 15 August 2019 - 11:05 PM

Dewed up optics is the first big guess (post #15 "I live on a lake").

 

Trying to focus too closely is the second guess.

 

Gross rotational errors of the secondary is the third guess.

 

The zeroth guess is obviously take the clear cap off the eyepieces!  (yes I was laughing because I too am an idiot)

 

1 and 2 are easy to fix.  Don't guess on 1, check it (#10), both primary and secondary, then all your eyepieces.. If the Moon fits in your widest eyepiece, it should be getting either bigger or smaller as you rack from all in to all out (hopefully smaller from all in to focus in the middle to bigger past that).  It should work at some point because of "stopped focusing" (#1).  If you needed extension tubes before, put them in.  If you can't fit the moon, use you highest power eyepiece and Vega or Arcturus.

 

Three is the most guessy part of collimation.  See post #14 here.  Sight tube, laser (that is itself collimated in a focuser that is aligned well), sight tube again, then autocollimator are the best ways to get both collimation of the elements with respect to each other, and alignment with the physical axis of the tube and focuser.  The focuser alignment is good enough becuase it worked at some point.  If this works, then turn evey knob randomly, then get more practice.

 

These are all big guesses.  Describe what image you see and how it changes as you rack in and out, and what you're pointing at.  With the eyepiece out (and a sight tube in so that your eye is centered in the focuser), sketch all the circles, parts of circles, and ellipses you see and describe them (or show the sketch).



#27 Redbetter

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Posted 15 August 2019 - 11:07 PM

If the focuser has full range of travel, and the collimation screws on the primary have not moved dramatically, then the next most likely source is some sort of missing extension piece.  I haven't used the Orions much but the one 10" I was trying to troubleshoot for poor images had some sort of extension piece from what I recall.  (It was focusing, but even with careful collimation the images remained soft.)  As has been said, this can be checked for by racking the focuser all the way out, and continuing to withdraw the eyepiece to see if it appears to be getting nearer to focus.  If it is then some sort of extension piece has been left out.



#28 SeattleScott

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Posted 15 August 2019 - 11:16 PM

Yes he needed an extension but he replaced the focuser with one that has more travel so problem solved.

Scott

#29 Redbetter

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Posted 16 August 2019 - 01:14 AM

Yes he needed an extension but he replaced the focuser with one that has more travel so problem solved.
 

The OP?  I don't see it in the thread and I haven't seen anything indicating the problem is solved yet.



#30 shrimpus

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Posted 16 August 2019 - 04:28 PM

So on reddit some guy told me to put a piece of construction paper behind the secondary and take a picture down the focuser and I saw this: https://i.imgur.com/Nn8S2jD.png

 

It seems like my secondary is all jacked up. How do I fix this?



#31 Redbetter

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Posted 16 August 2019 - 05:19 PM

Well, you told us it was collimated.  Obviously it is not. 

 

You need to start by searching through collimation threads and instructions here.  There are plenty of threads that cover it, and several experts here who will help guide you as questions arise.  But you will need to start providing more informative and accurate responses to questions about what you have done and what you are seeing so that people can help you better.   In this and the other thread about filter threads it has taken awhile to get basic information from you...which wastes both your time and those trying to help.



#32 N3p

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Posted 16 August 2019 - 09:16 PM

So on reddit some guy told me to put a piece of construction paper behind the secondary and take a picture down the focuser and I saw this: https://i.imgur.com/Nn8S2jD.png

 

It seems like my secondary is all jacked up. How do I fix this?

Look at this page it's a collimation tutorial, I find it well made, it's full of images, user friendly.

 

http://www.astro-bab...nian-reflector/



#33 ButterFly

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Posted 16 August 2019 - 11:52 PM

Well, you told us it was collimated.  Obviously it is not. 

.

Laser collimators do not reveal this kind of misalignment.  It helps to distinguish collimation and alignment.



#34 ButterFly

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Posted 17 August 2019 - 12:01 AM

So on reddit some guy told me to put a piece of construction paper behind the secondary and take a picture down the focuser and I saw this: https://i.imgur.com/Nn8S2jD.png

 

It seems like my secondary is all jacked up. How do I fix this?

Assuming you took that picture at the exact center of the focuser tube, it looks like the spider adjustments (if any) or the secondary hub were loose.  Check the big screw at the hub of the secondary as well.  If it turns too easily, I would add weak springs around the collimation screws to add some resistance.

 

Sight tubes (collimation caps, or simply old film canisters with a hole in the center) help sort these things out first, before collimation is checked.  You can always collimate misaligned optics, as precisely as you want.  The desire is to precisely collimate accurately aligned optics and tubes.

 

At f/4.7, secondary offset is relevant.  Take a look at this page for secondary offsets and how to set then align a secondary (collimating it to the focuser with a proper offset).  Here is a good video for the method of sight tube and laser collimator.  You can skip the picture taking with a phone.  Only after you are fine at both sight tube and collimator can you go to autocollimator.

 

The best advice I can give is to read and watch, then do nothing.  Find you local astronomy club and let someone show you, if possible.  It is much easier to learn to do something by watching someone who does it well.  Collimating a telescope is like tuning a cello.  If the strings were put on badly to begin with, the tuning won't hold long and will get way off.

 

If there really is no one to show you, remember to always keep the scope horizontal while working on the secondary if it can drop out.  A pillow in the tube is added cheap insurance.



#35 N3p

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Posted 17 August 2019 - 07:41 AM

Laser collimators do not reveal this kind of misalignment.  It helps to distinguish collimation and alignment.

 

Collimation is the process of alignment no?



#36 jtsenghas

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Posted 17 August 2019 - 02:05 PM

Collimation is the process of making the optical axes coincident, so that the primary is pointing at the center of the eyepiece via the secondary and the eyepiece is pointing at the center of the primary via the secondary. 

 

A prerequisite is that the secondary is rounded and centered in the focuser as seen from the height of the focal plane, which automatically offsets it correctly. Translation, rotation and tilt of the secondary may be required first. 


Edited by jtsenghas, 17 August 2019 - 02:06 PM.


#37 ButterFly

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Posted 17 August 2019 - 02:05 PM

Collimation is the process of alignment no?

Collimation with a laser is just adjusting the relative tilits of the mirrors with respect to the focuser tube.  The secondary here is just a flat mirror.  You can adjust the tilt so it's collimated, but if it is not in a proper position, you won't see the whole primary in it.  If the focuser is not perpendicular to the tube axis, the primary will not be either once you are done.  If the secondary is way off, you can end up looking at a mostly coma riddled area of the primary.


Edited by ButterFly, 17 August 2019 - 02:06 PM.


#38 jtsenghas

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Posted 17 August 2019 - 02:13 PM

The focuser needn't be exactly perpendicular to the tube axis for excellent collimation. Consider low rider scopes in which the focuser may be tilted upwards as much as 30 degrees and the secondary tilted a full 15 degrees from the usual 45 degree position. 

 

The secondary must be POSITIONED according to the aim of the focuser, wherever that is, and then AIMED so that the eyepiece sees the center of the primary.  Only then should the primary be aimed back. Since the primary may move a tad doing tha , the process is a bit iterative always ending with a primary tweak for tilt. 



#39 N3p

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Posted 17 August 2019 - 08:20 PM

hemm, I did a quick search on the www about collimation and on the very first link I found this:

 

http://www.markthomp...lescope-optics/

 

Its crucial to the quality of the view seen through a telescope to have the optics inside accurately aligned. The process of aligning them is called collimation and is a task that with a little practice you will be performing with ease.

 

There is no way to really tell if the focuser is truly perpendicular with the axis of the primary mirror. I need to slightly adjust the angle of my focuser in order to get things perfectly concentric with a view from the inside of a Cheshire Sight Tube. I highly trust that specific tool.

 

Considering that, I believe today, most likely, that my focuser is not perfectly parallel with the axis of the primary (or +- the tube), but the collimation is pretty neat inside the Cheshire and they gave me an adjustable focuser.. so I think there is a tolerance possible with the parallelism of the focuser.

 

I am not a physician, just connecting the dots.



#40 ButterFly

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Posted 18 August 2019 - 12:37 AM

 

There is no way to really tell if the focuser is truly perpendicular with the axis of the primary mirror.

You just measure it when you build it then test with a laser as you rack in and out (spot the opposite side of the tube).  If it's close it's nothing to worry about.  The only ill effect of being slightly off on alignment is field illumination - which you really won't notice for reasonable misalingments.  The scope worked at some point so it's good enough.


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#41 sixela

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Posted 28 August 2019 - 06:12 AM

Uuhmm -- collimation is what makes the focuser axis coincide with the optical axis. To achieve this the focuser axis need not be perpendicular to the tube at all -- wherever it points, it's something you can compensate with a different tilt of the secondary--, so this is not a mechanical issue (unless it's so severe that the front of the tube would start to vignette the light bundles).

 

hemm, I did a quick search on the www about collimation and on the very first link I found this:

 

http://www.markthomp...lescope-optics/

 

Its crucial to the quality of the view seen through a telescope to have the optics inside accurately aligned. The process of aligning them is called collimation and is a task that with a little practice you will be performing with ease.

 

There is no way to really tell if the focuser is truly perpendicular with the axis of the primary mirror.


Edited by sixela, 28 August 2019 - 06:13 AM.



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