To collimate or not to collimate?

[Dragons Egg]

What are the telltale signs when a telescope, a refractor in this instance, needs collimating? 

PS. I am aware of how to check whether a scope needs collimating: I just want to know what the signs are. Many thanks. 

[vlaiv]

High power view being not as sharp as it used to be would be first sign to do simple star test to verify collimation.

[John]

Lack of contrast on the planets. Not being able to get a clear split of double stars under reasonable conditions. Non-concentric diffraction rings on stars either side of focus at high magnification.

You can also check collimation with a cheshire eyepiece and the optical alignment of the focuser and objective with a laser collimator.

 

[PeterCPC]

http://www.opticalvision.co.uk/documents/202.pdf

Peter

[CraigT82]

2 hours ago, GrahamKnott said:

What are the telltale signs when a telescope, a refractor in this instance, needs collimating? 

PS. I am aware of how to check whether a scope needs collimating: I just want to know what the signs are. Many thanks. 

Simply defocus on a star, if the diffraction pattern is skewed and not concentric you've got work to do! 

Edited September 12, 2020 by CraigT82

[Dragons Egg]

So what would be the cause if the stars aren't uniformly in focus across the field? This is the problem I've had for several months now with my latest scope.  All stars now remain stubbornly out of focus towards a given edge of the viewing field, starting roughly 20 percent in from the field edge - whereas previously I received a tack-sharp view right to the very edge. In other words I can no longer get the entire field properly into focus. The end result can be observed by watching a selected star gradually lose its crisp point-like focus as it transits, in this instance, towards the right edge of the scope's FOV. I have checked for dirt or smears on the objective lens in the offending area, but can't see anything untoward there. I also checked the EPs, and they're fine. I will do the Airy test, also swap the scope's diagonal for my TV one; but if all that checks out fine as well, then I don't know what to think, other than accept that something serious has happened to the scope's optics. What exactly, though, has left me completely foxed. As a sidenote, I take great care of all my astro gear, as one does naturally. Thanks for the input.   

PS. Forgot to mention this, but while inspecting the scope's objective lens yesterday evening I spotted something I've not seen before: traces of what I assume are traces of glue lining part of the lens' protective outer shell? Just a thought.   

 

Edited September 12, 2020 by GrahamKnott

[fozzybear]

Maybe offer up what scope you are referring to, this might shed some light on it (sorry for the pun)

[Dragons Egg]

Hi fozzybear. It's an ES ED80mm apo (f/6). It's just over a year old too, and back then it gave amazing widefield views of star clusters etc, but no longer.  

[fozzybear]

A bit out of my league this is a triplet yes?

[Dragons Egg]

Yes, it's a triplet. 

[John]

Has it had a knock ?

If it used to perform and now does not, something must have changed

[fozzybear]

Just now, John said:

Has it had a knock ?

If it used to perform and now does not, something must have changed

agree with John something has moved

[Dragons Egg]

Hi John, although I'm still flogging my brains to remember if it's ever had a knock of any kind, it rather looks as if it might have had one, going on the evidence. Does this imply that the objective lens is now slightly out of kilter as a result? 

[John]

Triplets can be sensitive if one of the lens elements gets de-centered, ie: it's optical axis is not exactly in line with the other two.

Quite a tricky thing to diagnose and fix though.

 

[vlaiv]

1 hour ago, GrahamKnott said:

So what would be the cause if the stars aren't uniformly in focus across the field? This is the problem I've had for several months now with my latest scope.  All stars now remain stubbornly out of focus towards a given edge of the viewing field, starting roughly 20 percent in from the field edge - whereas previously I received a tack-sharp view right to the very edge. In other words I can no longer get the entire field properly into focus. The end result can be observed by watching a selected star gradually lose its crisp point-like focus as it transits, in this instance, towards the right edge of the scope's FOV. I have checked for dirt or smears on the objective lens in the offending area, but can't see anything untoward there. I also checked the EPs, and they're fine. I will do the Airy test, also swap the scope's diagonal for my TV one; but if all that checks out fine as well, then I don't know what to think, other than accept that something serious has happened to the scope's optics. What exactly, though, has left me completely foxed. As a sidenote, I take great care of all my astro gear, as one does naturally. Thanks for the input.   

PS. Forgot to mention this, but while inspecting the scope's objective lens yesterday evening I spotted something I've not seen before: traces of what I assume are traces of glue lining part of the lens' protective outer shell? Just a thought.   

 

This really sounds like field curvature. There are a few possible causes of why are you seeing this now and not before.

1. You used different eyepiece before that had shorter focal length / smaller field of view. Have you purchased new eyepiece with long focal length and wide AFOV recently? Does this happen with every eyepiece or just particular one?

2. It could possibly be that this aberration was there before but you did not pay attention - it is often the case of "once you see it, you can't unsee it anymore". This happened to me at some point in my observing career - once I began reading eyepiece reviews and noticed that people talk about edge of field aberration - suddenly I started seeing seagulls in my plossl eyepieces in fast scope - there were not there before (or I wasn't paying attention).

3. Deteriorating eyesight - or natural process with years. How old are you? At some point in life eyes loose their ability to focus close, or rather effective focusing range decreases because things loose their elasticity. It is actively happening to me last half a year - I need to start wearing glasses for reading or at least get longer hands

Field curvature causes edge stars to be out of focus, but if your eyesight is fine - your eyes automatically compensate and once you look at field edge - your eye does focusing and stars look in focus. Once you loose this ability to refocus fast with your eyes - then you start noticing field curvature - out of focus stars in outer part of the field.

[Alan64]

Triplet-apochromats are favoured by imagers, although they are used on occasion for visual-use.  Incidentally, your Tele Vue is a quadruplet.  Consequently, you'll want to handle that one as one would a newborn babe.

Then, for visual-use only, a doublet-apochromat serves admirably, and are, generally, problem-free.

Triplets, if misaligned, are usually not an at-home repair.  They are sent off instead, for servicing.

[fozzybear]

16 minutes ago, Alan64 said:

Incidentally, your Tele Vue is a quadruplet

The OP was referring to scope is an ES 80ED triplet and the diag is a tele vue

Edited September 12, 2020 by fozzybear
spelling

[Alan64]

1 hour ago, fozzybear said:

The OP was referring to scope is an ES 80ED triplet and the diag is a tele vue

I was referring to refractors only, and to the OP's Tele Vue NP101is, a quadruplet-apochromat.

[fozzybear]

5 minutes ago, Alan64 said:

I was referring to refractors only, and to the OP's Tele Vue NP101is, a quadruplet-apochromat.

? where does this post quote a TV NP101?

[Alan64]

1 minute ago, fozzybear said:

? where does this post quote a TV NP101?

 

[fozzybear]

3 minutes ago, Alan64 said:

 

? still confused this a post related to dew control...

[fozzybear]

Duh same user and a cross thread!!! lets go off tangent

[Alan64]

Just now, fozzybear said:

? still confused this a post related to dew control...

My reply was in reference to the problem with misalignments that occur with triplets, the subject of this thread at present, and other objectives with more than two lens-elements.

[Alan64]

5 minutes ago, fozzybear said:

Duh same user and a cross thread!!! lets go off tangent

You asked for information.  

[fozzybear]

2 minutes ago, Alan64 said:

My reply was in reference to the problem with misalignments that occur with triplets, the subject of this thread at present, and other objectives with more than two lens-elements.

Alan,

I understand now what would you recommend? Me if a triplet then send it back to the supplier to be recollimated if unable to do by oneself? or any other ideas?