To collimate or not to collimate?

[Alan64]

Just now, fozzybear said:

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?

I've already hinted that it requires collimation.  I, too, am suspecting that, and as you did, and in agreement with John.

[Alan64]

It is not impossible to collimate a triplet in a home setting.  The OP would need to research to go about that, if collimation is in fact required.

[fozzybear]

2 minutes ago, Alan64 said:

I've already hinted that it requires collimation.  I, too, am suspecting that, and as you did, and in agreement with John.

Alan,

Your quote on the TV kind of threw me  meant no disrespect

Andy

[John]

It threw me as well Andy !

With triplet lenses, my understanding is that the accurate centering of the 3 lens elements is critical as well as the tilt of the objective as a whole.

I have collimated doublet refractors quite a few times but if my 130mm triplet went out of collimation I'm pretty sure that I would seek professional help with that.

 

[Alan64]

This is the collimation-scene of an 80mm f/6, a crown-and-flint achromat, which did not require collimation upon arrival...

Well, perhaps a slight tweaking is necessary, particularly since it is at f/6.  You do want to push one for all it's got, in magnification, per its aperture.

Alas, it exhibits a blemish, and when having observed Sirius in this instance...

I see that all the way until it's at perfect focus.  It doesn't seem to degrade the images however; I hope, I trust.

On the other hand, the collimation scene of a 70mm f/13 achromat...

That's not so good.  But what I found, mainly, in that instance, was a wonky focusser. 

[Dragons Egg]

11 hours ago, fozzybear said:

? where does this post quote a TV NP101?

To prevent any further confusion, my other refractor is a newish NP-101is, and, yes, as has been rightly pointed out, I do treat it like a newborn babe - like the crown jewels, in fact. The pay-off were the fantastic high-power views it gave last night of Saturn and Jupiter at our local (semi?) dark site. And that was without any appreciable cooling down period either - the clouds were already rolling in while setting up the scope, but I'm going off topic here.  With regards to the ES 80mm apo. . .

 

14 hours ago, vlaiv said:

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.

1) It happens with all my EPs, ranging from a Delite 4mm down to an Ethos 21mm. As a matter of fact I started with wide AFOV EPs - the Ethos and two ES EPs (14mm/8.8mm 82 degrees), plus a barlow - and only later branched out into higher power EPs.

2) Yes, that's always possible, of course. Still, I am pernickety about my optics - as I am about other pursuits. Here's another thing: while aligning a new SW finderscope on the refractor in daylight last week, I couldn't help but notice how washed out everything now looked through the scope itself, while the same views seen through the 9x50 finderscope looked bright and crisply sharp by comparison. The contrast was startling in fact. I don't know what that brings to the table, but for me it was an early sign that something was no longer quite right with the refractor. 

3) I'll never see 60 again and my eyesight has plainly deteriorated as a result. Indoors certainly I now have to wear glasses when reading books or text on a screen. On the other hand, if the problems I'm having with the ES scope were purely due to failing eyesight, then wouldn't it also manifest on my other two scopes, likewise my binos?   

When you suggest field curvature as a possibility, does this imply that its effects would be 'global' - that's to say it affects the total viewing experience? Or could it show itself in just one part of the scope's FOV? - which is the case here. 

    

 

 

[kirkster501]

You can't collimate a refracting telescope without specialist gear, less so a triplet or Petzval design.  Back to the manufacturer or distributor.

[CraigT82]

17 hours ago, GrahamKnott said:

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.   

Sounds like the scope has taken a knock or the cell has somehow come loose? It's probably going to be a specialist job to inspect and repair. 

[Dragons Egg]

Yes, that looks to be the way of it. If there's any commiseration to be had, I originally bought the refractor last autumn to serve as a travel scope - it's so compact that it can easily pass muster as flight baggage, Well, of course, Covid 19 has put paid to all that  

[vlaiv]

16 minutes ago, GrahamKnott said:

3) I'll never see 60 again and my eyesight has plainly deteriorated as a result. Indoors certainly I now have to wear glasses when reading books or text on a screen. On the other hand, if the problems I'm having with the ES scope were purely due to failing eyesight, then wouldn't it also manifest on my other two scopes, likewise my binos?  

Field curvature of particular optics depends on its speed and design.

Triplet refractors of short focal length are particularly prone to having strong field curvature and field flatteners are a must in astrophotography with them.

17 minutes ago, GrahamKnott said:

When you suggest field curvature as a possibility, does this imply that its effects would be 'global' - that's to say it affects the total viewing experience? Or could it show itself in just one part of the scope's FOV? - which is the case here. 

Collimation issues will be global in nature - over whole field, but field curvature is strictly in outer field.

If stars are in focus in center of the field - as you move away from the center - they will become defocused. In fact - this gives simple test for field curvature:

1. focus on a star in center of the field and move it to outer field - it should become defocused

2. Now focus on it in outer field - if you are dealing with field curvature you should be able to focus it fine and once you move it back in center - it should be out of focus. This shows that center of the field and outer part have different focus position.

21 minutes ago, GrahamKnott said:

1) It happens with all my EPs, ranging from a Delite 4mm down to an Ethos 21mm. As a matter of fact I started with wide AFOV EPs - the Ethos and two ES EPs (14mm/8.8mm 82 degrees), plus a barlow - and only later branched out into higher power EPs.

This is somewhat concerning - field curvature should not behave like this so it could be pointing to another problem. If you use high power eyepiece - you are only looking at central part of the field and central part of the field should be in focus (or rather there should be very small defocus - look at diagram above - curved field starts easy then shoot off).

This means that you should not see the effect in 4mm eyepiece, unless eyepiece introduces field curvature of its own. Delite has comparatively smaller FOV than the rest of your eyepieces so we can't blame that on wide FOV.

 

Bottom line - try above focus / defocus test to give you some more clues if this is related to field curvature.

[Dragons Egg]

8 minutes ago, vlaiv said:

Bottom line - try above focus / defocus test to give you some more clues if this is related to field curvature.

I'll do that. Either way it now looks as if specialist remedial treatment is on the cards. 

Edited September 13, 2020 by GrahamKnott

[vlaiv]

14 minutes ago, GrahamKnott said:

I'll do that. Either way it now looks as if specialist remedial treatment is on the cards. 

If this is due to field curvature - then you can either accept that, change scope or get field flattener. Some people that are bothered with field curvature use photographic field flatteners adopted for visual use.