Nikon Fieldscope ED78 - Star test - advice please (1 Viewer)

[David M O]

I have the above Fieldscope that I have owned from new since 1996. For me, for the most part, it's been a fantastic piece of equipment - really bright and sharp image with the 30x wide angle eyepiece, not quite so bright but still sharp at the upper end of the range of zoom on the 25 - 56 x zoom. I've recently become more interested in waterfowl/waders and sea watching where a brighter and less restricted zoom eyepiece would be useful. As such I started looking into the alternative options available and have been reading through some of the threads on this forum. I have to admit, it has been quite an eye opener in terms of the recommended caution that I should take before buying a new scope - I never before appreciated how variable the quality of a premium brand scope might be.

So, I read up a little on how to do a star test and thought I would try it on my Fieldscope so that I know what do do when I come to inspect other scopes. Because the image is so sharp on my Fieldscope I fully expected to see something close to an identical set of concentric rings wholly within the bright spot. This turned out not to be the case. Unfortunately, I wasn't able to photograph the image I was getting so I've sketched out what I saw and attach that sketch. In essence, at one side of the focus point the concentric rings were displaced to the left of the bright circle. At the other side they were displaced to the right by an equal amount. If you rotate the attached image by 90 degrees you'll get the gist of what I'm seeing. The displacement was large - less than half (of the full circle) of the concentric rings was visible each time. If I moved my eye across the eyepiece a bit I could see a bit more of the concentric rings but not much more.

I have a couple of questions that I'd like your help with please:

1) Is it possible that I am doing the star test incorrectly and, in doing so, it is producing what appears to be an extreme result? If so, what might I be doing wrong? (I did try moving closer or further away from the star and also changed the eyepiece - neither seemed to affect the result I was getting).

2) If the answer to 1) is 'no' and the test is likely to be good, what does the result indicate is wrong? How should this affect the image of birds etc. that I see through the scope?

Grateful for any thoughts / suggestions you may have on this.

Thanks
David

 

[Binastro]

Hi David and welcome,

It seems that the scope is out of collimation.

The scope may have had a knock.

However, if the scope gives a good enough image as is, then just use it.

I don't know what the optical construction is.
It could be an objective lens and then a focus element and or negative element.
The fault could be in the intermediate elements.

I have often used somewhat faulty optics that give a good enough image.

Hopefully, some others here will give their opinion and whether it is worth fixing.

Regards,
B.

 

[David M O]

Hi David and welcome,

It seems that the scope is out of collimation.

The scope may have had a knock.

However, if the scope gives a good enough image as is, then just use it.

I don't know what the optical construction is.
It could be an objective lens and then a focus element and or negative element.
The fault could be in the intermediate elements.

I have often used somewhat faulty optics that give a good enough image.

Hopefully, some others here will give their opinion and whether it is worth fixing.

Regards,
B.

Hi Binastro,
Thank you very much for your prompt response on this, that is very helpful. In fact, after 25 years of no incidents, the scope did indeed get a knock last week - the wind blew the tripod over and it fell on the restraining knob on the collar. Result - the collar broke in 2 and a crack appeared in one on the intermediate elements. I didn't want to reveal this up front because my main interest was in whether or not I was totally awry with the way I was doing the star test and I didn't want to overly influence any response. It sounds as though what I found might well have been expected after the fall so I appreciate your response and diagnosis.

I thought I had lost the scope completely but I have been able to secure the collar/foot amazingly well, lashing it with elastic bands in lieu of working out whether I can rejoin the two halves more permanently. Looking through the scope I can see no difference as a result of the crack and I can still use it successfully - at least for the time being. This was the final straw in my decision to look for a new scope.

Given the extreme result of the test with this 'scope', I suspect I should find another to try out the method with to see if I get a more conventional set of images before I attempt to test one I might buy!

Thanks again for your time,
Regards
David

 

[Binastro]

Hi David,

It may be that an element has tilted or even the prism dislodged to move the image sideways or at an angle.

If the scope still works then use it, although further displacement could occur if elements are loose.

25 years isn't bad before a fall.

I have occasionally dropped eyepieces and once smashed a camera, despite being very careful usually.
Also dropped a 70mm monocular on granite when I slipped on ice.
Also a camera stolen.
It's part of life.

Regards,
B.

 

[wllmspd]

Sneakily star testing potential scopes would certainly help you get one that could take high powers. I had my old cheap 80mm astro refractor at 130x on Jupiter recently, red spot visible.
Sad to lose an old friend, good to hear it’ll still deliver some views till you find a suitable replacement.

Peter