How to check your new scope? (1 Viewer)

[Bird Reynolds]

Hi,

I am experienced in watching birds through low budget scopes. Now I purchased a Kowa 883 online.
The view is fantastic. But probably even the worst lemon of a high end scope would look great to me.

I just would love to have some tests at hand to rule out that I have received a slightly damaged (f. e. due to transportation) instrument or a lemon.

The exit pupil is round.
I performed a star test. The out-focus picture looked with its concentric round clear rings perfect, the in-focus picture blurry. I can barely see rings.
Here's a Post in thread 'Star Test Result Question' that kept me from despair. I tried again with a green foil in front of the led-star and the in-focus rings are a bit better.

Overall its not satisfying and I wish someone could help me how I could decide if it is a keeper or not.

Best regards

 

[Mono]

What did you buy the scope to do? The only thing that matters is if it does what you wanted it to do. You can get lost down a rabbit hole if you perform endless finer and finer tests until you find something that fails.

 

[jring]

Hi,

first of all, welcome to birdforum!

It is correct, that even a hypothetic perfect fast apochromatic refractor will show non-identical diffraction patterns which will look like some degree of over- or undercorrection. They can be mostly mitigated by using a green filter (as most optics for visible light are best corrected for green) and even better by taking images and using a green laser as a point source (DO NOT USE A LASER FOR STAR TESTING VISUALLY!!!).

The question is how much of the overcorrection you see is caused by that effect and how much is actually in the lens. Post 19 in the thread you linked above shows a very good star-test which I would love to see on any of my instruments... Use that for comparison...

Star Test Result Question

I am currently star testing two spotting scopes (Kowa TSN-883 and Swarovski ATX85mm) and I have a question about the results. Photos in a post below. The "star" is an LED flashlight covered with tin foil with a pin hole poked in it and placed 30m away (I measured). On the Kowa: the inside...

www.birdforum.net

What aberrations are acceptable also depends on the use case - and spotters used with their normal eyepieces and for birding with their relatively low magnifications tend to be more forgiving than astro scopes. So if you only plan to use your scope for birding without extenders or adapted astro EPs, the benchmark should be a to get a crisp image with an easy to find point of best focus at 60x on a cool and overcast early morning (for good seeing).

Joachim

 

[Tringa45]

You can get lost down a rabbit hole

But that's where some of us are at home .

John

PS: Welcome to Birdforum from one 883 owner to another and from one Tringa to another!

 

[Hermann]

A quick and dirty test is to try the scope on a day with good seeing. It should be sharp at the highest possible magnification. Zoom up from the lowest to the highest magnification. The image should get a bit darker, but it should stay sharp. In an ideal world you should have a scope of known quality for comparison.

Not as reliable as a star test, but you'll definitely see whether it's a true lemon or not.

Hermann

 

[Hermann]

What did you buy the scope to do? The only thing that matters is if it does what you wanted it to do. You can get lost down a rabbit hole if you perform endless finer and finer tests until you find something that fails.

Absolutely true. However, if you pay a lot of money for a top scope, you want to be sure it performs like it should, and sadly quite a few scopes don't. I've seen scopes from the top manufacturers where the image simply fell apart at 40-50x. If an 80mm scope shows less detail than a (good) 60mm scope, something is wrong, and I'd definitely want to know this before I paid a lot of money for such a scope.

Hermann

 

[Bird Reynolds]

Thanks for the fast replies!
Mainly I'll use it to identify birds of prey in an alpine environment, which is part of my job. So i am kind of a heavy user 😅

Even at 60x it is sharp from center to edge. I have to adjust the focus a bit after zooming. I think I'll keep it.