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Kowa 88A star test (1 Viewer)

mauhler31

New member
United States
Hi folks! I am new here and also new to birding as well. I got inspired to take on this new hobby on a recent trip where I took a wildlife tour and was amazed by the magnification and clarity you can get from scopes. After some research, I decided on the Kowa and was able to get the new 88A on sale. It just arrived today and I wanted to get feedback on these star tests and if there appears to be any glaring problems.

Please keep in mind that this is my first time doing a star test, so there could be some problems with my setup:
  • These photos were captured using a DSLR without camera lens. Unfortunately the DSLR digiscoping adapter was not in stock, so on the camera I connected an extension tube followed by a clear filter, and then used two tripods to line up the camera and scope eyepiece as close as possible. Since this was a loose fit, it's possible the camera was not perfectly aligned with the scope.
  • Used an artificial star (foil with pinhole + LED flashlight) indoors about 10m away

My sense of the results are:
  • The rings look mostly concentric, maybe slightly oval but I am not sure if that could be due to camera/scope misalignment.
  • The inner focus rings look pretty good.
  • The outer focus rings are difficult to see, and is brighter on the outside, possibly a sign of overcorrected SA. There's also redish/purple ring that I don't know what that could indicate.
  • There is a slight vertical line, possibly from the prism, which seems common to Kowas

Let me know what you think of the results and if there are other tests I should run. I hope to take the scope out tomorrow for some real-world evaluation.
 

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Hi and welcome to Birdforum!

You did a good job to get those pictures. Based just on these pictures I think there is indeed some astigmatism as inside and outside are oval at 90 degree opposite directions but it's not terrible amount. Definetely some spehrical aberration as outside figure has no clear inner rings as inside figure has, also inside center has big bright dot and outside has seemingly black dot. Maybe the center point is just a little downward from perfect center(?), which could indicate some coma. Prism roof line is showing as it seems to be with most 883 samples also. Colour of the rings (yellowish inside, purplish outside) may indicate chromatic aberration but could it be from the camera, can you test your scope viewing high contrast objects at max. magnification, dark branches against cloudy sky and see if there is some colour fringing at the dark edges? Some 883 have this colour fringing at high magnifications (including my sample) but it doesn't clearly jump out in star test, which has puzzled me (among many other things with optics, since I'm just amateur with limited knowledge and experience).

If these pictures are to be trusted, I think this sample is not a lemon but not a great one either, so it hits the average category.

If you could make resolution measurement or at least compare this sample to another, you might get some information how it is performing.

Regards, Juhani
 
Good job @mauhler31 and thanks for sharing!

I too look forward to hearing from the experts on this one.
- I wonder if the outside images were obtained after de-focussing for more than 4-5 rings?
- If you can, please try at longer distances (>30m) and try to have not more than 5 rings of defocus.

Cheers!
 
The outside rings size was determined by trying to match the same size as the inside rings. I think regardless of size it was very difficult to make out any outside rings.

I unfortunately don't have any other scope to compare to. This is my first scope and there are no Kowa dealers near me.

I did a couple more experiments today at max magnification using my smartphone adapter.

1. Resolution test indoors at 10m. Note I scaled down the image to print out at letter size and the brightness variation is in the actual paper print (this probably is not a super high quality printer). You can see chromatic aberration in the unfocused parts.

2. Outdoor tree branches at 10m. One in focus and one out of focus. The chromatic aberration is more apparent when out of focus.

3. Flowers at 15m. This was mostly to get a sense of color. It's hard to make a judgement without a second scope to compare to but it seems fine.

4. Tree branches at 50m. It looks sharp to my eye but I was fighting my phone's autofocus when trying to take the picture. The sun was also starting to set.

In general when looking through the scope directly, everything is sharp and I can't see any aberrations. Only when digiscoping and zooming in the picture afterwards I can notice these. I'll try next week to actually find some birds :)
 

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Hi mauhler and a warm welcome to you from all the Staff and Moderators.

I'm sure you will enjoy it here and I look forward to hearing your news.
 
Hi,

it seems I was called from gazing at stars to staring at star test pictures...

A bit of astigatism is visible - certainly less than the 0.37 lambda which would make it diffraction limited (when there are no other aberrations present).

Spherical seems very minor and as we know SA in star tests with fast ED doublets or triplets is a bit deceiving, so I would ignore it.

I think we have a quite good example here... as is shown by the digiscoping images which have nice detail.

Enjoy the view!

PS: Welcome to birdforum to the original poster!

Joachim
 
Thanks all, I'm glad to hear that the results are looking good. I'll try to take it out over the next few weeks and share some more pictures.

Regarding the overcorrected SA, it's occasionally noticable with the eye if I am intentionally looking for it, but only at certain distances between in focus and out of focus objects. It is more obvious in pictures, but I think it is manageable by post processing to add more blur to the background. I've attached two pictures, one without editing and one after using my phone's blur feature.
 

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