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asaint
Carpal Tunnel
Reged: 04/25/03
Posts: 2022
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Borg 76 ED
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edwincjones
Post Laureate
Reged: 04/10/04
Posts: 4430
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I miss the Small Scope forum because it covers the usefullness of small scopes for travel and grab and go.
I enjoyed your review.
What is the current status of Borg? They have downsized the bigger scopes and you do not hear much about them anymore.
Ed
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n w arkansas
Binocular, Solar, General Amateur Astronomy
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Ron B[ee]
Tyro
Reged: 04/27/03
Posts: 4719
Loc: CA
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Quote:
I miss the Small Scope forum because it covers the usefullness of small scopes for travel and grab and go.
Ed
Hello there Ed,
As "deceased"   moderator of the old Small Scope Forum, I'd like to say that you can still discuss, are encouraged to discuss and are appreciated to have discussed about small telescopes in the Refractor Forums since most of the discussion about small scopes used to revolved around refractors anyway (though small SCT and MCT were also frequently discussed).
Perhaps, you might even post a suggestion in the Suggestion Forum for a Small Scope sticky post in the Refractor Forum  . You didn't get this idea from me, eh Ed .
Ron B[ee]
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5-inch Tele Vue NP127 APO 
4-inch Tele Vue TV-102 APO 
8-inch f/6 Discovery PDHQ Dob 
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edwincjones
Post Laureate
Reged: 04/10/04
Posts: 4430
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I know the small scope forum is in the refractors, but the little guys seem to get lost in the big guys forum.
The change makes sense, but I just miss the "old ways"
Ed
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n w arkansas
Binocular, Solar, General Amateur Astronomy
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
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On the astigmatism issue - I have a borg 101, and when I got it it also had that "C" shape to the in-focus stars. The primary was out of collimation - there are 4 collimation screws hidden under the dew hood (you have to remove it to access them - they are very small allen setscrews). It took me about an hour of work outside doing a star test to get it collimated, but once I did the C shape was gone and the star test looked MUCH better (rings were better defined, and there was also less CA visible).
I have heard of several other people with similar experiences I think their cell design tends to slip during transport - I presume they collimate at the shop before shipping, but most people I have heard this from had the problem upon receiving them. Mine was used, and the previous owner said he hadn't noticed it (my case was significantly worse than the pics in the review, but similar shape).
You might want to check the collimation. I recommend getting 2 allen wrenches - I did it with 1, and that's largely why it took so long. I had to loosen one side, then switch to the other side to tighten, and often overtightened a bit, then back, etc. Two wrenches would make it much easier.
Jarad
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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The 76, as far as I know, cannot be collimated. However I did not notice the astigmatism when I took the photos, only later. It must also be noticed that the axes of astigmatis are in the direction of the spurious reflections.
On mild summer nights, when the scope is not subject to large temperature drops from its storage location to the filed, I see perfect round rings and a better resolution (the roll-off magnification si about 150x).
However, 120x or 150x does not make much difference. The big Jump, in my opinion, for planets is between 120x and 400x of the XT10i (my little dob). Of course I cannotg trasport the XT10 into the desert... but why should I move to the desert for planets?
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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... anyway thanks Jarad. I'll check whether there are screws (but I think there are not any) and whether they may be resposnible for the occasional astigmatism.
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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... ops, last comment: right click on Fig.5 and open it in another window (chose open image in another window or the like, the link does not work). You will see a very large picture with lots of details of the ring structure.
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 3858
Loc: Atlanta, GA
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Ooops, looks like you are correct - just checked the Borg website and only the 100mm and up cells can be user collimated.
It still looks an awful lot like what I saw, though. I suspect that the cell is either a bit out of collimation or pinching, or that it may be doing so only when the temperature shifts (i.e. maybe there are parts of the cell that have different expansion coefficients so it goes out of collimation when cold, but is in collimation at room temp, i.e. when they check it at the factory).
Jarad
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Yes, the amount of astigmatism the pictures show is not negligible. Without that, as said, the resolution improves noticeably. No difference are seen below 100x though. If you carefully look, besides occasional astigmatism, there is also brightening of the outer rings in the extrafocal image and -conversely- brightening of the central part in the intrafocal image (that in addition to softening inside focus).
I noticed that the differences in radial light distribution also vary with thermal stabilization (again they are a little less in pertfect conditions).
Overall I would rate what seen in the picture to be barely "diffraction limited" for what concerns SA (plus there is softening and the astigmatism). In better conditions I would say the scope becomes "diffraction limited", but only according to the less demanding Rayleigh criterion (not according to more stringent criteria . but these are my opinions). I have got mirrors that (although obstructed) produce diffraction disk with fainter rings. As I said the rings in this little scope resemble the brightness of a SCT-class obstruted scope (in turn brighter than the better low obstruction Newtonians).
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redvis
Pooh-Bah
Reged: 01/30/05
Posts: 1148
Loc: Claremont, CA
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Just a follow up - with my Borg 77ED if I have my tube rings too tight around the scope then the stars can look like they have astigmatism. I believe Borg even mentions somewhere on their site that tube rings should not be too tight. Once I loosened up the rings the optics became superb. This may be something to try.
Cameran
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Yes. If the tube is held too tight it also may produce astigmatism (I verified after reading the same instructions). Not only, I also keep the objective cell slightly loose (not tightly screwed into the tube).
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