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Airy disk question on cheap refractor

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#1 Guest_**DONOTDELETE**_*

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Posted 09 September 2003 - 12:01 AM

Hi. I'm new to this forum. Hope I'm in the right place. As if I didn't have enough hobbies, I'm thinking about getting back into this one after a long lapse. Last night I checked the circular diffraction pattern on my el cheapo 600 power (haha) Tasco 60mm f15 refractor that I bought at Costco 12 years ago (using a Meade 12mm MA eyepiece). I'm new to interpreting such things so I downloaded a copy of the aberrator program for help. The "disk" I saw had a very significant football shape which I couldn't duplicate with the aberrator. The diffraction rings were visible but compressed due to the football shape. Anyone ever hear of this kind of aberation? Just curious. Maybe I should give Tasco an award?

I'm thinking that an Orion Observer 70mm EQ is in my near future. I haven't seen anything better for $159. Anyone want to talk me out of it? Please?

#2 Trever

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Posted 09 September 2003 - 02:36 AM

As a current owner of a Tasco-like refractor(JASON) and a cheap Meade polaris refractor, I can honestly say that an Orion refractor should and will please you more often. Thats why I bought a better scope(Still on the way!). I can't tell you how many different shapes I had when I attempted testing the lenses with star images. Depending on what your price range or experience is, you may also want to consider a 6 inch Dob. You can probably pay 250 or so and maybe even cheaper used depending on where you look. As soon as I get my Dob, im pitching my JASON telescope as far as I can throw it. But there are many seasoned experts here that may differ from my opinion...

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#3 Bob Pasken

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Posted 09 September 2003 - 07:04 AM

You might try taking off the dew shield and loosing the lens retaining ring just a little. The dew shield should simply slide off with a gentle tug. The lens retaining ring should unscrew like a jar lid. Becareful not unscrew it to far. I like to point the tube vertical so the lens is pointing straight up in case I unscrew it too far. The lens cann't fall out that way. Very often the astigmatism is caused by the lens retaining ring pinching the lens. My son has a 60mm Orion and it came a bit pinched. loosening the ring took care of the problem. Turns out that the optics are about as perfect as you can get. The scope really performs well with the 25mm and 10mm eyepieces it came with

#4 Blair

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Posted 09 September 2003 - 07:20 AM

I've owned the 70mm; very good scope for the money. My problem with it was no dove tail mount in the focuser for a viewfinder. I tried putting a right angle 6X30 where the red dot finder was but that was too far away from the eyepiece. I had to raise up to look through the finder then sit down to look. If you can live with the red dot finder then buy it! :grin:

#5 EdZ

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Posted 09 September 2003 - 08:17 AM

Sounds like astigmatism. You can verify by checking to see if the elongated football shape of the image changes orientation by 90° from inside of focus to outside of focus.

My first question to you is do you wear corrective lenses? Do you have astigmatism in your eye? If so, you must wear your eyeglasses when checking star test images or the astigmatism from your eye will distort the image.

edz

#6 Guest_**DONOTDELETE**_*

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Posted 13 September 2003 - 10:30 PM

EdZ, the footbal shape does not change orientation inside or outside of focus. I wear contacts for myopia but I don't have significant astigmatisim in my eyes. Just to check, I walked around the eyepiece and the football shape stayed put. Also I adjusted the clamping force on the objective cell - no effect. I even rotated the objective. Next, I rotated the eyepiece - no effect. Last, I rotated the diagonal and THAT seemed to change the orientation of the football. With a star centered in sharp focus, rotating the diagonal causes the star to shift left or right out of the fov. Should that happen? I would think it would stay put if the diagonal mirror was oriented accurately. As the focuser is .965 and my eyepiece is 1.25, I can't simply eliminate the diagonal (which is also a .965 to 1.25 adapter).

I'm probably wasting my time with this scope but something in me is driving me to "tune" it to its best capabilitiy before I get another one. Thanks for all the help.

#7 jrcrilly

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Posted 13 September 2003 - 10:50 PM

Last, I rotated the diagonal and THAT seemed to change the orientation of the football. With a star centered in sharp focus, rotating the diagonal causes the star to shift left or right out of the fov. Should that happen


Nope. You've located the (or at least a) problem.

#8 EdZ

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Posted 14 September 2003 - 06:53 AM

I agree with John, I think you've located the problem.

You can't insert 1 1/4" eyepieces into the 0.965" tube, so see if the 0.965" sleeve will unscrew from the diagonal and GENTLY see if it will srew onto your eyepiece in place of the standard 1 1/4" sleeve. I have several eyepieces I do this with so I can use them in my 0.965" scopes.

If you can do this and can get an eyepiece into the focuser without the diagonal, you will be able to tell if it is the diagonal that has the defect. That's what it sounds like from what you are describing.

edz

#9 Guest_**DONOTDELETE**_*

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Posted 14 September 2003 - 12:13 PM

Edz, I tried what you suggested but it won't work. In the process of trying, I slightly disasembled the diagonal and found that it uses a prisim instead of a mirror. Am I wasting my time? Are quality .965 eyepieces (and diagonals) available? I read somewhere that they are popular in Japan. You don't want to know how much time I've spent in the last two weeks trying to figure out what new scope to buy. I've gone from one "decision" to another. Right now, I'm bouncing between the Orion SkyView Pro 120mm EQ and the AstroView 120ST EQ. I was thinking about the Observer 70mm EQ but now? Too many choices. I think we live in good times for finding a decent scope under $700. Hey, maybe I could use the Tasco as a guide scope for the f8.3...

#10 EdZ

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Posted 14 September 2003 - 01:45 PM

I still own four scopes that use 0.965". Two are Newtonians, so don't need a diagonal. I've had three 0.965 diagonals. All three are prisms. One provides a perfect image. One has astigmatism. The third was 20yr old original plastic. It still worked fine until the lock screw stripped. Maybe try posting a want ad for one over on Astromart. Shouldn't cost you more than 20$.

I've assembled a quality box of eyepieces for my 0.965 scopes. I have a 5mm and 6mm Vixen ortho and a few Orion plossls from 7.5 to 20mm. Plus, as I said before, the 0.965 sleeve screws off of the Orion plossls and will screw onto almost any eyepiece I own. Orion at one time marketed the Sirius plossls in both 1.25" and 0.965". It was the exact same eyepiece with a stepped down sleeve screwed onto it. The sleeve can be used to convert almost any standard 1.25" ep into a 0.965". So I've used most of my eyepieces in my small scopes. There are 0.965 eyepieces for sale all the time on Amart. The only thing you loose is maximum field of view maxes out at about 20mm. You might keep your eye on Amart ads for a few good eps.

Tak LEs fit 0.965 with no adapter. Just unscrew the 1.25" sleeve and the inside barrel extension inside that sleeve fitsa 0.965 exactly.

edz

#11 jrcrilly

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Posted 14 September 2003 - 05:19 PM

I'm thinking that an Orion Observer 70mm EQ is in my near future. I haven't seen anything better for $159. Anyone want to talk me out of it? Please?


There's an older Orion Observer 80mm EQ sitting here right now that I rescued from a garage sale so I have something to give or lend to the next new enthusiast I recruit. It doesn't look bad. I don't know why they dropped the 80mm or how different the newer model is, but I wouldn't be afraid to recommend one like this one.


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