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Alstar 1.25' 90-Degree Erecting Prism Optical Prism Inside Rather Than a Mirror which Makes Your Image Clear and Sharp

4.9 (6) Write Review
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$26.99

$ 13 .99 $13.99

In Stock

About this item

  • With this diagonal you can match the orientation of the star field with correct star maps and charts; It produces an image that is oriented properly; which means that the image is upright but reversed left to right
  • This is a great diagonal for daytime terrestrial use and night time astronomy applications for telescopes or spotting scopes
  • More comfortable viewing orientation as you observe from above; filter thread for any 1.25 inch filter; Accepts standard 1.25 inch eyepieces
  • High quality optical Prism inside rather than a mirror which makes your image clear and sharp
  • High quality optical Prism inside rather than a mirror which makes your image clear and sharp


Product Description

1.25" 90-degree diagonal features an image erecting prism that allows you to look into a telescope at a 90 degree angle.

  • It produces an image that is oriented properly, which means that the image is upright and correct from left to right (i.e., a correct, non-inverted, non-reversed image).
  • This is a great diagonal for daytime terrestrial use with small telescopes or spotting scopes. It can also be used for astronomy applications, especially when you would like to match the orientation of the star field with "correct" star maps and charts.
  • The prism housing is made of precision molded ABS plastic and the two end barrels are precision machined from aluminum.
  • The 1.25" nosepiece is threaded to accept standard 1.25" astronomy filters.

Please Note: This diagonal can only be used with refractor telescopes and some catadioptric telescopes like Schmidt-Cassegrain, Maksutov-Cassegrain, etc. It not meant for use with Newtonian / Dobsonian reflector telescopes or other variants of Newtonian telescopes.

Advantages in a nutshell

  • With this diagonal you can match the orientation of the star field with "correct" star maps and charts. It produces an image that is oriented properly, which means that the image is upright but reversed left to right.
  • This is a great diagonal for daytime terrestrial use and night time astronomy applications for telescopes or spotting scopes.
  • More comfortable viewing orientation as you observe from above, filter thread for any 1.25" filter. Accepts standard 1.25" eyepieces.
  • High quality optical Prism inside rather than a mirror which makes your image clear and sharp.
  • Economical 1.25" 90-deg prism diagonal for astronomical telescopes and spotting scopes.

Timothy Michael
2025-08-28 12:52:58
A really good value for the price.
Chris F.
2025-08-05 16:11:42
This prism works well at night, I used it on the moon and stars and it did alright. But I like to do bird watching in the daytime, sometimes a strange rainbow blob forms in the middle of the image? My guess is there's very little coatings on the glass. I have two William Optics prisms that don't do that. So I should have just bought the WO prism (cost twice as much) but I'm kind of picky, so if this is all you can afford them it's alright.Also I will add that all prisms give off a spike of light on bright stars, so I would recommend a good dielectric mirror for space viewing. I actually ordered another WO prism for my daytime viewing because they're much better quality because you get what you pay for these days.Finally..I've decided to assigned this diagonal to my 70mm solar telescope where it stays out of trouble with bright light and ghost images...it works great for solar 👍 Thanks
Kindle Customer
2025-05-23 10:51:32
Wanted a direct erecting 1.25" telescope prism and this does the job.
tommy sturgis
2025-05-06 18:56:16
This is good if you want a 45 and not a 90 degree.
AUTO R.
2025-01-28 16:23:46
good
Jason Malo
2025-01-13 20:40:09
A well made 90° Prism. The optics are clear. Definitely a good buy.