Orion CT80 80mm (3.25") f/5 Doublet Refractor Telescope

Above: The Orion 3.25-inch doublet crown and flint coated lens refracting telescope is f/5 and can show large sky areas. With a 1-inch or 25mm focus eyepiece gives only 16 power. A half inch FL EP (12mm) or a 2x negative projection barlow takes it to 32x or a 3x to 48x. It can be used as an RFT rich field telescope, a finderscope, guider and tracker or a main telescope. A plastic finderscope with a dovetail mount will keep it lightweight.

Orion CT80 80mm (3.25") f/5 Doublet Refractor Telescope
I'm really excited about these small telescopes that are lightweight, easy to carry and transport, and only take moments to easily set up. The class of these telescopes include tiny Dobsonians, tripod mounted OTAs and OTAs that share resources mounted onto an existinging telescope - for example to take advantage of a GOTO mount. Some examples are the Orion f/5 3.25-inch diameter refractor with a 14.7-inch tube, and the Acuter 60mm f/12.5 Maksutov which is a mere 8-inches long with the star diagonal attached. The CT80 is 2.25 lbs and the Acuter Mak is only 1 lb.

Orion CT80
The Orion CT80 is a 3.25-inch diameter f/5 coated doublet achromat refractor OTA with a 400mm focal length. A 1-inch focus eyepiece will give about 16 power and a very wide view of the sky. The OTA has a dovetail mount for a finderscope, and up until this time it was unknown how to mount the OTA. Upon close examination today, the OTA has a photographic standard 1/4" hot shoe mount for attaching to a tripod. An HF-3110 long four level tripod was found in inventory to make this telescope complete at no extra cost.

Left: the Orion CT-80 OTA is seen mounted on the HF-3110 tripod. The OTA is a bit heavy for this lightweight tripod but still fully functional.

The CT80 will also attach to the Celestron Nexstar 6SE using the Celestron camera mount. The CT80 weight is listed as 2.25 lbs. The optics are coated crown and flint glass mounted in a metal tube with a rack & pinion focuser. The tube is 14.7-inches long. Resolving power is 1.45 seconds of arc. It's short FL low power makes a fine addition to the Nexstar 6SE long focal length Schmidt Cassegrain scope and it gets to share the GOTO mount.

Left: a finderscope is added to the OTA using ring mounts on a dovetail shoe. The finder is all plastic with resin lenses and extremely lightweight. The rings and dovetail mount are metal and much heavier than the finderscope. however, these are capable of maintaining the rigidity needed for holding precise calibration.

Without central obstructions compared the Schmidt Cassegrain design, generally refractors are said to be equivalent to a Newtonian reflector or catadioptric about double the aperture. In this case, the 3.25 becomes a 6.5-inch telescope. The CT-80 has potential as its own telescope, and when mounted on the Nexstar - a guide telescope or dual telescopes on the single GOTO mount. At f/5 it can give 16x with a 1" EP. Many of these small telescopes are used as the main scope in astro imaging. Note: the 3.25-inch CT80 refractor is larger than a Celestron 2.99" reflector Firstscope.

Above Left: The tripod is an HF-3110 which is at just the right height when used with a 90-degree diagonal, but ends up being a bit wobbly along the axes until properly adjusted with four adjustment points. After moving the telescope, doing focus, a fair amount of time is required for the vibrations to subside. If not for testing, temporary usage and the ultimate in lightweight tripods for transportation, a larger and more sturdy tripod is recommended. 

Laser Light & Window Glass

Laser Light & Window Glass
Will green laser light penetrate glass?

This is an important question when operating an indoor telescope observing through window glass with a green laser finderscope. Will glass stop or significantly modify the beam rendering it useless as a nighttime finderscope to find celestial objects.

Introduction

There are many heated and optical physics, optronics,  and electro dynamic technical discussions that reach varying conclusions. The indoor observatory at HSO decided to test the laser beam at night and the results would speak for themselves and elicit the final say, conclusion and response. To the right of the telescope, the green gemstone laser was activated and a series of photos were take with the handheld iphone Xs Max. Would the light beam make it through the window glass and if it did, would it remain a coherent straight line? Would the laser light be significantly diminished and become unusable? Will powerful laser light reflect backwards and blind the observer?

Results

In the photo, the green line represent the laser pulse of light. At the bottom of the photo, the line begins inside the observatory room. About half way, a bright large circle of diffusion results where the beam strikes the single window glass pane first surface and enters the glass, and light scattering appears to take place. Then the beam penetrates the glass back side to exit to the outdoors and the straight line reaches skyward on the same straight and true intended path. The light is not noticeably dimmed in the process. There was no dangerous visible backtracking of the laser light and the safety of the observer was maintained by laser light continually moving in the forward direction.

Conclusion

Therefore an indoor laser can shine through window glass and be used as a finder to point to the location of celestial objects and to show where the telescope is pointing, safely by following the basic precautions. The light appears to continue moving in the forward direction. The only noticed different between the indoor and outdoor use is that indoors results in a large circle of diffusion appearing embedded in the glass or at the point of lasering entrance/exit - a bright green scattering of light - a round area of light where the laser beam enters the glass. As the laser is on only momentarily to locate an object, it won't interfere with astro imaging.

Change Finderscope Position

Change Finderscope Position

Decide the stance relative to the telescope, either from the back, left side or right side. This will determine which finderscope goes where.

Left: the challenge in this photo shows how the scopes are mounted without attention to observational positioning. Once the scopes are mounted, the positions can be decided. Using dovetail bases can allow for quick interchange. Currently the plastic finder is not interchangeable.

The most frequently used finderscope will be located in the most convenient location. Generally, even a back centered stance will favor the left or the right position. The photo shows three different finder scopes. At right, the Svbony has the ability to show the deepest magnitude. The laser can quickly point to objects at night without looking through it. The middle finder is very lightweight and and shows less magnitude - less confusing during heavily light polluted skies.

Top Photo: for indoor observatory one at Singularity Observatory, observing and astroimaging takes place from the left side.

Red Dot Finder

Red Dot Finder
What is this strange looking piece of plastic doing on my telescope?

Telescope companies began shipping telescopes with a red dot finder, a cheap piece of plastic that projects a red dot onto a small piece of viewing plastic. Using the human eye, objects visible can be found and centered. This is usually the Moon and brighter stars. For areas of heavy light pollution, it won't show any stars dimmer than what the human eye can see, so basic star hopping may not be an option. The illustration shows the Orion EZ Finder II for $39.99 and it's included on beginner telescopes. The same red dot finder sells for about $12 from China sources. The device uses a button battery and it's easy to forget and leave the finder on and then the battery burns out and needs replacing. Button batteries are not always easy to find at the store.

The finder design is not new. Many old timers will remember the Telrad finder from decades ago. It originated in the late 1970s when Steve Kufeld of Huntington Beach, California came up with the idea to manufacture an economical sight to help amateur and professional astronomers find their way across the night sky. "The Telrad is a different kind of device for aiming a telescope. It has been very popular with amateur astronomers since its introduction in 1978. The Telrad works on an entirely different principle, and has some distinct advantages over its optical finder scope cousin. 

Using a lens and light from a red LED and a glass beam splitter, the Telrad projects a nearly parallax-free, illuminated red bulls-eye into the observer’s eye. When the observer looks through the glass window, the bulls-eye appears to be projected onto the sky." Source

While the red dot finder is a great invention, and good for bright objects and beginners, the telescope may need another finder for dim objects. I recommend a 5x24 finderscope with lenses to see dimmer stars over the human eye, but not too many stars making the star field confusing. A 6x50 finderscope might show too many stars. It depends on the location and application. One could keep the red dot finder for locating bright objects and add another finder to the telescope for seeing dimmer stars when starhoping.

More about red dot finders, etc.

https://www.cloudynights.com/documents/9finders.pdf

Orion Starblast Telescopes have a red dot finder

https://otisastro.blogspot.com/2021/06/new-orion-telescopes-on-order.html

https://otisastro.blogspot.com/2021/06/orion-starblast-telescopes-order-update.html

Celestron FirstScope 5x24 Finderscope

https://otisastro.blogspot.com/2021/04/plastic-telescope-takes-cake.html

Plastic Telescope Takes the Cake!

The World's First All Plastic Commercial Telescope Takes the Cake!

Designed for the Celestron FirstScope as a finderscope

Leave it to Celestron partnered with Chinese constructionists to make perhaps the world's first commercial all plastic telescope and at the lowest pricing imaginable. As far as we know, everything from the tube, mount, ring, dust caps, eyepiece, and even the lenses are plastic!

The 5x24 telescope is originally designed for the Celestron FirstScope telescope and attaches to two bolts on the tube. With an aperture of 24mm or about one inch, and magnifying 5x, the scope is intentionally made of plastic to keep it extremely lightweight, an absolute requirement when attaching as an accessory to a small lightweight telescope without the means for adjusting balance.

What is the performance of an all plastic telescope? Surprisingly good. Who would have guessed? The photo at left was taken with a camera phone by holding it in front of the eyepiece end of the scope and shows crosshairs and a crisp colorful view of city buildings located far away. It was shot through a curving fence. The image through plastic lenses has very minimal distortion and any slight achromatic fringing is negligible. The photo shows any spherical aberration is also negligible and hardly noticeable. Plus the price is right - at only $8.69 and free shipping when purchased from Svbony Optics.

Cleaning Note

When cleaning a plastic optic, do not contact the surface with anything, especially a lens cloth, tissue or any other means of wiping the lens. Soft plastic will easily and permanently scratch. Clean by using gentle puffs of clean dry air.

Specs at a Glance

Function - Finderscope for Celestron FirstScope, Spotting Telescope

Aperture - 24mm

Magnification - 5x

Weight - A Few Ounces

Size - About 15cm Long

Dust Caps - Included

Construction - Plastic

Crosshair Reticle - Yes

Finish - Glossy Black

Mounting Bracket Type - Celestron FirstScope

Focus - Adjustable

Image - Inverted

Cost - $8.69