Spectacular Focal Reducer Discovery for FirstScope

Spectacular Focal Reducer

Discovery for Celestron FirstScope

by Mike Otis

Experiment Gone Fantastic!

Change your telescope's world with a simple Fresnel card. Turn your telescope into a very fast f/.5 system!

Over the years the flat plastic Fresnel lens about the size of a credit card has significantly improved and now forms much better grade optical images. The card is designed to be used as a low cost flat magnifier and conveniently carried in a wallet like a credit card,

I bought several flat Fresnel lenses for experimentation in the lab. I noticed a long focal length of several inches and the last night out with the Celestron FirstScope, I held one up to the rack and pinion and pointed the telescope at the Moon. I was surprised to see a very tiny Moon, as if the Fresnel lens was acting like a massive focal reducer.

I set up a rig to measure the focal length of the Fresnel and got 6 to 8-inches. The Celestron FirstScope has 2.99-inches aperture with a focal length of 11.8-inches at f3.95. Using the upper limit of the Fresnel focal length, I got f/3.95/8-inches = f/.5 and a magnification of 11.8/8=1.5x. Technically, the flat Fresnel performs like an 8-inch focal length eyepiece giving 1.5 power. After learning the very bright tiny dot in the telescope was the Full Moon and not a reflection, I realized the lens was acting like a spectacular focal reducer. Having a Celestron FirstScope converted to a very fast f/.5 system is like having an all new telescope, very fast lens, or a Schmidt Camera.

Aberrations

System aberrations include small effects of flexure - deviations from a plane surface, and achromatic aberration due to using the single plastic element.  When taking images for processing, the blue end of the spectrum is easily subtracted or filtered out.

Fresnel Lenses - Tips & Techniques

I also discovered the opposite effect of the focal reducer is possible. Make a 2x or greater Barlow by stacking two or more credit card lenses and observe the overall magnification increases.

Extra Experiments

* Try making a telescope using the Fresnel lens as the primary objective

   and stack two for the eyepiece

* Record the magnification increases when stacking cards for a Barlow effect

* Prop one up on the telescope tube for use as a finder card

* Place over the hand controller to enlarge the display

Testing Svbony Optics on the First Scope Telescope

Testing Svbony Optics on the First Scope Telescope
Tests included the 2x Barlow, .5x focal reducer, full focal length set of Svbony eyepieces (both Plössl and Aspheric) on the Celestron 3" f/4 Dobsonian First Scope Telescope. The results are surprising!

I UPGRADED my Celestron Signature Series FirstScope telescope using optical components from Svbony, including finderscope, focal reducer, barlow and new eyepieces. The results were exciting and surprising to say the least. Let's take a look at what was learned in the testing process.

First let's talk about the setup. I set the telescope on a small round table indoors and imaged through the closed clean patio window. The single pane of clear (white) parallel float glass is relatively optically pure and works well for quick tests and good focus inside the convenient environment of a bug free and perfect temperature telescope control room.

I selected a set of two types of eyepieces from Svbony, a reliable supplier of telescopes and accessories. The first set are Plössl and the second set are Aspheric. Note: different EPs are known to have different performances given short or long focal length mirrors. 

Svbony Set One

40mm Plössl

32mm Plössl

Svbony Set Two

23mm Aspheric

10mm Aspheric

4mm Aspheric

Focal Reducer

.5x

Barlow

2x

The results are surprising. The set of Aspheric oculars, with a more simple glass design, was vastly superior, giving sharp focus bright images across the FOV. The views were dazzling and exciting!  Colors were exceptional, images formed clearly without distortions or aberrations, and the low cost of each eyepiece fit well with First Scope Telescope (a set of all three eyepieces currently costs $26.18, just a little over $8 each!). These EPs have outperformed other brands costing several hundred dollars each.

Another positive and greatly welcomed factor is the light weight of each ocular - weighing almost nothing - it does not upset the predetermined non-adjustable balance of the OTA. Aspherics won out over numerous other brands, for example, a 4mm Celestron ocular known for producing dim images has extremely tiny lenses that don't admit much light at all and it was very difficult for visual observing with an almost non-existent exit pupil. However, the Svbony Aspherical 4mm eyepiece was a pleasant surprise with its large diameter lenses, a comfortable exit pupil for visual observing, and exceptionally bright clear images especially for a short focus EP at high power.

However, given the fast focal length f/4, the Plössls, both 32mm and 40mm were a different story. Visual observing produced a black round spot in the middle of the fov, i.e. a shadow of the secondary, which can happen when the exit pupil is wider than 7mm. Clearly, the long focal lengths of these Plössls do not match the short focal length of the mirror. Therefore in conclusion, the Svbony Aspheric eyepiece set is the real winner and highly recommended for use with the F/3.95 Celestron First Scope Telescope.

Focal Reducer & Barlow Testing

Bonus Tip: Those thinking about trying a focal reducer ($9.45) with any of the tested eyepieces will soon realize the telescope does not have focus in range. However, the 2x Svbony Barlow ($7.90) is another big winner and works exceptionally well, even at the highest power when changing the 4mm into a 2mm. With the telescopes 76mm (2.99") aperture and 300mm (11.81") focal length at f/3.95, this gives 150x with tac sharp clear images. The Svbony 2x Barlow worked well with all three aspheric eyepieces (including the 23mm and 10mm) and is highly recommended for the Celestron First Scope.

Top left - Svbony .5x focal reducer attaches to the eyepiece. Left below - Svbony 2x Barlow also threads into a 1.25" eyepiece. The finderscope will be reviewed at a later date.

Celestron Firstscope Telescope Specs

Design - Newtonian Reflector

Style - FirstScope Signature Series

Manufacturer - Celestron

Mount - Dobsonian

Drive - None

Diameter - 2.99 inches (76mm)

Focal Ratio - f/3.95

Focal Length - 11.8" (300.2mm)

Prime Focus - 12x

Eyepieces - Original Celestron eyepieces melted

Resolution Rayleigh - 1.83 arc seconds

Resolution Dawes - 1.53 arc seconds

Limit Stellar Magnitude - 11.9

Light gather power compared to human eye - 118x

Adjustments - None

Highest Useful Magnification - 150x

Lowest Useful Magnification - 12x

Owner Added Accessories

* Svbony Aspheric Eyepiece One - 62 Deg., 23mm (.9") = 13x

   with 2x Barlow = 26x

* Svbony Aspheric Eyepiece Two - 62 Deg., 10mm (.4") = 30x

   with 2x Barlow = 60x

* Svbony Aspheric Eyepiece Three - 62 Deg., 4mm (.16) = 75x

   with 2x Barlow = 150x

* Svbony/Celestron Finderscope - 5x24, Plastic, Crosshairs, Focus

* Celestron iPhone Software - Sky Portal

* Computers - Apple Mac

* Image Processing Software - Apple System Photos, Mooii Tech PhotoScape X,

   GIMP, Soggy Waffles Paintbrush

* Apple Camera - iPhone

* Zwo ASI224MC Camera

* Svbony Focal Reducer - NA

* Svbony Barlow - 2x

* Svbony - Filters

Celestron FirstScope Test Accessories

FirstScope Project: Test Accessories

Finally this is what we are all waiting for! Project Test, More Eyepieces, Focal Reducer, Barlow Lens, Focus Test, RFT Results

First, for our main tests we used the 10mm 62 degree Aspheric eyepiece from Svbony, which is the best performer in the Celestron FirstScope. The Celestron is an RFT with a focal length of only 12-inches and a defective secondary mirror. Nevertheless, the small Moon appears extremely bright.

In keeping right on budget, none of the accessories exceeded $14 in cost.

The big problem encountered was the handheld iphone could not adjust for all the light coming through the eyepiece. A Moon filter could not reduce the light in any significant amount and was set aside. Next I tried a polarizing filter. Still the moonlight was far too intense. Finally the tests were shot with the overexposed Moon. The first image shows the Moon image scale through the 10mm 62 degree Aspheric eyepiece.

The next image shows the Moon with the same eyepiece and an attached Svbony .5x focal reducer screwed to the bottom of the eyepiece.

In the third image, the eyepiece and a 2x Svbony Barlow were used. The Barlow screws to the bottom of the eyepiece. The change in image size is apparent.

Next, a 32mm and 40mm focus Plossls were test and they did not do well with the scope. The Moon had the image of the mirror and eyepiece superimposed over it. These fast eyepieces just don't work in this telescope. The final photo shows another type of obstruction defect that occurs, blocking out most of the Moon.

In all tests, the full uncropped size of the iPhone camera is retained.

Finally a really big problem surfaced. Due to the fast f3.9 focal ratio of the mirror, and perhaps inclusive of the subpar optical system, the focus could not be achieved over the entire region of the Moon. Thus focus had to be interpolated and approximated. Even a Bahtinov mask could not help this probem.

In conclusion, aside from the inherent problems with the Celestron FirstScope:

* Aspheric eyepieces with medium focal lengths do well with the f3.9 system
* Images are small and bright
* The focal reducer is working well
* The 2x Barlow is working well
* The telescope produces Moon images that are too bright for iPhone imaging
* The scope may have its place with deep sky objects

All experimental project Moon images were shot on the night of Friday, September 4th, 2020.

Celestron FirstScope - Index
https://otisastro.blogspot.com/2020/08/celestron-firstscope-index.html