Kind of Weird…

Meade 90 mm on Bresser mount

With haze from high winds and the moon I did not open up Temple 10 awhile back. Temple 28 was awaiting a new power supply. So I got out the Celestron Firstscope 80mm F/11 and a generic F/8.8 Meade 90mm I bought on ebay about 8 years ago and a Meade 70 mm. I am working on doing a telescope review of my 70, 80 and 90 mm scopes.

After looking at the moon I decided to try something. If you put on the Star Analyzer (diffraction grating) you can actually see the amount of Chromatic aberration in the shape of the spectra. Since an achromat normally only brings 2 wavelengths (green and red) of light to focus you see what is called a fishtail at the blue end. This is a cone shaped smear of violet and blue light. The spectrum is clean and tight in green/yellow and depending on lens makeup and quality of manufacture often has a slight widening of the spectra in deep red.

Celestron 80 mm Firstscope.

The 80 mm Firstscope has a fishtail in blue which extends to the edge of green, which is common to achromats but goes nice and smooth right to the near infrared region before it widens a bit. So it indicates that using a Minus V filter would probably negate a lot of the bad effects of CA but not all. The 70 mm Meade F9 had a very similar spectra to the 80mm except that it showed signs of pinched optics.

This is what is weird. My assumption going into this testing was that out of the 3 telescopes I tested the 90 MM F/8.8 Meade would be the worst. I bought it off of a vendor on Ebay and paid about 40 bucks for the Optical Tube Assembly. Turns out it is the best. When looking at the moon or Venus there is only a hint of purple and you have to look for it. So I tested this scope with the grating and found a fishtail in violet but not blue. This means that it has much less CA than my other scopes! The red end looks like the 80mm and is better than my 6″ achromatic refractor. Over all there is substantially less visual CA in the 90mm than the 80. Yet the 80 mm is an F/11 and should have better color correction. That very small shift in the wavelength makes a lot of difference. So a Minus V filter would really reduce the CA in this scope.

Meade 90mm scope F/8.8 with Solar filter on it. 

In the 3 telescopes I tested in and out of focus the Meade was also the best. The 80 and 70mm both have about 1/2 wave of spherical aberration. The Meade is at a 1/4 wave.

So after writing most of this I went out and did the tests with the grating again and then just looked at some bright stars with a homemade 26mm eyepiece. The Meade 90 mm really does have a nice view. In fact I put a 4mm planetary eyepiece on it with the almost full moon and saw just a tinge of blue but really nice crisp views of the partially illuminated craters. When a 90 mm F/8 can look this good I think I got the luck of the draw on this telescope!

So a 40 dollar telescope that I have had for years turns out to be the best of the 3 refractors I am testing, go figure!