Pulcherrima!

Beauty and the beast: my 130mm f/5 Newtonian versus a 90mm f/5.5 ED refractor

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Date: Wednesday March 28 2018

Time: 22:00UT

Temperature: −2C

Seeing: III, bright gibbous Moon, small amounts of cloud cover in an otherwise clear sky.

It is often claimed that refractors give more aesthetically pleasing images of celestial objects than reflectors. But how true is this statement? Last night, I learned yet another instructive lesson that shatters this myth once and for all.

Earlier in the evening, I fielded my 8″ f/6 Newtonian against a very good 90mm f/5.5 ED apochromat. The target was Theta Aurigae, then sinking into the western sky and so past its best position for observing. Seeing was only average. Both telescopes had been fielded about 90 minutes earlier with the optics capped, so both were completely acclimated. I charged the apochromat with a 2.4mm Vixen HR eyepiece yielding 208x. The 8 inch Newtonian was charged with a 6mm Baader orthoscopic ocular delivering 200x.

Examining the system in the 8 inch reflector showed the primary star as a slightly swollen Airy disk but the faint companion was clearly visible. In contrast, the view through the 90mm refractor showed a less disturbed primary but the secondary(for the most part) couldn’t be seen!

Question: How can an image be deemed more aesthetically pleasing when a prime target (the secondary) in that said image can plainly be seen in one instrument and not in the other?

Date: Thursday March 29 2018

Time: 00:05 UT

Temperture:−3C

Seeing; II/III, slight improvement from earlier, otherwise very similar.

Later the same night, I fielded my 130mm F/5 Newtonian along side the 90mm refractor and  turned my attention to a spring favourite; Epsilon Bootis, now rising higher in the eastern sky.

This time, I charged the refractor with a 2.0mm Vixen HR eyepiece yielding 250x. The Newtonian was fitted with a Parks Gold 7.5mm eyepiece coupled to a Meade 3x Barlow lens giving a power of 260x.  Examining the system, I was quite shocked by the difference between the images; the refractor did show a dull, greenish companion but it was entangled in the diffraction gunk from the orange primary. What’s more, the entire system was surrounded by chromatic fog owing to the imperfect colour correction of the refractor (an FPL 51 doublet). In contrast, the 130mm f/5 Newtonian image was far superior in every way; the Airy disks were smaller, tighter and more cleanly separated, and with zero chromatic fog to be seen. The Newtonian image remained just as stable as in the refractor image throughout the observation! The components also displayed their pure colours (as only a reflector can yield); the primary orange and the secondary, blue. In a phrase, the differences between the images was like night and day!

Conclusions: The 130mm Newtonian provided a much more aesthetically pleasing image than the refractor, which was compromised by its smaller aperture and less than perfect colour correction. As a small portable telescope, the Newtonian is far more powerful and is capable of delivering images that are simply in a different league to the refractor.

ED 90 Refractor: Proxime accessit.

130mm f/5 Newtonian(Plotina): Victrix/Pulcherrima!

 

Postscriptum: As always, I would encourage others to test these claims. Truth matters.

 

 

Neil English is author of Grab ‘n’ Go Astronomy.

 

De Fideli.

 

 

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