Comparing premium refractors and reflectors

WAITING FOR THE SHADOW

Astronomical Observing and Photography

Comparing a premium 6 inch refractor & a premium 6 inch newtonian reflector

Joe Cali and Phil (surname withheld - Phil never publishes his surname online)

Observations : Saturday 16 & Sunday 17 February 2019. Article written Sunday 17 & Monday 18 February 2019.

This follows on from the article, "Last Friday Night - Simple Pleasures"

On various forums, I have often seen questions posted or discussion asking for comparisons or performance equivalence between a smaller aperture refractor and a much larger reflector. The comparisons are often made between a $10k-$20k premium refractor and a $400-$1000 mass produced fast f ratio reflector.

I read this and wonder : -

Why are premium reflectors with custom figured mirrors never considered or used in these comparisons?
Why are low end fast f ratio reflectors always used in the comparison?
Would anybody in their right mind, be weighing up or choosing between a $400 reflector and a $20000 refractor?

I have, on occasions, read claims equating a 6-inch refractor to a 12 inch reflector. My mate Phil and I decided to put this to a test pitting two instruments of premium optical quality, identical aperture and very similar focal length, one a triplet Takahashi TOA150 APO refractor, one a very accurately figured home made Newtonian reflector optimised for visual observing.

Phil owns the property and the observatory and gives me unfettered access to the property. Sometimes we observe together, sometimes each on our own depending on what else we have to do and what we want to do astronomically. Phil is purely a visual observer. I do visual, astrophotography and am gearing up to do video astrometry to return to asteroid occultation timing after several decades out of the game. Phil called me at lunchtime Saturday about an hour after I woke from my Friday night observafest. Weather forecast was for another clear night Saturday night. I wasn't sure if I could do another all-night session after only 3hrs sleep but I also didn't want to waste it.

We agreed to check in with each other later in the day after I had another sleep. I put all my widgets & batteries on to their chargers, did some chores, grocery shopping then managed another nap in the late afternoon. I met Phil on site towards the end of twilight.

I set up my EM200 mount using my A.P.P.S. Jig It helps me to set excellent polar alignment of the scope in under 5 minutes. Phil's observatory usually has a Mewlon 300 on the EM-400 mount. He has had it demounted for mirror cleaning and maintenance recently and has his Takahashi TOA150 is on the mount at the present time. The TOA is a 6" f7.5 APO triplet, my scope is a 6" f7 parabolic Newtonian. Same aperture, 50mm difference in focal length. We thought this was a good opportunity for a comparison of these two instruments.

Bias
Phil is an unapologetic fan of Takahashi and other premium optics. He admitted with a big grin on his face that, "I already owned the Mewlon 300 when I bought the TOA150. I didn't really need it, I just wanted one."

Readers will obviously tell from my description of the build process for my reflector in the thread, "Last Friday Night - Simple Pleasures" that I am a fan of quality reflectors. It doesn't mean that I don't appreciate premium refractors, just that I am open minded as is Phil. I am also a physicist and therefore a believer in the laws and limitations of physics. We are also both acutely aware of how bias can skew this type of comparison, I drafted this article and Phil has proof read, and agreed on the descriptions and language used in describing comparison observations in the hope that our respective opposing biases and agreement on language cancel each other out.

After proof reading Phil sent me an email, to say that he didn't have any changes and this article represents our observations fairly. So we are in complete agreement over the results and the descriptive language presented. This is as good as we can manage to remove bias in a qualitative study like this.

Observations
The 10-day-old waxing gibbous moon was about an hour past transit and washing out everything else. We spent the first couple of hours observing fine rills and crater details around Mare Humorum, Gassendi crater and Dopplmayer crater all of which were sunlit but just near the terminator.

Providing the exact same eyepieces were used in both instruments, we found the views were very similar in both instruments after allowing for the slight (50mm) difference in focal length. The TOA could out resolve the reflector on all subjects but the difference in resolution was very marginal. We had to look long and hard at the finest of details to see a difference. Using eyepieces of same focal length but different brand/types/quality could elicit much greater apparent differences in views, clarity, crispness between the instruments, than the marginal difference that existed between the two instruments when identical eyepieces were used. Both instruments were resolving both the sunlit side and the shadow of the straight ridge in Dopplmayer marked in the attached photos. Each feature, the sunlit side and the shadow, measured off LROC images, was approximately 1km wide. On Saturday night, the Moon was 361000km away from Earth's geocentre, perhaps 352000 km from our topocentric location to the lunar surface suggesting that both instruments were resolving close to 0.7 to 0.6 arc sec, the TOA a little better than the Newtonian.

As the Moon was getting lower in the sky, we pointed the scopes at the Orion nebula. The Moon was still washing out the sky. Here we found something very odd. My reflector which has a very small 1mm 100% illuminated field and a 19mm 75% illuminated field, could pick up more detail in the outer arms of the nebula where vignetting is greater while the TOA showed more contrast and detail between bright nebulosity and interstellar dust in the bright nebulous area around the trapezium but less in the outer arm.

We then looked at some open clusters. With fainter stars and no diffraction spikes from the Newt spider, the views were near identical. We examined the omega Centauri globular cluster. Here we disagreed slightly. Phil felt that stars were very slightly better resolved across the core whereas I felt the views and core star resolution were near to identical.

Screen capture from Starry Night showing the face of Jupiter at the time of observation.

We also looked at Jupiter that was still at low altitude and rising. The image was jumping around a lot due to the low altitude. Phil said he could see three bands in the TOA, two in my reflector. I could only see two in each instrument. The spider diffraction was quite strong in my reflector. I later discovered that two vanes were slightly twisted and therefore quite thick.

With atmospherics playing the part they did, Jupiter was only this may have been down to Phil being luckier with moments of stability that I was. Phil has better eyesight than I do although at the magnifications and exit pupils being used, my astigmatism shouldn't come into it.

By now it was 3:45am, with the Moon gone, the sky had just become very dark and transparency was as good as I've seen it. We compared views of Eta Carina. Phil said he could see some very faint stars near the star Eta Carina slightly more clearly in the TOA, than in the reflector. Although he did his best to explain where these stars were, I could not see or tell which stars he was referring to.

Phil had had a very long day and he had to call it quits and head home. With my two daytime sleeps, I was still feeling fine and continued observing after Phil departed.

As he was leaving, I observed at the ghost of Jupiter (NGC3242) then began a galaxy quest. I started on my old friend, the Sombrero Hat Galaxy (M104) in Virgo. Next, I looked at the Leo triplet, M65, M66 and NGC 3628. A 6 inch, f7 telescope gives a very different view to an 18 inch, f5.6 when observing galaxies. The 18 inch has 2.5 x the focal length and consequently smaller exit pupils. Nonetheless, the wide field high contrast views it offers afford the ability to view galaxy groupings in the same field albeit with less detail and brightness.

I moved on to explore the Virgo galaxy cluster. I started in the middle with Markarian's chain. After spending some time cruising along the chain, I began a square spiral slew slowly working my way around dozens of brighter galaxies in the Virgo cluster. What a window on the universe! Magnificent! I continued observing until about 20 minutes before twilight when I sensed that I needed to pack up and drive home before I became too tired.

Later on Sunday Phil and I again spoke on the phone. We agree that while there were small differences in our perceived observations as can be expected, we agree on these aspects of the newt-tak comparison -

Overall conclusion :
The difference is much much smaller and more finessed than the "folklore" on various astronomy forums where equivalences are sometimes drawn between a 6" premium refractor and 10-12 inch reflectors. If those comparisons are to be believed, the reflectors used in those comparison must be very very poor-quality examples of Newtonians.

A 6 inch newtonian reflecting telescope and a six inch APO refractor telescope are both six inch telescopes. You can't beat physics regardless of the pedigree. The spider and secondary obstruction give a slight degradation of performance that gives the premium refractor a slight advantage on most targets.
Looking at the Moon, the same fine details could be observed with both instruments, the refractor showing the very smallest details only very slightly more clearly than the reflector.
Both instruments were resolving lunar surface details of approximately 1km or 0.6-0.7 arc sec.
The Takahashi TOA slightly outperformed the Newtonian on almost every test subject.
The Takahashi produced a slightly brighter image, but that brightness difference was on the limits of perception.
Different quality eyepieces have a much bigger effect on and can result in much bigger differences in the quality, clarity and brightness of the final image than does the difference between the primary optics.
Diffraction from the spider vanes is most noticeable when observing bright planets such as Jupiter or when looking for a faint companion to a bright primary. With a Newtonian fitted with rotation rings, the tube and diffraction spikes can be rotated to a position angle where the diffraction spikes don't interfere with the detection of a faint companion. I discovered a slight twist in the Newtonian spider-vanes after the test.

Joe Cali and Phil