The Reality of Planetary Amateur Telescopes

Updated January 2007

 

During the (2003) perihelic Mars Opposition and the opposition of 2005 and subsequently, I have had the opportunity to compare views through various telescopes under conditions of average seeing for mid latitude Canadian conditions with some interesting results.

My own telescope is a TeleVue 102mm f/8.6 apochromatic doublet http://www.spacealberta.com/equipment/tv102/tv102.htm and in updating this article I have compared images and observations through this telescope and the two that I previously owned, a 150mm f/8 Synta achromat and  a Sky Watcher (Synta) 120mm f/8 refractor, comparison was also made with a 3” f/15 30 year old Japanese achromat. http://www.spacealberta.com/equipment/80mm/80mm.htm

Both a Sirius Optics Minus Violet 2” filter, and a Baader Fringe Killer filter were used on the achromats, along with various Televue Plossl and Radian eyepieces. This telescope is tested with a ronchi grating at 1/8 wave. The mounting details are available elsewhere on my website.

Under the seeing conditions of this Mars opposition that varied from IV to II on the Antoniadi scale, I was able to get 3 nights of almost being able to reach the resolving limit for the 150mm lens. This would equate to conditions giving about .8 to1 second or slightly less of arc seeing. The colors and amount of detail visible on Mars was astounding to say the least. At the time I was able to compare these views with  Meade and  Celestron 8” Schmidt Cassegrain telescopes and a couple of  Dobsonian style reflectors of up to 12” aperture. To be truthful, there was just no comparison to the refractor views. The larger aperture of these telescopes gave a brighter image to be sure with more background stars visible, unfiltered views showed much more irradiation making the viewing of fine detail very difficult and they were definitely more affected by changing transparency and scintillation. At least one of the Newtonian reflectors needed collimation as the images were showing slight coma.

I have had many amateurs argue that fact that a larger aperture is always better than a small one and that may be true if you compare different sizes of the same design, but there are differences in performance between designs that are obvious to the eye. I have been a planetary observer for at least 25 years, using both refractors from 60mm to 150mm and an 8” SCT over the years. I have come to the following conclusions as to why refractors outperform other designs almost twice their aperture on the Moon and planets.

I realize that many amateurs form an emotional attachments to their instruments and when approached with a ronchi grating to check the figure of their objective refuse to test their scopes for fear of negative results, never the less, I think that a few cold hard facts need to be stated. Many people have approached me over the years, when they wished to take up astronomy, asking “What kind of telescope should I buy or build?” Many already think that they should get the biggest reflector that their budget can afford, which is fine if you want to complete your Messier and NGC certificates and observe casually.

But… for high definition planetary and lunar observation, the sharpest crispest images that can be obtained under realistic viewing conditions is required.

A key factor is what kind of atmospheric transparency and seeing are you going to encounter where you live and intend to observe from?

 

The following table is based on information from the Astronomy weather website; http://weatheroffice.ec.gc.ca/astro/seeing_e.html

 

 

Telescope (aperture)

Dawe’s Limit (arc sec.)

Raleigh Limit (arc sec.)

Minimum Seeing Required

2” (50mm)

2.28

2.8

III

2.4” (60mm)

1.9

2.3

III

3.1” (80mm)

1.47

1.75

III

4” (102mm)

1.14

1.37

III

5” (127mm)

0.91

1.1

III

6” (152mm)

0.76

0.92

IV

8” (203mm)

0.57

0.69

IV

10” (254mm)

0.45

0.55

IV

12” (304mm)

0.38

0.46

IV

16” (406mm)

0.28

0.34

V

18” (458mm)

0.25

0.31

V

20” (500mm)

0.22

0.28

V

 

Where V=<0.4”, IV=0.4” – 0.9”, III=1” – 2”, II=3” – 4”, and I=>4”.

 

Typically, resolution over most urban areas will not usually exceed II and from most inhabited rural areas depending on terrain, III

There will be the occasional night where for a short period, under the right conditions, seeing can be II or better. I have seen 5 of these nights over the last 4 years. Judge for your self how this will affect your choice of aperture, will you actually ever get to see your scope resolve to it’s limit?

 

Given that the final resolving power of your instrument is dependant on how clean the wave front of the incoming light is after passing through differing air layers that are affected by temperature, air movement and suspended particles including meteoric and volcanic dust. The site that you have chosen to observe from will be one of the two most important factors in image resolution, the other being observer skill and experience.

 

A search of the internet on the average seeing conditions of both professional observatory sites and the average urban and rural seeing reveals some interesting numbers. Taking into account the location and elevation of sites such as Pic du Midi, Mauna Kea, and the central Chilean mountains the average resolution is between .64 and .85 seconds of arc with better nights of .32 to .5 seconds of arc with the very occasional exceptional night of .25” at the very best.

In contrast, most urban sites will exhibit between 2 to 4 seconds of arc, due to suspended particulates and condensates from aircraft, automobile, industrial sources, and in the winter, heat and smoke from heating sources. Most urban areas create enough heat to form their own micro climates with inversion layers of either warm air trapped under a dome of cold or vice versa which occasionally yields freezing precipitation. Rural sites located within an hour or two of urban centers give better seeing and transparency as most effects are weather and climactic as opposed to local effects, so that ‘high’ seeing is more of a factor. It is best to avoid changing conditions of low pressure weather systems and elect to observe from a rural location during a relatively stable high pressure situation when the jet stream is not directly overhead. Typical seeing from a location such as this usually averages 1 to 3 seconds of arc with the occasional night of 1 second or less.

 

The second factor of observer experience and skill is dependant on learning to see. The way this is done is the same as it has been since visual telescopic observing began, and that is by spending long hours at the eyepiece and recording what you see as you see it by sketching and making notes. This is the method used by all the famous observers of the past…Cassini, Huygens, Schiaparelli, Herschell, Dawes, Denning, Moore. etc. and it still holds today. Consider the work of Walter Scott Houston who wrote a deep sky column for Sky & Telescope magazine for decades, used a 4” refractor for most of his observations. Consider also Stephen James O’Meara, probably the best visual observer of our time who uses an older model Televue 4” Genesis refractor. Too many rush to build or buy the biggest reflector that they can afford hoping that more aperture will show them more in the sky, but every observer who wants to develop their skills will have to serve this appenticeship. There are no shortcuts.

 

After some research on the physics of light and practical experimentation, there a few key factors to consider in planetary telescope design:

 

Irradiation & Glare:

Experience has taught me that actually ‘throwing away’ some light through the judicious use of neutral density, blocking, and colored filters can greatly improve the amount of detail seen through the eyepiece. In conjunction with a well baffled instrument, the amount of glare can be reduced to reveal subtle planetary features when the atmosphere allows.

 

Central Obstruction:

This factor has been beaten to death as a chief cause of poor planetary performance, while it is an important factor, it is not the only one.

Generally an obstruction area of over 12% of the primary mirror area will result in noticeable decrease in performance. This factor applies not only to catadioptric scopes, but particularly short focal ratio Newtonians where a larger secondary is required to field the fat light cone from the primary, the situation can actually be worse than in a compound scope because of the lack of tube baffling and the additional secondary support structures.

 

Tube currents:

A real problem with closed tube instruments, especially larger ones as it goes hand in hand with longer cool down times for larger pieces of glass, negating any chance of taking advantage of good conditions, ventilator fans and tube baffling will partially help but larger mirrors can take over 6 hours to reach thermal equilibrium. Open tube designs introduce heat from the observer as well as incident light into the optical path.

 

Dirt:

I have seen some really filthy reflectors, every particle scatters light and degrades the final image.

 

Temperature Equilibrium:

The larger and thicker the optical components, the longer to reach thermal stability, a cooling fan may be required for larger pieces of glass especially in tubes of wood or composites which insulate and tend to increase cool down time.

 

Collimation:

Alignment of optical components must be accurate to deliver decent images, some designs require collimation after being moved every time, becoming a frustrating experience every time you want to observe. Every new scope must be roughly collimated to center components using a Cheshire eyepiece if possible and if you have access to a laser collimater, use to fine tune your alignment. Star testing can be a way of adjusting the final alignment to produce perfectly round star images.

 

Optical Figure:

Star testing can show whether your optics are astigmatic or over or under corrected, for example many SCT’s show either under corrected or rough figures of their primary mirrors. Use of a ronchi grating can also be done to check mirror figure.

 

Wave Front Interference:

I became aware of this factor when testing machined surfaces for flatness with optical flats. Usually a monochromatic light source is used and the reflected image shows shadow bands as the reflected waves interfere with the incoming waves. By reflecting an optical path back on it’s self, wave front interference is created that adds to the degradation of the final image. This is one of the reasons that a larger reflector mirror is required to create the same resolution as the smaller lens.

 

Shear size:

A monster scope is heavy, hard to mount and unless on a commercial mount, difficult to add tracking capability, making it even heavier.

Even SCT’s over 8” in aperture become a two person setup.

 

Advantages of the Refractor:

Central Obstruction and secondary supports: None.

Tube currents: The light path immediately bends away from the wall of the tube and is thus not as affected by air traveling along the tube walls if the tube diameter is slightly larger than the aperture.

Cool down is rapid and complete in an hour or less.

Dirt: a sealed system usually keeps dirt out of the tube.

Collimation: Once obtained usually does not require any major adjustments unless the scope is disassembled or subjected to very hard use.

Optical Figure: This is the only practical drawback as at least 4 surfaces must be accurately figured making the cost per inch high, but an excellent lens will give you planetary images that knock your socks off. False color can be a problem in cheaper optics but is overcome with additional elements such as a chromacor or filtered with a Minus Violet or Planetary Contrast filter.

Wave front interference: None unless a poor quality diagonal is used, some users eschew the use of a diagonal at all, preferring not to degrade the image as delivered by a superb lens.

Size: When long focus achromats were popular, the length of the tube was prohibitive, however with modern optical glasses and design, the refractor can be easily portable in apertures up to 6”.

Apochromatic refractors have greater image contrast due to focusing all visible light wavelengths to the point where perceptible color fringing is negligible, whereas achromats will have a haze of unfocused colors that degrade overall contrast. Use of blocking filters reduces this unfocused light but does not add any further information to the image although the impression is that the image is sharper.

 

Ask anybody who has seen Mars or Saturn through a good refractor, a smaller instrument of good optical quality can be quite satisfying and certainly easier to transport and setup, the rule being the scope that is easier to setup will be the one you use most often.

 

In conclusion then, the telescope one chooses is dependant upon the type of observing that you will do during your lifetime as an amateur. Once beyond the Messiers, NGC’s, and planetary nebulae, what then? Whatever you choose as your ultimate observing goal, the previous thoughts and points are some thing to keep in mind, the best advice being to first develop your skill as an observer, pick the right location and conditions in which to observe, and choose and instrument that performs well within the limits of transport, setup and the seeing conditions you are observing under.

 

References:

 

http://www.televue.com/engine/page.asp?ID=142

 

http://cleardarksky.com/so/

 

http://www.saao.ac.za/~erasmus/overfsee.htm

 

http://medusa.as.arizona.edu/graham/astro.html

 

http://www.ing.iac.es/Astronomy/development/hap/dimm.html

 

http://www.rca-omsi.org/seeing.htm

 

http://en.wikipedia.org/wiki/Seeing_disk