Refractors vs Reflectors?

I am a newbie with my first scope, a Meade 114NT.

From my reading I have come up with the following conceptions/misconceptions:

A few comparison points of a reflector vs a refractor:

1> Color is perfect in a parabola Newt. No achro, super achro, apo quantum price jumps. Newt wins in both price and performance for CA.

2> I believe that cool down is almost a non-issue in a small ~4.5" Newt. A draw for small reflectors?

3> Obstruction/spider diminishes a Newt by how much? I dunno. A refractor wins, but an 80mm refractor to a 114mm reflector? Would not the reflector win here too?

4> Portability: A small ~4.5 Newt mini-Dob is as portable as a small refractor and mount, and perhaps easier to set up. A draw?

5> Viewing comfort: Would you rather have your head level or looking straight up at the zenith? Reflector wins big here.

6> Contrast: The claim is that you get black skies and terrific contrast with a refractor. I have no experience here for comparison.

Given the above, why aren't sub 120mm refractor lovers using 4.5" Newts?

Stu

Stu,

Some of your points are valid but not absolute. Cooldown can be an issue with any scope but a small reflector is more likely than a small refractor to have problems. Viewing angle and eyepiece position are not a problem with a refractor if you use a good mount.

However, the biggest attraction the refractor has is the very wide fields of view with little or no edge distortion. All Newts suffer from coma by design and the amount of coma is dependant of F ratio, not aperature. Add to that the need to collimate a Newt to get even a small distortion free field and the refractor wins. Plus lenses last forever if not abused. Mirrors need recoating.

I have both types of scopes. I prefer the refractor in smaller sizes.

clear skies,
dan

Stu:

This is the sort of question that one can only answer by looking through the different scopes, there are many issues here, some of which you have not addressed.

1. Chromatic Aberration... A Newtonian is better here.

2. Cool down: I have owned 4.5 inch Newtonians, currently own a 130mm F5 Newtonian that is a favorite scope. To get the good views, these scopes definitely need to cool down. In the relatively mild climate of San Diego, to operate at 200x+, it takes at least 1/2 hour. Thermal issues are big with Newtonians, just touching the metal tube with a warm hand will initiate a current that can last a minute or more.

Refractor definitely wins here.

3> Obstruction: This is really only part of the story. The mirror coatings are not as efficient like a refractor, with new standard coatings you are looking at 88% transmission and in reality probably less. Two mirrors mean plus the CO means that the images are not as bright, a 114mm Newtonian with a 25% CO will have the thru-put of a 97mm refractor. There is also more scatter from a mirror than there is with a refractor.

Definitely the refractor wins here.

4. Portability: It really depends on the scope. A 4.5 inch F8 Newtonian is about 36 inches long, too big for an air travel. A 4 inch F6 Achromat is around 24 inches long and with a simple Alt-Az tripod takes up less space.

Personally when space is at a premium, I take a refractor. My last car trip to the Navajo Reservation I took my ED-80. I would have to give portability to a refractor.

5. Viewing comfort also depends on the scope. I like DOB mounts but I also like a 600mm focal length refractor on an ALT-AZ mount. With a long refractor, one can be down on the old knees and for that reason, my 102mm F10 Achromat rarely gets used.

This is really a personal preference.

6. Contrast... I think, because of there is more scatter with mirrors than with lenses, I think this has to go to the Refractor.

Some things not mentioned:

7. Newtonians are a compromise. As you found looking at Newt, you can either have a large fully illuminated field of view and thus a reasonably wide field of view or you can have a small central obstuction and thus better planetary contrast but you cannot have both, it's just the manner of the beast. With a small refractor like the ED-80 with a 2 inch focuser, 4 degree FOVs that are fully illuminated and sharp edge to edge are possible. With a Newtonian, this is just not going to happen. Coma with intude as well as the problem with the diagonal size.

So in conclusion, first let me say that a 4.5 inch F8 Newtonian was for many years considered a serious scope and IMHO a scope capable of providing some very nice views. These days, with 10 inch Newtonians for $500, first scopes are pretty outrageous.. I enjoy viewing through smaller Newtonians and often spend the evening with my 130mm. A 4.5 inch F8 Newtonian can provide great views of many targets. The scope that really got me going was a 125mm F8 Newtonian...

But refractors have several things going for them. They are more robust than a Newtonian, do not require frequent collimation, cool down times are short, (even an ED-80 needs to cool though) and they can provide bright fully illuminated views from very low magnifications to high powers. Refractors are the most efficient design so that on an inch for inch basis, you see more. Furthermore, a small refractor does a nice job during the day.

So in answer to your question, "why aren't sub 120mm refractor owners using 4.5 inch Newts", you really need to see for yourself. I will say sometimes when I am viewing through my 30+ year old 6 inch F8 Newtonian, I wonder why people are using long focal length refractors.

My personal preference is larger Newtonians, smaller refractors.

Best wishes and clear skies ahead!!!

Jon Isaacs

Viewing comfort: Few people outside of Japan use a refractor straight through. We use diagonals. These can be easily rotated to any comfortable angle.

"Given the above, why aren't sub 120mm refractor lovers using 4.5" Newts? "

Because if we were, we wouldn't be refractor lovers, would we?