Posts Made By: Roland Christen

>>I would love to hear everyones comments, and if possible, some input from Roland, if you can spare a moment.>>

OK, I'll comment. What is the point of comparing these two scopes? The 130F6 EDF was designed for imaging. It has a fast focal ratio and wide field, all in a compact package. There are other ways to accomplish that with Petzvals for instance, but it usually results in a heavier and longer tube. The 130 F6 is really too short to be a planetary scope, but it really shines as an imager, especially H-alpha work where you need highly concentrated photons because the filters are super lossy. It's also a nice little grab & go scope for its aperture, and does a decent job on the planets. It's a superb deep sky instrument because the lens is so light efficient - lots of light throughput - only 2 air-glass surfaces - very little light gets lost in the optics, etc.

If you are not into imaging, then I would look elsewhere for something longer. In fact, we will soon be introducing some scopes specifically designed for the visual astronomer with excellent imaging capabilities also included, but not as fast as the EDF series.

Roland Christen

Since I make various size scopes, both refractor and reflector, here is my opinion:
It all depends on your local seeing conditions. I'm assuming that you are talking planetary and not deep sky. There are many times here in the midwest that my 5" F8.35 refractor shows clean crisp views of planets and double stars. It is usually a lot cleaner than my 8" F12.5 Mak-Cass and my 10" F14.6 Mak-Cass. The reason is that there is normally always some upper level disturbance in the air here. It is unusual for the air to be really steady in the Midwest.

My 6" refractor beats all three in image clarity 90% of the time at my location. It has just enough more aperture to edge out the 5 incher, but not so much that it is affected by the local seeing. Going to a 7" refractor at my location usually produces less satisfying views than the 6", but better than the 8" Mak.

Going down to Florida in the Keys, the opposite is true. The 10" Mak-Cass beats all the others 95% of the time.

All five of these instruments are figured to very high Strehl and are well below the diffraction limit with ultra-smooth optics.

Roland Christen

That's fine, Mark. Buy what you want.

We are a small company and I am the only optician. I cannot compete with mass market optical firms that sell thousands and even millions of optics. The point is not whether I would make a cheap scope for you. The point is I will not buy Chinese made refractors for resale here in the US even if they are excellent quality (which I don't think will happen).

I have my own priorities in a more rarefied market that you probably will never enter or even be aware of. Yes, I could cut costs to the bone by using inexpensive die cast parts for the focuser and lens cell, but this would never work for the majority of our customers who like to hang $10,000 CCD cameras, heavy like hell, on the end of the focuser and expect less than .0001" of motion. In order to do that, I have to start with lots of metal and do some rather precise machining and fitting of parts. Heck, it takes a good hour or more to assemble just one focuser! The 4" focuser on the end of our 6" refractor weighs 9lb, that's more than a typical 4" Chinese telescope. Where do you want me to start chinzing on build quality?

You guys have no idea what it takes for some of this equipment. I have customers out there that worry about 2 arc seconds of periodic error in their mount tracking. Compare that to a typical Chinese made equatorial that produces random fluctuations of 50 arc sec or more! Totally useless for some imaging applications.

You think you can take just any stuff and do anything and everything there is to do in this hobby. Well, I got news for ya. There's more to this hobby than just looking at planets.

Rolando

Here is an image of just the luminance portion to show the extent of the nebula.

R.C.

2" diagonals tend to be flatter over a small area than an equivalent 1.25" diagonal over the same area. Assuming they have the same wave rating, the 2" diagonal would be 60% flatter over the small area that a typical star would occupy.

RC

This seems to come up again and again. It is an example of a slick marketeer taking advantage of the general ignorance of geometric optics by the amateur public.

For a star diagonal where the mirror is near to focus, each star occupies a tiny fraction of the overall surface of the mirror. To see how small that area is, place a 4mm eyepiece into the eyepiece end of the diagonal, then look into the other end and ballpark measure the size of the mirror that actually gets used. It will be around .1 to .2 inches. Let's assume that the diagonal is only 1/4 wave across the entire surface. That central .1" will be 1/40 wave flat. And that is a fact. How much flatter do you want this device to be?

Any deviation from flatness in a star diagonal of this type will not change the final figure - it will not add spaherical aberration. Instead it shows up as an increase in astigmatism (stars will be little ovals instead of little round marbles at high power). At what point does this occur and would be visible in the eyepiece? It will begin to show up if the surface of the star diagonal is approximately 1 wave.

Note, this does NOT apply to a Newtonian diagonal, reason being that because of its location far from focus, it really does need to be 1/10 wave or better across the entire surface.

In the attachment the image shows an interferogram of a 2.3" x 3.1" coated quartz flat used in the Maxbright diagonal. The full aperture of a 2" star diagonal is only 1.8" (you cannot have zero wall thickness on the 2" adapter). The actual area used by any given star is shown in the highlighted test area and the actual wavefront numbers can be read on the right.

Roland Christen

In the case of lenses it's usually the result of some accident which causes a scrape or chip on the front surface. These lenses can sometimes be rescued by regrinding, polishing and refiguring. Also, sometimes overzealous cleaning can damage a coating and introduce sleeks or rub marks.

Roland Christen

>>Haven't heard anything about it but there's an interesting graph comparing the color correcting properties of FK01 verses BK7 (on one end) and CaF2 (Calcium Flourite) used in Tak scopes at:
http://www.optique-unterlinden.com/tab_fluo.htm>>

Those curves are misleading. One can make any kind of color correction including zero color with either FK01 or Fluorite. Color correction depends strictly on the mating element, not on the fluorite or ED material.

Roland Christen

>>It has been said that the AP's APO's are fully corrected in the Red and Green but off some in the Blue. Thus making a RGB image combination more difficult since the stars in the Blue channel will be larger looking bloated compared to the others.>>

First question: Compared to the other whats?

Second question: Have you actually ever done RGB imaging? What do you mean by the blue being off? Don't you realize that even if the blue colors have a slightly different focus, that when you do RGB, you always focus each color for smallest spot diameter?

Thirdly, the blue focus on the EDF lenses is the same as the red focus. It is the green that's slightly different. The reason for that is that the lens design was chosen to have 3 color crossings very far apart so that both the violet colors at 400 nm and the infrared colors out to 1000 nm will have the same focal points.

Fourthl: In order to have essentially zero color visually, you will have to sacrifice the deep IR at 1000 nm. Our new triplet lens does that - no color visually but the deep IR at 1000nm will need to be filtered out for the best Luminance image. RGB will not be affected, just like the present EDF design.

Would you like to see the two color curves?

Roland Christen

Hey, nice. You never showed me that one

Roland Christen