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Off Axis Mask for Planetary compaired to APO

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#1 orion61

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Posted 05 December 2012 - 11:04 AM

How well do you think an off Axis mask performs compaired to an APO refractor.
I know a lot to do is quality optics, I had a 13.1" Coulter with OK optics but I thought the views were pretty good.
Has anybody set up next to a refractor and compaired?
I could get about 4" clear with mine.

#2 Nils Olof Carlin

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Posted 05 December 2012 - 01:27 PM

Many years ago, I set up my Coulter 13.1 stopped down to some 5", next to an AstroPhysics 5", testing it on Saturn low over the horizon - mine was the clear winner, but I don't know much about the refractor, it may have been in poor shape.
It is cheap and easy to decide for yourself, so by all means, try!
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#3 Jon Isaacs

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Posted 05 December 2012 - 02:12 PM

Has anybody set up next to a refractor and compaired?



I frequently setup my larger reflectors next to my 4 inch apo but I never use an aperture mask.. the reflectors provide significantly better views of the planets, as one would expect. In my thinking, the advantage of a 4 inch apo is that is compact and does as well as a 4 inch scope can do but a decent 10 inch has a real advantage over any 4 inch...

Jon

#4 Dwight J

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Posted 05 December 2012 - 02:21 PM

Interestingly enough, I used a 5 inch off axis mask on a Coulter 13.1 as well with good results but not quite the match of a nearby 4 inch refractor (a Carton F13 achro). This I think was mostly due to the mirror still not cooling enough. The old Odessey design did not allow for any air to get to the mirror. I then sold the 13 and used the mask on an astigmatic C14 for high power viewing, rotating it around untill the astigmatism was minimal. It disappeared at the 11 o'clock position, providing us with excellent views. I tried it on a Meade DS 16 with a 6 inch mask but it was not a good as a nearby C8. Rotating the mask around while checking the view in various positions can improve things rather than just applying the mask. You still need good optics and cooling, especially the boundary layer on larger mirrors, to get good results with a mask.

#5 jgraham

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Posted 05 December 2012 - 04:13 PM

In the olde days the use of an off-axis mask often did more to mask the poor quality of the mirrors than anything else. Dropping the brightness down also helped to make the planet appear better with higher contrast. I've recently had an interesting experience with my LightBridge 16. The view of Jupiter is so bright that it's a bit uncomfortable to to look at and the brightness also degrades the contrast. However, when I use my binoviewers I found that is knocks the brightness down enough that itss very comfortable and the contrast is much improved. As a bonus, there's no reduction in resolution when using binoviewers. During moments of steady seeing the view is wonderful.

As for using a aperture mask... why not give it a try, what could possibly go wrong?

#6 Jarad

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Posted 05 December 2012 - 04:22 PM

How well do you think an off Axis mask performs compaired to an APO refractor.



Well, if the scope meets the following requirements, they will be very similar to an APO of the same aperture. Those requirements are:
1 - good optical quality over the unmasked area.
2 - thermal equilibrium
3 - good collimation

The more important question, though, is how it performs compared to the unmasked aperture. The main reason a masked aperture out-performs the same scope un-masked are:
1 - The primary mirror has zones or TDE or other optical error that is in the masked-off section.
2 - The brightness of bright objects is so high that your eye is losing contrast.
3 - Really bad seeing.

The only one of these that I would routinely use a mask for is #1 - you really may see more detail with a smaller aperture of higher quality. For #2, I would use something like a neutral density or variable polarizing filter to tone down the brightness (or a binoviewer as the previous poster suggested if cost is not object). For #3, you generally still see a bit more detail at full aperture, although the turbulence will be more obvious and annoying than in a smaller aperture which may produce a more aesthetically pleasing and steadier image.

There may be other reasons I have forgotten, I am sure someone else will list them.

Jarad

#7 Daniel Mounsey

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Posted 05 December 2012 - 04:50 PM

How well do you think an off Axis mask performs compaired to an APO refractor.



Well, if the scope meets the following requirements, they will be very similar to an APO of the same aperture. Those requirements are:
1 - good optical quality over the unmasked area.
2 - thermal equilibrium
3 - good collimation

The more important question, though, is how it performs compared to the unmasked aperture. The main reason a masked aperture out-performs the same scope un-masked are:
1 - The primary mirror has zones or TDE or other optical error that is in the masked-off section.
2 - The brightness of bright objects is so high that your eye is losing contrast.
3 - Really bad seeing.

The only one of these that I would routinely use a mask for is #1 - you really may see more detail with a smaller aperture of higher quality. For #2, I would use something like a neutral density or variable polarizing filter to tone down the brightness (or a binoviewer as the previous poster suggested if cost is not object). For #3, you generally still see a bit more detail at full aperture, although the turbulence will be more obvious and annoying than in a smaller aperture which may produce a more aesthetically pleasing and steadier image.

There may be other reasons I have forgotten, I am sure someone else will list them.

Jarad


Jarad, you covered the reasons quite well. :waytogo:

#8 Daniel Mounsey

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Posted 05 December 2012 - 06:26 PM

Just an FYI,

I've conducted countless visual comparisons using an off-axis masks to the finest apos on the market of equal aperture, same scopes, different nights. I've had situations where I've posted my results and sometimes it was a bit better in the apo and sometimes a bit better in the off-axis mask. The #1 thing about a refractor is its "consistancy".

After all is said and done as long as the optics and coatings are good quality and thermals are out of the picture, the off-axis reflector will perform 100% as good on planets as the best apos in the world. I've seen it with my own eyes in side by side comparisons against premium apos. The reflector is a force to be reckoned with when it's operating properly.

If everything else is good, full aperture in a good medium size or large reflector is a Godly sight to behold on planets. If you already know you have a premium reflector and it's not holding against the apo in your comparison, just rack the focus out on both scopes. As long as you know you have a good reflector, you will usually find that the star test in the reflector has a bit of thermal noise, causing the problem.

Thermals HANDS DOWN! are the Achilles Heel of the reflector. This is why the views of planets sometimes look equally good or worse than the apo. The refractors images are usually very "dormant" compared to a reflector that isn't modified well.

One only has to imagine what’s going on inside the reflector. A refractor receives its photons with little or nothing in front of its objective except atmosphere. The photons are undisturbed. The reflector on the other hand has to contend with thermal garbage between the primary and secondary and are often hindered before the photons even reach the primary. Then they have to travel back. It's like a plane going through turbulant atmosphere and it isn't a smooth ride.

Pons who is Newton master always told me. He said just the slightest wave over the primary is all it takes to remove the contrast of those tiny, delicate planetary features and rob you. In today’s common truss designs, those waves come from your own body currents, ground currents, boundary layer currents etc. This often happens even while using an off-axis mask.

Observers can preach the gospel all they want about aperture and planets, but until they face this reality, it isn't as simple as just buying a larger reflective telescope, and the larger the telescope, the larger the noise gets amplified with reflectors. The mask can often help while observing planets and double stars by reducing the noise.

#9 JohnMurphyRN

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Posted 05 December 2012 - 06:57 PM

The view of Jupiter is so bright that it's a bit uncomfortable to to look at and the brightness also degrades the contrast. However, when I use my binoviewers I found that is knocks the brightness down enough that itss very comfortable and the contrast is much improved. As a bonus, there's no reduction in resolution when using binoviewers. During moments of steady seeing the view is wonderful.


+1!

#10 orion61

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Posted 05 December 2012 - 09:10 PM

I bought a cheap DC fan from Radio Shack and mounted it in the bottom of my 6" F10 Super Planetary Newt with rubber bands, ran off a 9V battery and All thermals went away, no vibration, My biggest problem is that it ran so quiet I kept forgetting to turn it off, 9v's are evpensive.

#11 Daniel Mounsey

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Posted 05 December 2012 - 11:19 PM

The view of Jupiter is so bright that it's a bit uncomfortable to to look at and the brightness also degrades the contrast. However, when I use my binoviewers I found that is knocks the brightness down enough that itss very comfortable and the contrast is much improved. As a bonus, there's no reduction in resolution when using binoviewers. During moments of steady seeing the view is wonderful.


+1!


+1 as well.

#12 bartine

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Posted 06 December 2012 - 10:50 PM

You need to get one of these to save on 9v batteries - a variable AC adapter with a 9V plug.

Crank up the voltage to your fan to cool quickly
Turn it down to maintain the clear views. I had one and gave it away with a scope I sold - I'm getting another one!

http://www.thelabora...?zmam=176627...






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