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Friendly Giant
super member

Reged: 12/11/07

Using an aperture mask on a dob new

new

#2631830 - 09/08/08 10:58 PM

Hello:

As someone who is relatively new to astronomy, I bought a 10" dob about a year ago and still believe that it was a great choice for a first scope - ample light grasp, etc...

This type of scope is good for DSOs, but occasionally I like to also observe the moon and planets. I was considering buying a small (80-100mm) apo refractor. Standard wisdom has it that the crips images and higher contrast make lunar and planetary views more pleasing.

Then one day I heard about a quick hack that intrigued me. Someone said that you can create an off-axis aperture mask for a dob. He suggested that the performance you get would approach that of a long-focal length refractor (without CA

). In my case, that would give me 4" of clear aperture (no obstruction), and "convert" my 10" f/4.6 dob into a f/12 scope.

The price was right, so I decided to try it. I took a pair of scissors to a piece of cardboard and in about 15 minutes had an aperture mask for my dob. That night I tried it out against the moon. I must say that I was impressed. I saw a lot of subtle detail on the moon and I believe the blacks were blacker than they were without the mask.

I'm sure other folks have tried this. What are your impressions? I'm actually thinking of forgetting the refractor for now and using my money to buy an Ethos eyepiece

Thoughts?

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star drop
Snowed In

Reged: 02/02/08

Loc: Snow Plop, WNY

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2631852 - 09/08/08 11:13 PM

Resolution is greatly reduced using an off axis mask. Dark looks darker because less light is getting to your eye. I spent a lot of time cutting one out from a sheet of aluminum and I am glad that I did not waste any paint painting the inside flat black. My mask with its ten inch hole went out in the recycling bin years ago.

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Friendly Giant
super member

Reged: 12/11/07

Re: Using an aperture mask on a dob new

new

[Re: star drop]
#2631875 - 09/08/08 11:31 PM

Quote:

Resolution is greatly reduced using an off axis mask. Dark looks darker because less light is getting to your eye. I spent a lot of time cutting one out from a sheet of aluminum and I am glad that I did not waste any paint painting the inside flat black. My mask with its ten inch hole went out in the recycling bin years ago.

Thanks for your candid response. Yes, resolution will obviously drop when you're looking through a smaller aperture. You won't get the same resolution looking through a 4" hole in a 10" mask as you do using the full 10" aperture.

The salient question is: Is the view in the 4" hole close to what one might expect from a 4" refractor? I don't have a 4" refractor to compare it to, but I thought someone else might have done a direct comparison.

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Bill Weir
Carpal Tunnel

Reged: 06/01/04

Loc: Metchosin (Victoria), Canada

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632009 - 09/09/08 01:16 AM

In a way it will be better than a refractor. No chromatic aberation will be introduced.

Bill

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632101 - 09/09/08 04:56 AM

Quote:

Standard wisdom has it that the crips images and higher contrast make lunar and planetary views more pleasing.

Standard myths, you mean. Refractors only have higher contrast *at equal aperture*.

Physics dictate that when you put a mask over your scope, the contrast transfer function *decreases* over all frequencies (except perhaps when the seeing is very bad).

*Do* get a variable polarising filter - that lets you cut down on the light throughput without sacrificing resolution.

Quote:

I'm sure other folks have tried this. What are your impressions?

I have one. I never use it anymore, especially not since I've made an apodising mask for the full aperture to look at planets in bad seeing. The only thing for which it's still useful is to look at the moon in very bad seeing.

Mind you, it does help things like focusing (the depth of field is much larger), does help the off-axis performance of cheaper eyepieces, and obviously cuts down on irradiation on bright targets at low power, but it's a bad indirect way to fix these problems. A two-speed focuser, better eyepieces (or using more eyepieces in a barlow for higher power) and filters solve these more directly, and without sacrificing image quality.

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Lamb0
Carpal Tunnel

Reged: 07/25/07

Loc: Fairbury, Nebraska

Re: Using an aperture mask on a dob new

new

[Re: Bill Weir]
#2632104 - 09/09/08 05:03 AM

Quote:

The salient question is: Is the view in the 4" hole close to what one might expect from a 4" refractor? I don't have a 4" refractor to compare it to, but I thought someone else might have done a direct comparison.

The short answers are:

Chromatic aberration---: absent (like an extremely good apo)

Resolving capability----: similar

Contrast-----------------: similar - no central obstruction

Light gathering---------: Almost , this depends on the mirrors' reflectivity - maybe 89% times 89% or ~79% total for Aluminum coated mirrors, versus ~98% for a coated refractor times 89% for an Aluminum coated diagonal for a total of ~87% - YMMV

Coma--------------------: still present, but more complex than a simple Newtonian design. Less than the full aperture scope would otherwise have, but more than a simple Newtonian with a similar long f/ratio due to the mask being off axis. Instead of the coma increasing linearly with the image radius, the increase is approximately proportional to the image radius times {[(the ratio of the exposed masked radius's radial arc length of the primary divided by the mask radius's radial arc length of the primary) times (the arc distance as measured from the center of the primary divided by the total radius of the primary)]}integrated from the inner arc length from primary center to the outer arc length from primary center ...whew!

Perhaps someone else can simplify that!

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Friendly Giant
super member

Reged: 12/11/07

Re: Using an aperture mask on a dob [Re: sixela]
#2632192 - 09/09/08 07:54 AM

Quote:

I have one. I never use it anymore, especially not since I've made an apodising mask for the full aperture to look at planets in bad seeing. The only thing for which it's still useful is to look at the moon in very bad seeing.

Can you elaborate? What's an "apodising mask"?

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632251 - 09/09/08 08:55 AM

Something which enhances contrast at medium frequencies at the expense of higher frequencies, to be used when the seeing doesn't let you see the smaller detail anyway.

The most common version uses a bug screen, which causes a rainbow like diffraction pattern (but not around a small zone around the object that is a light source), making it OK to use on planets but not on the moon.

Note: for obstructed apertures, it's *really* important to design it according to http://home.digitalexp.com/~suiterhr/TM/apodize.htm . If you don't, the combined effects of the central obstruction and the mask may make matters worse rather than better.

http://www.csastro.org/gallery/article4.htm

for pictures -- but don't use the percentage values there in an obstructed scope!

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Jarad
Postmaster

Reged: 04/28/03

Loc: Atlanta, GA

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632268 - 09/09/08 09:05 AM

Here are my thoughts on aperture masks and small refractors:

Aperture masks: They reduce the amount of air you are looking through, and eliminate diffraction spikes from the spider. This produces a steadier, and sometimes more aesthetically pleasing view (especially when the seeing is poor). It also produces a less-detailed view (you lose resolution, and the reason it looks more aesthetically pleasing is that you no longer resolve some of the atmospheric turbulence).

I rarely if ever use aperture masks, since the full aperture always gives more detail. However, since they cost the price of a piece of construction paper, I would say go ahead and make one and try it to see if you like it.

Small refractors.
A 4" refractor should not outperform a 10" reflector on any object under any condition unless something is wrong with the reflector (poor optics, miscollimated, not thermally cooled down, etc.). The advantages it does have are that it can give wider TFOV views than the larger scope (but stopping down the larger scope will probably NOT accomplish the same due to the longer FL), and that it is light, portable, and does not need a long time to cool down and come to thermal equilibrium.

This makes it ideal for 3 things:
1 - quick looks when you don't have time to set up, collimate, and cool down the larger scope.
2 - Wide field views of objects like the Veil, North American Neb, Double Cluster, M31, etc.
3 - Astrophotography.

Those are things that you cannot get by simply masking down a larger instrument.

So if you want to get one of those 3 advantages, you should still get the small refractor. If you just want to reduce diffraction spikes and atmospheric effects (at the cost of less resolution and detail), then the aperture mask will work.

Jarad

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mcoren
Pooh-Bah

Reged: 01/11/06

Loc: Virginia, USA

Re: Using an aperture mask on a dob new

new

[Re: sixela]
#2632277 - 09/09/08 09:09 AM

I find that the two biggest "improvements" from homemade aperture masks are due to a reduction in glare (which swamps the eye's ability to discern fine detail) and edge effects on inexpensive, mass-produced mirrors.

A 10" dob sends a lot of light into your eye, so the glare can be cut down with a variable polarizer, as Sixela suggested. That's the best "planetary filter" I know of!

On my XT10, the mirror has a slightly turned down edge. Until I decide to get it refigured, I use a mask to cover the outer 1/2" or so for when I do high-power planetary viewing. I experimented with various poster-board cutouts and determined that was sufficient to dramatically improve the star test. Note that this leaves my secondary mirror in the optical path (and the size that much larger relative to the exposed primary aperture), so I take issue with stories of secondary mirrors making reflectors unusable for high power planetary viewing!

The slight aperture mask combined with 1/2 of the variable polarizer is a great combo for me.

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walt r
Post Laureate

Reged: 02/13/07

Loc: Doylestown, PA

Re: Using an aperture mask on a dob new

new

[Re: mcoren]
#2632372 - 09/09/08 10:15 AM

Yep, I have tried the offset aperture mask many times. The full aperture has always shown more details. The few times it seemed to help was due to the dimming of bright objects (like the Moon or the 'double-double'). I found in those cases a dense filter (UHC or ND) gave a larger improvement.

If the mask improves the details then look for a TDE, bad collimation or other optical problem. Since a mask is cheap and easy to make I do recommend trying one but also recommend trying a filter if the object is bright.

Edited by walt r (09/09/08 10:18 AM)

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Bill Weir
Carpal Tunnel

Reged: 06/01/04

Loc: Metchosin (Victoria), Canada

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632572 - 09/09/08 11:59 AM

Quote:

Quote:

I have one. I never use it anymore, especially not since I've made an apodising mask for the full aperture to look at planets in bad seeing. The only thing for which it's still useful is to look at the moon in very bad seeing.

Can you elaborate? What's an "apodising mask"?

They work wonderfully, by moving the edge if the first diffraction ring further out from the bright object. This means they are also quite useful for splitting difficult double. Things like Sirius B.

I made one for both of my dobs. Once I was at a public event set up next to a TV 102. We were looking at Venus, Saturn and Jupiter. My inexpensive Orion with a 99 cent accessory put a real frown on the face of the guy with the Televue. The view through my scope, of the thin crescent of Venus close to the horizon , blew the Apo away. This setup also gave a 9" Mak a real run for it's money.

Bill

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KaStern
professor emeritus

Reged: 04/18/06

Loc: TwilightZone

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2632698 - 09/09/08 12:47 PM

Hello Friendly Giant,

with the 4" mask you will loose a lot of resolution and a lot of
contrast transfer possibility too.

Especially the latter is not obvious every time, since the atmosherical conditions
vary and the air is not allways so stable that you can get the theoretical
resolution and the theoretical contrast transfer of your 10" scope.

With the mask installed you avoid two marginal contrast lowering things:
1) the central obstruction diffraction effects
2) the spider obstruction diffraction effects

Both are marginal against the loss of contrast by downsizing the aperture
from 10" to 4". With the mask installed you keep two problems coming
along with the 10" Newt:
1) the thermical problem caused by the big glass mass down in your tube
2) the focal ratio of the scope is the same as a 4"f/12 only in terms
of light gathering, resolution and contrat transfer, but NOT in terms
of the angles of the light rays coming into your eyepiece.

The mask makes an off-axis-Newtonian. The Light rays for the side
where the mask is situated come in like the rays of a 10"f/4.8 Newt.
Simple eyepieces will suffer, you should use better suited designs.

The off-axis coma of the newt stays to be there with the mask installed.
But it will be distrubuted asymmetrically. And it will be much less visible:
1) since the stars are much dimmer with the mask
2) the total amount of the mirror surface near to the edge is largely reduced
3) the coma differs depending on the position of the star, tangential
and sagital. Please see a website like this one:

http://www.telescope-optics.net/

All in all you combine the downsides of small aperture with the practical
downsides of the optical system Newtonian and cure only the relatively
marginal negative effects of obstruction.

Quote:

Standard myths, you mean. Refractors only have higher contrast *at equal aperture*.

Yes, and only top quality refractors with low colour aberration have
slightly better contrast transfer.

Quote:

I'm sure other folks have tried this. What are your impressions?

I have looked through a 10"f/6 Newt with 4" off-axis mask.
I was not impressed at all.

Clear skies, Karsten

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hoof
Carpal Tunnel

Reged: 04/07/05

Loc: Redmond, WA

Re: Using an aperture mask on a dob new

new

[Re: KaStern]
#2632899 - 09/09/08 02:30 PM

I experimented with off-axis masks with my 12.5" F/5 truss dob I used to own. The only "benefit" I ever noticed was that my cheap eyepieces were suddenly transformed into spectacular eyepieces (at F/12+ I'd expect them to). The images were darker, showed less detail, and never looked better than full aperture.

One interesting thing about the off-axis mask was when you focused on a star. Defocusing caused the star to actually shift in the view. It makes sense when you think about it, but when I first saw the effect, I thought the 'scope was moving!

Incidentally, the Orion 3.6" off-axis 'scope they used to sell is a physical implementation of the off-axis concept.

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jg3
sage

Reged: 05/27/07

Loc: near Auburn, CA

Re: Using an aperture mask on a dob new

new

[Re: hoof]
#2633458 - 09/09/08 07:45 PM

The only sight that benefits from a 5-inch off-axis mask on my 12.5" f/6.3 scope is the moon in mediocre seeing (most of the time here). It has never helped me with planets.

Theoretically, MTF improves at no spacial frequency at all when masking down like that, even accounting for the obstruction when unmasked.

I also made an apodizing mask following Suiter's articles and spreadsheet. That makes a world of difference on planets in any seeing, especially the low-contrast Mars and Saturn. That's even after controlling for brightness by neutral and/or contrast-enhancing filtration, and whether bino or mono viewing.

Suiter's material unfortunately gives only minimal guidance on what to try to achieve when toying with shade-ring dimensions. After fiddling with his spreadsheet a bit, I linked it up to the Solver tool in Excel, and experimented with different objectives and starting values. Best for planets is to maximize MTF at a low spacial frequency point. It's surprising how much contrast does *not* depend on resolution. Good for doubles is to minimize intensity everywhere outside the disk - you can suppress the first ring quite a bit, but at the expense of contrast at low spacial frequencies. The spreadsheet suggests maximizing encircled energy within the disk, but you can't do much there. I'll report more details if popular demand emerges.

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Friendly Giant
super member

Reged: 12/11/07

Re: Using an aperture mask on a dob new

new

[Re: jg3]
#2633851 - 09/09/08 11:04 PM

Wow, I'm pleased with the contributions by everyone in this discussion. Since the general concensus appears to be that nobody believes in any benefit from the off-axis mask, I decided to look again tonight. The sky was clear with a half-moon, so I focused primarily on Jupiter and the moon. I viewed first with the 4-inch off-axis mask, then without the mask (but using a polarizing filter to achieve approximately the same light intensity when viewing).

I found that I really couldn't see better detail without the mask compared to with the mask. They really were comparable in resolution. Although some may suggest that I might want to revisit my collimation (which I accept), I suspect that the seeing simply didn't allow the full aperture to show its full potential resolving power (looking through a lot more air). Does that sound reasonable?

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Rob S
professor emeritus

Reged: 03/16/07

Loc: NZ

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2634170 - 09/10/08 03:52 AM

Another place an apodization mask is in place in currently available optics:

http://the135stf.net/index.html

Rob.

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sixela
Postmaster

Reged: 12/23/04

Loc: Boechout, Belgium

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2634184 - 09/10/08 04:32 AM

Quote:

I found that I really couldn't see better detail without the mask compared to with the mask. They really were comparable in resolution.

Well, what power were you using? At low power, you can't see the resolution limits of a scope. You need at least 200x before you'll begin to clearly see the difference between a 4" aperture mask and something bigger, especially on the moon (which has very high contrast features that are easy to detect).

But the difference is there - physics dictate it, and my personal observations agree. Try looking for small subtle colour and shading differences in the Mares rather than hard contrast details on shadows, and you'll see the difference sooner.

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Friendly Giant
super member

Reged: 12/11/07

Re: Using an aperture mask on a dob new

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[Re: sixela]
#2634225 - 09/10/08 06:25 AM

Quote:

Well, what power were you using? At low power, you can't see the resolution limits of a scope. You need at least 200x before you'll begin to clearly see the difference between a 4" aperture mask and something bigger, especially on the moon (which has very high contrast features that are easy to detect).

But the difference is there - physics dictate it, and my personal observations agree. Try looking for small subtle colour and shading differences in the Mares rather than hard contrast details on shadows, and you'll see the difference sooner.

These are good points. Actually, I was using 240X (perhaps a little too low to make the difference in resolution apparent). I looked in the Mares as well as the hard contrast areas, but may not have concentrated on the Mares enough.

OTOH, I did see more detail looking at Jupiter at 240X. Since Jupiter is a lower contrast object, perhaps this was a better test.

Again, the seeing may have also prevented the full aperture from showing all that it can do also.

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walt r
Post Laureate

Reged: 02/13/07

Loc: Doylestown, PA

Re: Using an aperture mask on a dob new

new

[Re: Friendly Giant]
#2634570 - 09/10/08 10:19 AM

On the planets use higher power (without a mask) than the seeing seems to support then keep observing. There will be moments of steadier seeing where much more detail is seen (sometimes on only part of the planet at a time). With the mask the moments of improved seeing are never seen.

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