39 replies to this topic

[jayscheuerle]

I've heard that using an offset mask on a large newt will give it apo-like views as it does away with the diffracted light from the spider and CO, but it seems to me that the entire edge of the offset mask would just be spreading the diffraction around, not unlike a curved spider, instead of eliminating it. I had a mask at one time and just found my views to be dimmer and less detailed.

Does anyone have the down-low on the physics for this one? Thanks! - j

[Jason D]

dimmer and less detailed.

Compared to APO or the unmasked reflector?
I assume you meant the first since the explanation for the latter is simple: Less aperture means dimmer and less detailed.
Jason

[Nils Olof Carlin]

Jay,
if you had a telescope with an aperture with no edge, you wouldn't have to worry about diffraction!
Seriously, the diffraction is determined by the aperture as such, not by the edge - and an offset, obstruction-free part of a mirror will give the same diffraction pattern as a refractor of the same aperture, ignoring any optical imperfections. The result of "spreading the diffraction around" is usually known as the Airy disk...


The usefulness of an offset mask has been debated over and over again - the consensus seems to be that with a good reflector, the stopped-down view may look just as pleasing as the apo at least if you just look at the center of the field - the edge may look different. But the view with the full mirror always gives better, or at least equal, resolution than the stopped-down one, seeing good or bad. Or than the APO, so why pay extra?

I personally suspect that there is more to gain if the mirror is a bit less than perfect in the first place. A part of the full mirror may be expected to have much less wavefront deviation from ideal than the full mirror has - think of an edge defect, only a tiny part of it will be within the aperture used. Or a bit of astig.

If you (any of you out there) happen to have a large reflector, the cost of a piece of cardboard is so small that you really ought to make a mask and try for yourself. Try on bright doubles, and on the moon (for low-power views where resolution is no issue but intensity is - take it off for high power, detail viewing).

Nils Olof

[dave b]

the view is better without the mask

read this article by our own darren drake about how aperture and resolution is king:

http://www.astromart...?article_id=473

[KerryR]

(edit: oops. This is pretty much what everyone else said. They posted while I was typing. Forgive the redundancy...)

I don't know much about the physics, so someone else who does will likely clarify, but here's my (lack of) understanding:

I think the edge of the mask would have the same effect as any egde in any aperture-- it does indeed difract light all around, but this is always the case, whether the object causing the diffraction is a spider (curved or not), secondary, or aperture edge of a newt or refractor, or a misplaced baffle-- all edges cause difraction if they are in the light path.

I believe that's what causes the difraction rings in the in or out of focus star image-- the edges of the aperture. I think the Airy disk a tiny 'image' of the front aperture (or whatever part of the optical train that is the last 'visible' edge).

The front aperure would put out the same sort of difraction as a curved spider, though the aperture without the curved spider (or secondary for that matter) would have only the difraction caused by the aperture itself, all difractions perpendicular to a line tangent to the curve of the aperture (which really just means they're spread out away from eachother, as opposed to lined up with eachother as is the case with a straight spider), and therefore have theoreticly better contrast. In practice, it's probably hard to see the difference.

In my 16" newt, I find that planetary images have much more resolution and detail at full aperture, difraction spikes and all, than they do when using a 6" off-axis mask, courtesy of the added aperture.

The images are sort of apo-like with a mask, but the focal length is so much longer that it's really not the same beast as an f5 or even f9 apo-- the FOV will be whatever it is in the full aperture scope (minus whatever you loose to fully illuminated field at the field stop, I think), so it's not as "fun" as the equivalently sized refractor with a shorter FL.

[jg3]

The "edge theory" of diffraction commonly circulated among amateur astronomers is an incorrect extrapolation of the classic slit experiment in college physics classes, and how it explains vane diffraction patterns. This is forgivable because it's almost impossible to generalize what any shape aperture will diffract into; it takes a somewhat cumbersome numerical model to predict the diffraction pattern from any but a few specific cases.

A counterexample of the "edge theory" is an eccentric aperture mask with two equal circular holes instead of one. The diffraction pattern has spikes, despite no straight edges.

An eccentric aperture stop-down mask for large reflectors can help with bad seeing and/or local air "thermals" on the mirror or in the scope. If things appear blurred with the full large aperture, they may appear reasonably sharp but wiggling around when stopped down - fine for visual observing. But they will not appear as sharp as with the full aperture (obstructions and all) in a moment of still air.

My own experience with an eccentric mask (on my 12.5-inch reflector) is it only helps with the moon during poor seeing. It has never helped my planetary views.

Closer to the original question, the effects upon contrast by the central obstruction can be more than made up by getting a bigger scope. A good 12.5-inch reflector with a CO has comparable contrast to a 10-incher without, and is more easily obtained.

[walt r]

I have also found that even during poor seeing my 18" scope stopped down to 6" never gave views better than at full aperture. During a Jupiter viewing session, the view with the 6" mask was consistent, only so much detail could be seen and the view did not change. With the full aperture Jupiter showed about the same amount of detail most of the time but during moments of better seeing much more details and contrast was seen than when the 6" mask was in place.
I have the opinion that it is a myth that an offset mask will improve the details or contrast.

Cutting a piece of cardboard to make an offset mask is easy and cheap. I encourage anyone with a large scope to try one.

If it does improve the view then look for a TDE or other defect on your mirror.

[Bill Weir]

Build one of these http://www.kitgear.com/apodiz/ it will increase the planetary contrast when using a reflector. Personally I like using the black metal screening. The person who copied these plans out of an RASC Journal has his explanation off a bit on how it works but that isn't important. They do work.

Also very useful on double stars. The apodizing mask on my 6" dob made the difference in clearly seeing Sirius B the other night.

Bill

[jg3]

I made a very effective apodizing mask, but I hesitate to mention it because doing them right is fairly technical. They are liable to not work if not designed accounting for the obstruction ratio and the actual transmissivity of the screen.

Dick Suiter (physicist and author of "Star Testing Astronomical Telescopes") has graciously posted resources that I consider required reading:

http://home.digitale.../TM/apodize.htm

To Suiter's material I'd add a couple things:

1) For measuring screen transmissivity, an alternative to a light-meter is a camera and a longish focal length eyepiece, to get a picture from which you measure the screening proportions;

2) He gives little guidance on the objective goal to seek on the spreadsheet. For planets, I got great results maximizing encircled energy within disk plus the first diffraction ring, or raise the low-res MTF. For doubles, it's possible to cut the near rings way down even though this smears light around. (I warn you, it's technical.)

[HandyAndy]

Hi,

I tried a 6" F5, 10" F6.3, 16" F5 and off-axis 6.5" F12.2 on Jupiter and Saturn.

The 6.5" was better on Saturn, I presume as it was better at showing the low contrast details on Saturns disk.

The 16" with 0.3ND filter was best on Jupiter.

Cheers. Andrew.

[sixela]

Build one of these http://www.kitgear.com/apodiz/ it will increase the planetary contrast when using a reflector.

Except A, B and C depend on the obstruction ratio and what you want to achieve, and on the screen transmittance (as jg3 correctly pointed out). Pick them wrongly and the filter will be counterproductive (even though it may still help in bad seeing).

Mine did wonders on Saturn yesterday. Couldn't see the Cassini gap without it, could see it with the mask (if anyone reads this years from now: don't forget what the rings look like in 2009 ). There was also a lot more structure visible in the edges of the cloud band south of the rings.

[Bill Weir]

Build one of these http://www.kitgear.com/apodiz/ it will increase the planetary contrast when using a reflector.

Except A, B and C depend on the obstruction ratio and what you want to achieve, and on the screen transmittance (as jg3 correctly pointed out). Pick them wrongly and the filter will be counterproductive (even though it may still help in bad seeing).

Mine did wonders on Saturn yesterday. Couldn't see the Cassini gap without it, could see it with the mask (if anyone reads this years from now: don't forget what the rings look like in 2009 ). There was also a lot more structure visible in the edges of the cloud band south of the rings.

In the origional RASC Journal article that this person used, the central obstruction is taken into account. It is what I used to built them for both my 6" and 12.5".

With me last night, the apodizing screen on my 12.5", made the difference in being able to pick out Mimas right next to the ring, when observing Saturn.

Bill

[BradleyB]

I've built an offset mask for my 12.5" and found that when the seeing is quite poor the 5" offset gives much better resolution. On the double double with poor seeing (most of the time here) at 12.5" I could barely tell that they were doubles. With the mask it became quite clear with a good split. This is not due to a bad mirror, when the seeing is ok or better the 12.5" aperture blows away anything smaller. I can also use the mask in various orientations using different parts of the mirror with no different effect. Even if I place it so that some of the spider vanes is in the view it does not make much difference. Those of you farther south will probably never have seeing so bad that a mask will help. But for those of us up north it does help at times. Yes it is dimmer and no help on faint objects, but for doubles at least it can make a positive difference.

Brad

[walt r]

I've built an offset mask for my 12.5" and found that when the seeing is quite poor the 5" offset gives much better resolution. On the double double with poor seeing (most of the time here) at 12.5" I could barely tell that they were doubles. With the mask it became quite clear with a good split.
Brad

I've also found that a mask cleaned the split on the double-double when the seeing was poor. But I than tried the full aperture and a neutral density filter to get the same dimming as the mask. Well, yo and behold the split was just as good and maybe even a little better with the filter.

My conclusion is that the apparent improvement from the mask is solely due to the dimming of the relatively bright stars and not from the reduction of the aperture. I have read that others have found the same results.

Next time the seeing is poor give a filter a try.

[astroghlo]

I did use them but they just dimmed the image. If the seeing is poor I'll just bring out a smaller scope,such as I'm doing tonight.

[BradleyB]

I tried a .25 ND filter last night on a double star and compared it the mask. The ND filter does not cut as much light as the mask does, so I'm not sure it was an equal comparison. It's a 12.5" with 5" mask. The filter helped a little, but not anywhere near as much as the mask does. The seeing supported about 250x on the moon for best detail, and the mask still gave much better splits on double stars. Now the view of the moon was better at full aperture than with the mask. Not sure why.

Brad

[walt r]

Thanks for your report Brad.
If the ND filter dimmed the double as much as the mask I'm sure the split would have been as good or better than with the mask. Its for the some reason the Moon was better at full aperture---greater resolution. The same goes for planets.
If you have a UHC filter try that to dim a double if the .25 ND doesn't dim enough.

[jayscheuerle]

A mask isn't about dimming the views, even though it does. The mask gets rid of the central obstruction (a filter doesn't) and slows the scope down, along with reducing the aperture to be less effected by poor seeing. Those are its supposed advantages. A filter will allow you to get the full resolution out of the full aperture of your scope, while an offset mask will not. A ND filter will simply enable you to keep from wiping out your night vision with the Moon or even Jupiter.

Comparing the ND filter to an offset mask is an apples to oranges comparison. - j

[walt r]

Hi Jay,
Yea we did get a little off your original topic but not by much. I believe that a basic part of your question is 'will the VIEW be better'? A comparison of a masked down aperture to a full aperture ND filtered view is relevant in trying to understand why a masked view might look better.

As to offset mask = APO view one could do this theoretically or empirically. The theory says that a central obstruction (and spiders) decreases the signal of low frequency spatial details as shown of an MTF (Modulation Transfer Function) plot but does not decrease the high frequency details. However, it is generally accepted that for central obstructions under about 22% diameter there is no visual degradation. Loss of low frequency details can be seen with CO's of greater than about 25-30%. So on most larger Newtonians the CO is less than 22% and an offset mask has no advantage.

Empirically is a bit difficult due to the offset mask changing other characteristics of the optical system. The higher f/ number produces a dimmed view of the object (and reduces the limiting Mag of stars) and the smaller aperture increases the size of the Airy disk which reduces the resolution. This is one reason to compare a full aperture view dimmed with an ND filter. If the ND filtered view equals the dimmed mask view then it was not removing the CO that improved the view. An ND filter does help split bright doubles like the Epsilon Lyra on nights with poor seeing and is also helpful to reduce glare on other bright objects like Mars during opposition. Try it.

Two very good books that cover the details of how different optical system perform are: Telescope Optics by Harrie Rutten and Martin van Venrooij and Star Testing Astronomical Telescopes by Harold Richard Suiter

[jayscheuerle]

An ND filter does help split bright doubles like the Epsilon Lyra on nights with poor seeing and is also helpful to reduce glare on other bright objects like Mars during opposition. Try it.

So the ND filter dims the flares dancing around the targets in poor seeing, leaving the cores more visible? Makes sense.

It took me over a year to get a secondary that would allow me to split the double-double in my 12"LB, so I'll happily give it a shot! - j

[Bird]

Hi Jay, let me add my $0.02 worth :-)

I'll take a different tack to everyone else who's replied, by taking your question a bit more literally - ie "Will an offset mask of X inches on a newt give me the same view as an APO of X inches aperture?".

Well, theoretically the answer is yes, in fact under ideal conditions (properly cooled mirror and scope etc) you will get a better view than the APO can manage for two reasons - the light has to pass through a set of lenses in the APO but not in the newt, and so the newt gives a slightly brighter (not that you can notice) and less abberrated image.

Of course, the question everyone else here is trying to answer is *why* you would do this with the newt. Opinions vary, many people suggest that the image will always be better in the unobstructed aperture. Certainly the unobstructed view will be brighter and have a smaller airy disc, ie better resolution.

FWIW I agree with this opinion, but I can't really say I've tested this scientifically under perfect conditions, so , as they say, YMMV.

cheers, Bird

[Gandalf223]

I made an offset aperture mask for my 8" Dob because I didn't want to wait for a lunar filter to get here. Total cost: maybe a dime's worth of tape. It works great for lunar viewing -- the system is f/16 with the mask -- and now I'm not sure I need to bother with the ND filter.

I haven't done any kind of diffraction analysis, but I suspect the tradeoff (getting rid of diffraction from the secondary spider, vs more diffraction from the smaller aperture) is a fair one.

I do have a question; maybe someone who is more conversant with Newtonian optics can answer this: With an offset mask like this, we are using a section of the primary that is to one side of the mirror, not around the center. What happens to the Newt coma in this case?

[walt r]

The coma is nearly the same as with the full aperture.

[Bird]

Even though the aperture mask is only allowing the edge of the mirror to capture light, as long as that light is on-axis then there is no coma.

cheers, Bird

[Jarad]

Bird is right that there is no coma on-axis (there isn't coma on-axis even at full aperture). Walt is partly right - the off-axis circle will still have coma, but it will be less than with the full mirror. That's because most of the coma comes from the edge of the mirror, and the offset circle uses less of the edge than the full mirror does.

It will also be more than than the coma of a standard untilted design of the same f-ratio would be, because the offset circle uses more of the edge and further out toward the edge than a centered circle would.

So if you make an offset mask for an f4 newt, turning it into an f16, the coma will be like something in the f6-f10 range. Reduced, but not gone.

Jarad