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Why does a mask make a Dob give sharper images?

mdstuart

By mdstuart
in The Astro Lounge

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jetstream

jetstream

Posted
Posted

Just been using my 90mm offset aperture mask on my 250px Dob (black paper coated foam  - £1.99 per A4 sheet similar to the board used by Astronymonkey). Significant benefit on Jupiter, Mars and the Moon in tonight's conditions.  

I believe that you experience a benefit from the 90mm mask,but don't understand it.My 90mm refractor is a good scope,but has never bested my 10" dob,obstructed with a large 63mm secondary.It is interesting to me to hear of these experiences.A friend has just recommended to me a non conventional mask for my 10" dob, that should give about 120mm clear aperture,will be fun to try.Great that your mask worked!

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sbj

sbj

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"Is that not just the same as leaving the tube cover on but taking off that little round cap thingy or is a mask a completely different thing ? "

That's what I was thinking. Is it not?

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Tiny Small

Tiny Small

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I would assume that the decrease in CA is from the mask induced increase in focal length,as is the increased "depth" of focus.But I could be completely wrong :smiley:

You are completely correct. I expected it to improve the frac but I had no idea by how much. I was very pleasantly surprised.

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John

John

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You are completely correct. I expected it to improve the frac but I had no idea by how much. I was very pleasantly surprised.

It has removed most of the CA because the effective focal length has increased from F/5 to around F/10 assuming the diameter of the hole in the cap is around 50mm. So what you have created is a 50mm F/10 refractor - very little CA but rather low on contrast and resolution as you might expect a 2" scope to be. 

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faulksy

faulksy

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i made one last night thanks to shane, i just cut a 140mm disc out of the flimsy 00 tube covers dead easy with those covers, dont no what my f/l is now but hopefully try it tonight, it was to windy last night. thanks shane

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ronin

ronin

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On a refractor a lot of the CA and SA comes from the edge region of the lens, a spherical surface is not the ideal profile and the edges are the "most wrong" bits. So you have removed the most wrong bit of the lens and that also means less refraction so again less CA.

Likely similar on the reflector, a parabola is again not the idea shape for a mirror, so the egde cause contribute greater aberrations in the final images (less sharp) and the mask effectively prevents the edge oif the mirror being used.

In easy terms the Hubble is not a parabolic mirror, they are just not good enough, neither will the James Webb be.

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John

John

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On a refractor a lot of the CA and SA comes from the edge region of the lens, a spherical surface is not the ideal profile and the edges are the "most wrong" bits. So you have removed the most wrong bit of the lens and that also means less refraction so again less CA.

I think this is correct for SA (spherical aberration) but I believe the bulk of CA comes from the refractive properties of the glass types used in the objective lens elements. Poor figuring can make it seem a little worse but even a perfectly figured achromat objective would show CA commensurate with it's focal ratio.

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AlexB67

AlexB67

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I tried the smaller hole in the cap of my 10 inch a couple of times some time ago on the moon Apart from darkening the image, I only found a loss in resolution the biggest thing affecting views, small lunar features just vanished from the views, not that it was unpleasant to look at, but a scope my size I am not intending to do it with  10 inch aprture.  With a bigger hole even  a drop from say 10 to 4 inches is a significant drop in resolution and don't think makes up for the energy loss and diffraction issues that come with an obstructed scope, unless seeing is totally  limiting the views anyway.  I have yet to be convinced in my scope, If I had a bigger Dob different story perhaps.

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faulksy

faulksy

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what a interesting find, first of was jupiter, tried with a few eps. 14mm delos and 6mm delos then tried 5 and 6mm bgo,s. first test was without the mask, jupiter was very bright could see the main bands and some of the fainter eq belts also a transit of some kind, or maybe a barge didnt record the time. but main note was deffraction spikes, now tried with the mask defraction spikes gone 100% planet looked slightly duller and very slight difference in detail but nothing to shout about.

next up was mars this was around 23.30 same eps without the mask. image was boiling and shimmering away very slight detail just about see syrtis major and the mare ery but that was it, found the 5mm bgo a bit much.

then tried it with the stopped down mask, i rotated the mask until i knew the spider wasnt in the way and bang lovely detail, massive difference in every way polar regions were visible and lots of dark features everywere tried all the eps and found the 5 and 6mm bgos were just about handling the conditions which wernt great.

so a result for the moonshane mask massive difference on mars hope to try it on saturn in a bit. so i will report back again if i manage the ringed planet

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TwoPi

TwoPi

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You've got to laugh. I've had my 200P for over a year now and I've only just worked out from reading this thread that the main tube cap has a removable smaller cap built in. What an idiot! :tongue:

I always wondered what the two raised discs were for - I use them to put the finder scope and focuser caps in so they don't get lost in the dark.

I wonder how many other 'secret' features my scope has? :rolleyes2:

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