Aperture masks really work on Jupiter

[Doc]

Last night I was viewing Jupiter with my dob but the 16" of aperture made it way to bright. I used the aperture mask I made and stepped it down to 110mm of aperture making the dob a F16.6 and wow is all I can say.

The detail was amazing, different bands clearly visible, even saw a few of those little festoons.

Unfortunately no transit or GRS was happening.

Never really looked at Jupiter to much but now pretty hooked on it.

Anyone else use an aperture mask?

[Keiran]

Hi doc,

The other month when I went to my dark site, I was observing jupiter through my 10" dob, not quite 16", but that was bright enough. I cant imagine how blinding that must have been through yours?.

Anyway, I put the cap back on it and removed the little cap from it, creating an apeture mask, and what a difference it make

A big thumbs up from me

Keiran

[Matthew.Blake]

Why not just use a neutral density filter so you don't lose any resolution?

[Doc]

I use a neodydium filter as well but it's still very bright, 16" of aperture is quite alot of light.

[lukebl]

That's interesting. Would that affect the imaging quality too? For instance, if I stopped my 250mm Newt down to 110mm, would I get sharper images?

[Doc]

Not quite sure. What it does is eliminates the diffraction spikes, it also makes the background darker and the image is bolder and alot more detailed.

[lukebl]

Not quite sure. What it does is eliminates the diffraction spikes, it also makes the background darker and the image is bolder and alot more detailed.

Hmm. Have to give that a try. One avi with a mask and one without, for comparison.

[Hypernova]

So would I see an improvement on planets if a place the standar dust cap on my dob and remove the smaller cap covering a 3 inch-ish hole?

[Doc]

Don't know Tom, mine improves because of the glare I get with 400mm of aperture.

[Matthew.Blake]

Surely a smaller aperture reduces your resolution due to diffraction effects though! Moon & Neutral Density Filters - Variable Polarizing Moon Filter looks like just the thing, no loss of resolution but dealing with the glare problem.

[Hypernova]

Using a the apature mask to make a 70mm opening would make my'scope an f17. Even with the reduced apature, would the increase in fo=cal ratio make a difference for the better or would the image end up being blurry and dark?

[Doc]

Full aperture is a better way to go because in the fleeting moments of good seeing full aperture shows a lot more detail and provides a lot brighter image. However the off axis mask does make the views "refractor like" in that the stars become pinpoints and the effects of diffraction are eliminated, although lunar/planetary images are dimmed considerably I still say on those nights of not so good seeing the images appear bolder and more defined.

[themos]

Surely a smaller aperture reduces your resolution due to diffraction effects though!

Dobs already suffer from diffraction effects from the secondary obstruction. I guess as long as the new off-axis aperture is larger than your secondary, you'd be quids in.

[Doc]

I've designed mine with three apertures:

80mm F22.8

110mm F16.6

145mm F12.6

[John]

Surely a smaller aperture reduces your resolution due to diffraction effects though! Moon & Neutral Density Filters - Variable Polarizing Moon Filter looks like just the thing, no loss of resolution but dealing with the glare problem.

Thats my thoughts on this as well. I could stop my 10" /4.8 newtonian down to a 4" F/11.7 but as I've already got a 4" scope I might as well just use that !.

I must have funny eyes but I've never found glare from objects an issue. OK the brightness of Jupiter overpowers some of the contrast when you 1st look at it but I've found that my eye soon adjusts and soon the glare is not noticable, even with my 10" newtonian.

[perrin6]

I get much better planetary & star images using a 12 inch (non-off-axis) mask on my 13.2 inch. There are several reasons:-

1. The Focal ratio is increased which improves images as the mirrors figure doesnt need to be so accurate with long focal ratios to achieve good images.

2. The narrower the aperture the less atmospheric distortion is picked up.

3. The outer part of an astronomical mirror's figure is often less accurate than the inner, as it is more difficult to create. (It is often wise to mask off the outer 5mm as standard practise because an innaccurate figure in the outer part (usually a 'turned-down edge) reduces contrast over the whole image. You can find this out by star-testing your mirror.

I use a 12 inch mask for planets & double stars & the moon, and only the whole aperture for extended objects ie. nebulae & galaxies.

Alan

[earth titan]

I recall an evening I spent using my 6" newt where I had accidently left the main end cap on. I didn't notice because one of the off axis aperture covers had been left off to allow condensation to dissipate from the night before.

I didn't notice for the whole evening, but I thought Jupiter looked clearer. Never gave it another thought after the 'doh!' exclamation as I packed up for bed but this post has made me think.

With the new 12" dob I'm finding Jupiter really bright and lacking detail. Now I've seen your excellent homemade Blue Peter job on your previous post, it looks as though I have another job to do in these here cloudy evenings.

Off I go to rescue the cardboard from the recycling bin!

[Doc]

Original thread with photo's here:

http://stargazerslounge.com/members-equipment-gallery/99274-my-new-multi-aperture-mask.html

[RobH]

Interestingly enough, at Salisbury in August, I got a much better view of Jupiter through my 12 inch lightbridge than the 20 inch Obsession next door.

I'd put this down to being less affected by atmospheric turbulence.

The best views I've had of Jupiter are with my 6 inch TMB refractor though, but I'd say that the quality of the lens is a big factor there.

Cheers

Rob

[Talitha]

I made an aperture mask years ago for the 8"SCT because someone said it would make Jupiter look better. TBH though, the only thing i noticed was that it made Jupiter appear darker and dingy.

My eyes must be just like John's, because the glare from Jupiter doesn't bother me either. Naturally, i need to get used to it, but it only takes a matter of seconds. It's odd too, because i'm fairly light-sensitive.

[Keiran]

I think they really come in to their own on a full moon.

Keiran

[Moonshane]

I did just the same thing although mine was a bit more expensive - it's called a 6" f11 dobsonian the images are far superior on Jupiter than the 12", much more contrast and detail.

inspired by Mick's effort I made an aperture mas for the 12" a while back but never really used it and haven't on Jupiter at all. I'll try and do a side by side before he dips off for the winter (Jupiter not Mick).

[Matthew.Blake]

I did just the same thing although mine was a bit more expensive - it's called a 6" f11 dobsonian the images are far superior on Jupiter than the 12", much more contrast and detail.

inspired by Mick's effort I made an aperture mas for the 12" a while back but never really used it and haven't on Jupiter at all. I'll try and do a side by side before he dips off for the winter (Jupiter not Mick).

Will you do a side by side of aperture mask vs ND filter with a larger aperture or aperture mask vs no mask?

[Moonshane]

sure will - if the sky ever clears!!

possibly not this weekend but next weekend as the next week my daughters are in plays every night!

[Astrokev]

Interesting thread.

In my limited experience of trying masks, I don't think there is a definitive answer on whether a mask will always improve an image, since it depends so much on the state of the atmosphere.

Matthew.Blake - I gather from your posts that you are not yet convinced that a mask can improve an image? Whilst, theoretically, you are correct in saying a larger aperture = more resolution, in practice this doesn't always hold true. As has been said by others, the practical resolution attainable on any given night depends largely on the steadiness of the atmosphere. A larger aperture is more prone to turbulence than a small aperture simply because you're viewing through a wider column of air. Any turbulence in the air column will degrade the image. However, when viewing through a narrow air column (ie. with a smaller aperture) there is more chance of getting "good air", which will therefore produce higher actual resolution. If the air is very turbulent then all apertures are likely to be similarly affected, and a mask will give no improvement.

The bottom line is to "suck it and see". If it improves the image - fantastic! If it doesn't, take it off!