# Moon Filter: how much to spend?

Gold Member
2020 Award
Here's another question for you experts.
I screwed in my new Baader Hyperion Zoom in a brief clear period and the Moon looked absolutely blistering. I am so pleased with this lens. It is so much better than the kit lenses that came with the scope. (Dobs 200p).
'Everyone' says that a Moon filter will improve the visibility of features and cut out glare. The image is quite blinding in my eyepiece and it obviously needs stopping down a bit, if only for comfort.
Looking at prices, I can see that they go from under a tenner (eBay) to hundreds of quid. The figure of 13% seems to show up in the slightly more expensive ones. The pictures on eBay ads seem to show lower density than that (?).
Can you give me suggestions for the sort of spec I could get away with without breaking the bank? I am pretty fussy about Image Quality in my photography so I would spot a bad one. (That's what impressed me about the Hyperion EP.)

you might want to check this website out! but the problem is most of these specs cost over £100

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Gold Member
2020 Award
you might want to check this website out! but the problem is most of these specs cost over £100
Yes. Well. That's the problem. Andy Resnick put me right about a Solar filter requirement, telling me that a survival blanket would see me ok 'till I graduate to the big boys' league. I was sort of looking for a similar tip about the Moon filter problem. I realise that an old sock will just not do the job!! If a neutral filter will do the job then what about a cut-out screen over the 8" end of the tube? A central hole would upset the resolution but perhaps a ring aperture round the outside would maintain the required width. Nothing wrong with trying an experiment with a biscuit tin, I guess. I can't think, offhand, what the resulting distortion would be. The secondary reflector doesn't do too much harm. (Subjective view with no quantitative knowledge. Something PF is not keen on) Someone has bound to have tried something like that.????

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davenn
Gold Member
Andy Resnick put me right about a Solar filter requirement, telling me that a survival blanket would see me ok 'till I graduate to the big boys' league
well actually I mentioned it as a great plan and posted a pic of one earlier in that thread than he

for the cheapest for the moon, initially just stop down the aperture of the scope, a piece of cardboard etc across the front
and a say 3" diameter hole in it ( assuming your scope has a 5" or 6" main mirror)
good advantage of stopping down the aperture is it will improve the image sharpness and give better resolution .... I also mentioned that in post #23 in that other thread when commenting about using the mylar

piece of cardboard and some tape .... a couple of bucks

Dave

Andy Resnick
Gold Member
2020 Award
well actually I mentioned it as a great plan and posted a pic of one earlier in that thread than he

for the cheapest for the moon, initially just stop down the aperture of the scope, a piece of cardboard etc across the front
and a say 3" diameter hole in it ( assuming your scope has a 5" or 6" main mirror)
good advantage of stopping down the aperture is it will improve the image sharpness and give better resolution .... I also mentioned that in post #23 in that other thread when commenting about using the mylar

piece of cardboard and some tape .... a couple of bucks

Dave
Sorry for non-acknowledgement.
Are you saying that the 200mm mirror would have worse aberrations than a smaller one? That would be due to the accuracy of the mirror grind? Your solution is obviously worth trying. Must start eating Cornflakes again.

davenn
Gold Member
Sorry for non-acknowledgement.
no probs, just send me a slab of beer

Are you saying that the 200mm mirror would have worse aberrations than a smaller one?
it's not an aberration thing, it's an optical thing, lower f-stop ( more open) say f4 will always produce a softer image than a high f-stop, say f10 - 15 which will be sharper. This is particularly noticeable when using camera and lens. I see no reason why it wouldn't be any different for a telescope. It's just a big telephoto lens

Dave

davenn
Gold Member
am looking for some references .....
never had to before ... hahaha ... just one of those things I discovered as have others .....
there is one disadvantage as discussed in at least one of these links ... that is if you push it too far you run into diffraction problems
and that again will produce image softness

http://www.steves-digicams.com/know...how-to-use-aperture-to-get-sharper-shots.html

http://photo.stackexchange.com/ques...her-aperture-less-light-make-an-image-sharper

http://improvephotography.com/686/tips-sharper-photography-sharpness/
Sharpness Tip #3: Determine your sharpest apertures

Most lenses will have a "sweet Spot" the f-stop that produces the sharpest image. It will vary from lens to lens,
make and model

Dave

davenn
Gold Member
Ohhhh and just a though that I didn't mention in this or the other thread .....

The hole you cut in the cardboard ( or whatever you put over the front*) must be offset from the centre ....
cannot be in the centre as the secondary mirror will block the light getting to the main mirror

* ... I found a plastic flower pot base that fitted nicely over the front of my scope and cut an off-centre hole on it

http://www.bing.com/images/search?q=Plant+Pot+Saucers&FORM=IRIBIP

Dave

Gold Member
2020 Award
With a camera lens, I have always understood that f8 is a good 'optimum' (average lens, I guess) that is a large enough aperture for the least worst diffraction and small enough to avoid the aberrations of wide aperture lenses. I was assuming that a mirror could be better than a lens for aberrations but, if not, the f8 might still be the optimum.
I happen to have a purpose made off centre hole in my dust cap and that is around 30mm. Perhaps that's one of the reasons it's there. But f40 seems a very slow f number. 150mm would give me f8.
If what you say is true then it's really the light gathering power of my 200mm mirror that makes it worth having.

Andy Resnick
'Everyone' says that a Moon filter will improve the visibility of features and cut out glare. The image is quite blinding in my eyepiece and it obviously needs stopping down a bit, if only for comfort.
Most lenses will have a "sweet Spot" the f-stop that produces the sharpest image. It will vary from lens to lens,
make and model
There are a few reasons why stopping a lens down +1 or so will increase the *acutance*, I'm not entirely sure that the resolution (defined as say, the cutoff frequency) is increased. It's important to remember that most people refer a high acutance, low resolution image over a low acutance, high resolution image:

http://www.cambridgeincolour.com/tutorials/sharpness.htm

First, lens designers and manufactures define a maximum aperture beyond which the image quality is degenerated beyond acceptable limits due to under-controlled aberrations, thus the maximum aperture coincides with maximum aberrations. Most aberrations can be reduced by decreasing the aperture stop diameter (spherical aberration, coma, field curvature), but some are aperture-independent (distortion, for example). Stopping the lens down will reduce many of the aberrations, and that is often perceived as an increase in sharpness. It's not entirely clear that this is also an increase in resolution, since the resolution is set by both the aperture diameter and aberrations.

Astrophotography has additional degradations caused by the atmosphere. Thermal lensing is common to super-tele work, but glare is often a new one- non-image forming off-axis light scattered through the aperture, 'fogging' the image. Stopping down the aperture will reduce glare, and this will make your blacks more black: hence, the acutance of the image is improved. Reducing the aperture diameter of a telescope will also make you less susceptible to seeing, caused by atmospheric turbulence. To give you an idea, I will stop down my 5.5" refractor by 1/4 stop (5.1") when there is particularly poor seeing/lots of glare and easily notice the improvement.

It seems that the Hyperion eyepiece accepts 2" filters, if you have a correctly sized weak ND filter from a camera lens, try popping it into the Hyperion and seeing if that helps. A polarizing filter may be even better. And assuming that is the aperture plane, try a home-brewed aperture stop there: paint a narrow-ish ring around the edge of a blank/UV filter and see if you notice an improvement.

Gold Member
2020 Award
There are a few reasons why stopping a lens down +1 or so will increase the *acutance*, I'm not entirely sure that the resolution (defined as say, the cutoff frequency) is increased. It's important to remember that most people refer a high acutance, low resolution image over a low acutance, high resolution image:

http://www.cambridgeincolour.com/tutorials/sharpness.htm

First, lens designers and manufactures define a maximum aperture beyond which the image quality is degenerated beyond acceptable limits due to under-controlled aberrations, thus the maximum aperture coincides with maximum aberrations. Most aberrations can be reduced by decreasing the aperture stop diameter (spherical aberration, coma, field curvature), but some are aperture-independent (distortion, for example). Stopping the lens down will reduce many of the aberrations, and that is often perceived as an increase in sharpness. It's not entirely clear that this is also an increase in resolution, since the resolution is set by both the aperture diameter and aberrations.

Astrophotography has additional degradations caused by the atmosphere. Thermal lensing is common to super-tele work, but glare is often a new one- non-image forming off-axis light scattered through the aperture, 'fogging' the image. Stopping down the aperture will reduce glare, and this will make your blacks more black: hence, the acutance of the image is improved. Reducing the aperture diameter of a telescope will also make you less susceptible to seeing, caused by atmospheric turbulence. To give you an idea, I will stop down my 5.5" refractor by 1/4 stop (5.1") when there is particularly poor seeing/lots of glare and easily notice the improvement.

It seems that the Hyperion eyepiece accepts 2" filters, if you have a correctly sized weak ND filter from a camera lens, try popping it into the Hyperion and seeing if that helps. A polarizing filter may be even better. And assuming that is the aperture plane, try a home-brewed aperture stop there: paint a narrow-ish ring around the edge of a blank/UV filter and see if you notice an improvement.
Some interesting points in that; thanks. Things are popping into place about the comments I've read about the suitability of different telescope f's. The Airy disc is not the only thing that counts, it seems. Light gathering is what you need for dim, diffuse objects and acutance is what you need for bright planetary features. I was just thinking in terms of bang per buck and the 8" mirror should be good for finding some of those galaxies on a suitable night and when my navigation is up to it.
I will try some cardboard stops and also that hole in the dust cover.
I will look into getting a suitable photographic ND filter, too. Could well be cheaper for the same thing without without Astronomy written on the side. My only polarising filters are too big, I think.

Chronos
Gold Member
An eyepiece moon filter is available from almost any telescope accessory vendor and typically costs less than $20 US. Even a decent pair of sunglasses will help in a pinch. Science Advisor Gold Member 2020 Award An eyepiece moon filter is available from almost any telescope accessory vendor and typically costs less than$20 US. Even a decent pair of sunglasses will help in a pinch.
Yes, I have seen several. The pictures show them as having different colours but that can also be for other reasons than dimming an already too bright object. Some of the advertised filters seem to claim that they eliminate 'certain' wavelengths without altering the perceived colour or reducing the brightness. What is actually required - apart from just reducing the brightness of the image you see?

russ_watters
Mentor
Yes, I have seen several. The pictures show them as having different colours but that can also be for other reasons than dimming an already too bright object. Some of the advertised filters seem to claim that they eliminate 'certain' wavelengths without altering the perceived colour or reducing the brightness. What is actually required - apart from just reducing the brightness of the image you see?
The only thing actually required is reducing the brightness and other than perhaps a polarizing filter, I wouldn't think any others would do anything useful. Are you sure those other filters are specifically advertised as moon filters?

Anyway, I use this one and like it a lot:
http://www.telescope.com/125-Orion-13-Transmission-Moon-Filter/p/5662.uts?keyword=moon filter

Gold Member
2020 Award
Are you sure those other filters are specifically advertised as moon filters?
I have to admit that they are on eBay and the statements seem to imply that some filters are good for 'anything' Clearly, they are not all to be relied on!!!
But there are a number of filters available , e.g. Nebula Filters. What do they do?

russ_watters
Mentor
Definitely different filters have different purposes, and it would be a rare filter that you would use all the time. The moon filter is the only one I use for visual (sun filter kinda, but I don't do direct observation with it).

For photography, there is one filter I always use: a UV/IR cutoff filter, because it removes extraneous light outside the visual range that can register on a camera and mess with the colors.

I do LRGB imaging, which means I use red, green, blue and a "luminance" filter, which is just a second UV/IR cutoff filter (I use two for for consistency, since the others are always paired with one).

Light Pollution/Sky Glow filters mostly cut out a specific wavelength corresponding to sodium vapor streelights.

Nebula filters cut out everything except a specific frequency or two corresponding to the nebula's color. Common ones are Ha (Hydrogen alpha) and O III.
https://en.wikipedia.org/wiki/H-alpha
https://en.wikipedia.org/wiki/Doubly_ionized_oxygen

davenn