Jump to content
  • Join the online East Midlands astronomy club today!

    With active forums, two dark sites and a knowledgeable membership, East Midlands Stargazers has something for everyone.

F ratios.


Daz Type-R
 Share

Recommended Posts

So I'm with the family in the middle of nowhere and we are all sat watching tv, which is boring, so I started pondering the below question / statement.

For visual use only, how important are high F ratios for planetery viewing?

My scope is an F6 (1200\200) so if I was to put the dust cap over the end of my scope and remove the small cap (I'm assuming the hole size is 50mm) that would give me a F24 (1200\50).

So for VISUAL use only, would I notice any difference?

God I'm bored!

Link to comment
Share on other sites

Interesting question!

Using the same eye piece the magnification would be the same (focal length of scope / focal length of eye piece), but presumably the brightness would be much less. I can't see any advantage of doing this with the planets.



Jd

Link to comment
Share on other sites

There would be advantages yes. In general, the higher the ratio the better the contrast. For planets with good detail such as Jupiter or Saturn (the rings counting as detail) you will get a much better view as the contrast will be greatly improved. You will also notice a marked improvement on the moon too. Green filters further enhance this as the eye is far more sensitive to green than most other colours, so this too helps to increase contrast which will bring out more detail.


Link to comment
Share on other sites

Awesome, cheers for the replies guys.

I know high F ratios are preferable for planetery viewing but wondered if this method would work or if you need to get a high F ratio using the full size of the primary and a really long tube.

Next clear night we get I'm going to give it a try.

Link to comment
Share on other sites

Yes, it will be interesting to know if the theory Mike talks about translates into improvements in the visual experience; you should do a blinded test on various people and ask them which view of Jupiter they prefer, number 1 (200mm aperture, f/6), or number two (50mm aperture, f/24).

 

JD
 

Link to comment
Share on other sites

Ok, but it's still unproven that the improvements in contrast of Jupiter which will result by reducing the aperture to 50mm in Daz's kit will be beneficial to his viewing experience.

Jd

Link to comment
Share on other sites

If you want to argue with science and hundreds of years of astronomy feel free :lol: It is very much well proven that increasing focal ratio will help with planetary observations. Doing in this way is no different to buying a f24 50mm scope.


Link to comment
Share on other sites

I'm not arguing the science, i'm questioning if the benefits detected by the human eye of improved contrast (by increasing the f ratio in this scenario) out weigh the draw backs of reducing aperture and presumably resolution by reducing it from 31415mm2, to 1963mm2.

Jd

Link to comment
Share on other sites

Guest ecopley

This is interesting. Given that I can't argue with the science (as has been so accurately pointed out!) may I suggest that the f/24 set up might have better contrast at a given brightness.

What I mean is that looking at, say Jupiter, the contrast in the bands in the f/6 set up would appear greater because it's brighter.

So the bright bands might be, say, 20% brighter (any amount will do, I'm just trying to make a point) than the dark parts. In the f/24 setting they might be, say, 60% brighter but since the dark bands are REALLY dark in the f/24 setting, that doesn't translate into much actual brightness.

You know - 60% of bog all light is still bog all whereas 20% of quite a lot is a big difference.

Make sense?

Link to comment
Share on other sites

It does make sense yes. It may not suit all people either, but the contrast will increase. However as you rightly point out, it will require a bright target in the first place. When viewing the moon though, you hardly notice much difference in the brightness of the light bits, but the dark bits get darker.


Link to comment
Share on other sites

I think about it from a photography (and a low level physics) perspective and also like to relate to the pinhole camera and only normally think of it in relation to depth of field which I don't expect to be such an issue in astronomy since everything is to all intense purposes at infinity in an optical sense.


 


Anyhow, limiting the size of the hole/iris reduces the need to focus the light using a lens as the rays cross more precisely at a pin hole meaning the image is crisper and in focus over a much greater range of depth/distances. The trade off is of course less light getting in hence the need for longer exposure or a more sensitive sensor. In this sense I slightly question the idea that it improves contrast absolutely and I wonder if a compromise is more what is required depending on the brightness of the object being viewed.


 


However it seems well documented that faster scopes are less forgiving in terms of lenses used for visual work so I guess there is some truth in it. Does it come down to clarity and edge sharpness? Resolving power in some regard? (That of course does some what depend on aperture!)


 


Just some thoughts.


 


Steve

Link to comment
Share on other sites

i always thought the small 50mm cap in the front of a newt cover was designed for Moon observations, or very bright objects ideally to cut down the brightness of the Moon or object when viewing, even a half Moon with an 8" mirror or grater can be painful on the eyes.


be interesting to see the difference it does make.


Edited by red dwalf
Link to comment
Share on other sites

Guest ecopley

I think that with very bright objects there is a different effect on the go. If you look at the moon through a large scope with dark adapted eyes there won't be much contrast in anything because your eyes will be... I'm not sure what the technical term is. I think struggling to cope probably covers it but I really want to say something a little stronger.

This might not be quite right but I believe the cells in the retina can detect single photons. NB, that's not the same as seeing single photons, it's lots more complicated than that! So one's visual range is from the lowest level possible to staring at the sun, as newton did - to find it what would happen.

My point is that, contrast for visual observing isn't like contrast on a camera. It depends on too many other factors.

I thought about this more last night and my intuition tells me that observing with a f/6 scope with a 50mm endcap hole would not give an f/24 view but a very dim f/6. Whether the darker image would have more contrast I couldn't say. Again, my intuition tells me that if noone has discovered something so simple in the last 450 years then perhaps don't be to surprised if it doesn't deliver great results. On the other hand, clearly if we're all missing a trick then I'd be fascinated to hear about your experience. My end cap has no hole. Maybe I need to drill one!

Link to comment
Share on other sites

Guest ecopley

See, I told you I rush into things.

I don't see the need for a corrector plate any way. They should just make the mirrors properly in the first place. I mean, really, how hard can it be...

Link to comment
Share on other sites

Well I have been reading this thread with much interest.


The one point that has been missed here is the scope.


The mirror in this 200 newt is an F6.


Surely no matter how you change the aperture it is still an F6.


To Get an F24 you need a mirror configured to F24.


Link to comment
Share on other sites

F ratio is simply focal length divided be apature. Your focal length isn't changing so reducing to 50mm apature will increase the focal ratio.

Corrector plates are the starting point from which your light is being focused, they mean yo get an extra 50% or so focal length without the need to increase the tube length. Plus they hold the secondary in to the end of the ota, you'd be stuffed without it or else have to have a spider and live with diffraction spikes.

Link to comment
Share on other sites

Well I have been reading this thread with much interest.

The one point that has been missed here is the scope.

The mirror in this 200 newt is an F6.

Surely no matter how you change the aperture it is still an F6.

To Get an F24 you need a mirror configured to F24.

In a word, no.

Link to comment
Share on other sites

I tried this with the 200P DOB the other night as one of my experiments / messing about.

Jupiter was less bright but I did feel that the contrast of the bands was slightly improved / less flared out.

Mmmmm...... Perhaps I'll try it again.

Ade

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.