My head hurts!

[Dendroica]

I think I’ve got my head around the difference between aFOV and tFOV but I’ve been thinking  ...maybe too much!

I’m probably going to embarrass myself and show my lack of understanding with these  questions … anyway here goes ...

 

The formula for working out tFOV at least superficially only seems to involve parameters concerned with the eyepiece field stop diameter and primary's focal length. To my way of thinking a 400mm mirror would gather light from a wider angular section of sky than an 100mm mirror? So does the aperture not play a role in potential FOV?

 

Secondly, am I correct in thinking (in a Newt) the dimensions of the secondary are governed by:

1) It’s distance from the primary?

2) Is it bigger, smaller or always the same dimension as the cone of light heading for the primary’s focal point?

3) What factors come into play when designing the dimensions and relative position of the secondary?

 

Go easy on me please !   

 

Dave

[ronin]

tFOV seems independant of the mirror size, can half see why but suspect it is not correct.

Also the mirror is to collect light and that light is what is termed collimated, and collimated light is a "little" pecular in what happens.

Easier to accept no it makes no or little difference. No point in losing sleep.

Secondary size is governed by how far from the primary but also how far from the primary governs where the prime image has to be located. A smaller one higher up would not "push" the prime image out far enough for the eyepiece to use it. Also it is sort of safer to have it a bit oversize - easier to get the position right.

The "lower" down it is the bigger it needs to be to deflect all the light from the primary, the cone diameter is bigger as you move towards the primary..

Factors are usually the bigger it is the more it gets in the way and causes more "diffuse" images (less sharp). Also the bigger the secondary then the bigger the bits holding it in place need to be. Takes more to hold a 250gm glass secondary in position then a 150gm glass secondary.

[Stu]

I think the way to consider the aperture 'conundrum' is that basically the light rays are parallel as they are arriving at the mirror so the actual size doesn't affect the fov, just the amount of light gathered.

The FOV depends upon the focal length of the scope and, as you say, the fieldstop of the eyepiece, that is correct.

The secondary size depends upon a number of factors. The focal ratio of the scope is one factor, as a short focal ratio scope needs a larger primary than a long ratio scope. Often long focal ratio newts have very small secondaries (because of the longer lift cone) which give them excellent contrast for planetary and lunar observing.

The secondary can be larger, smaller or exactly the same size as the light cone. Generally though it should be slightly larger I would say so as not to waste light. My 300mm f6 scope has two secondaries which I can swap between. One is 60mm and it fully illuminates 2" eyepieces. The other is 50mm and does vignette on 2" eyepieces but is fine for 1.25" and because it is a smaller obstruction it gives better contrast for planets.

The other factors relating to position will be to do with where you want the focal point to be, ie how far out from the OTA. This requirement will be related to whether, for instance you are doing imaging or want to use a Binoviewer.

Hope that's of some use

Stu

[Dendroica]

tFOV seems independant of the mirror size, can half see why but suspect it is not correct.

Also the mirror is to collect light and that light is what is termed collimated, and collimated light is a "little" pecular in what happens.

Easier to accept no it makes no or little difference. No point in losing sleep.

Secondary size is governed by how far from the primary but also how far from the primary governs where the prime image has to be located. A smaller one higher up would not "push" the prime image out far enough for the eyepiece to use it. Also it is sort of safer to have it a bit oversize - easier to get the position right.

The "lower" down it is the bigger it needs to be to deflect all the light from the primary, the cone diameter is bigger as you move towards the primary..

Factors are usually the bigger it is the more it gets in the way and causes more "diffuse" images (less sharp). Also the bigger the secondary then the bigger the bits holding it in place need to be. Takes more to hold a 250gm glass secondary in position then a 150gm glass secondary.

Many thanks Ronin. I hadn't thought the higher up the tube the smaller the shorter the cone of light would be and so too short to reach the eyepiece.

[Dendroica]

I think the way to consider the aperture 'conundrum' is that basically the light rays are parallel as they are arriving at the mirror so the actual size doesn't affect the fov, just the amount of light gathered.

The FOV depends upon the focal length of the scope and, as you say, the fieldstop of the eyepiece, that is correct.

The secondary size depends upon a number of factors. The focal ratio of the scope is one factor, as a short focal ratio scope needs a larger primary than a long ratio scope. Often long focal ratio newts have very small secondaries (because of the longer lift cone) which give them excellent contrast for planetary and lunar observing.

The secondary can be larger, smaller or exactly the same size as the light cone. Generally though it should be slightly larger I would say so as not to waste light. My 300mm f6 scope has two secondaries which I can swap between. One is 60mm and it fully illuminates 2" eyepieces. The other is 50mm and does vignette on 2" eyepieces but is fine for 1.25" and because it is a smaller obstruction it gives better contrast for planets.

The other factors relating to position will be to do with where you want the focal point to be, ie how far out from the OTA. This requirement will be related to whether, for instance you are doing imaging or want to use a Binoviewer.

Hope that's of some use

Stu

Thanks Stu. I'm beginning to get it but ...

Why does a longer focal length/smaller secondary give better contrast? Thanks for any answer ...

[YKSE]

Why does a longer focal length/smaller secondary give better contrast? Thanks for any answer ...

These links give good explanations, kind of heavy reading though.

Focal length:

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/forum/AA_error_article.pdf

Secondary size(Central Obstruction)

http://www.astrophoto.fr/obstruction.html

[Stu]

Great links YKSE, says it all