theoretical and usefull max focal length

[torana68]

Hi,
Not the best day out there so Ive been inside thinking again. I want to make an 8" F9 but then I was thinking F10? at what point do I loose out, there is only so much light so there must be a point where I should stop . Where does the point lie? where would a longer focal ratio begin to deliver poorer preformance? (planetry only and ignore length issues for the theoretical max)
Roger

[Rod]

Hi Roger,

I would think about it more in terms of how narrow a field you can tolerate. Theoretical performance itself does not decline at any point - I suppose you could argue performance continually increases as aberrations (mainly off axis coma) become less as the F/ratio goes up.

I tend to think of half a degree as the minimum field width I would want.

Rod.

[torana68]

thanks :) without getting the calculator out whats that in terms of focal length with an 8"?

[Rod]

You calculate the scope's field of view by dividing the eyepiece Apparent Field of View (AFOV) by the magnification the eyepiece has in the system of interest. So for an 8 inch telescope with say a 40mm (1.6 inch) eyepiece and a 50 degree AFOV, an F20 (efl=160 inces) scope would have a field of half a degree with a magnificaton of 100.

To put it another way:

eyepiece AFOV/Magnification = scope FOV

so in this example:

50 (AFOV)/100 (Mag)=0.5 (FOV)

Of course different eyepieces will yield different results but hopefully that explains how to do the calculation.

Hope that answers your question.

Rod

[Moon]

Roger
You ask a good question. I also wonder why we don't see huge focal lengths being used for planetary imaging. For example I was surprised to learn Bird was going for a 16" f/4.
http://www.iceinspace.com.au/forum/showthread.php?t=45124

I guess the physical dimensions required for long focal lengths start to be the limiting factor. Are there any 16" f/10 newts out there?
James

[torana68]

F20 huh :) I like to play but thats a bit long, interesting to think you could go as far as you can focal length wise without a loss of image quality (from F7 for example), Im thinking F10 but for the diference in length is it worthwile stretching to F11 or 12 (wont be going further than that), dont know if anyone has experience of this in the age of the F4 :)
Roger

[Rod]

Hi Roger,

An 8 inch f10 would be a nice instrument - no visible coma, dark field, small diagonal etc. You will easily get a 1 degree field. The only advantage (to me) in going to F12 would be the possibility of leaving the mirror spherical, if you are making it yourself. I would have to do the figures but I think an 8 inch F10 needs parabolizing.

I am fairly sure Roger Davis (from Bintel) once built a long focus newt around F9. You could PM him about what he thought of it.

Rod.

[Wavytone]

Just take a look through a long focal ratio Classical Cassegrain or Maksutov with a small secondary - at least f/12 to f/15 - to see what it's like.

There's not a lot wrong with a long focus Newtonian apart from size, however there is an issue with making the secondary large enough to fill the field of view of a low power eyepiece without too much vignetting.

I made a 6" f/18 folded newtonian in the late 1970's in an era when high power eyepieces were truly awful... it worked pretty much as expected, lovely high power images but there was really only high power...

IMHO with the improvements in eyepieces in the last 30 years, f/15 is a bit of a waste and f/8 is the ideal focal ratio for a Newtonian. Secondary is small enough not to matter, but still capable of low power if you have a 40mm eyepiece.

[torana68]

thanks all, Im thinking of F10/11, doesnt seem like a lot of reason to go further?
Roger

[Rod]

I agree. Unless you are very tall, or like ladders, then I would think F10 (in an 8 inch) is about the maximum I would go for.

Rod.

[astro744]

I built an 8" f9 once and used a 1.25" seondary, (15.6% obstruction). Performance on planets was exceptional and the length of tube was similar to say a 12.5" f6. I think any longer becomes more difficult to mount and operate.

My best view that I recall was a clear disk of Ganymede in the pre dawn hours one very frost morning a long time ago. Normally one can see Ganymede as noticeably bigger but a clear disk is not always noticeable. I think perhaps exceptional seeing rather than an exceptional telescope may have been more the case but I was very happy with the size and length of the telescope.

Surface accuracy is more important than f ratio so keep the length manageable. Longer does mean a smaller diagonal is possible and for super planetary performance aim for a obstruction of around 15% (by diameter). Note the 15-20% range is still very good to excellent on planets.

Perhaps choose a tube diameter and secondary size you want and then work out what f ratio best suits. If high contrast is all you need for planets and on axis galaxies then a smaller than normal secondary is all you need. Just make sure you get at least 100% illumination on-axis and at least a few millimetres off-axis. The 100% illuminated field diameter is normally set to match a full lunar disk as seen in a given focal length telescope. but you could go smaller if planets are all you want to look at. Use NEWT or Mel Bartels secondary sizing program to work out what you need. See http://www.bbastrodesigns.com/diagonal.htm

[Wavytone]

Hmm sounds tempting. I'm tempted to try a 10-12" f/9 dob, after seeing the reincarnation of Mental's 17". Enough aperture to match the 1-night a year when seeing matches it's resolution, and small enough to be convenient.

[torana68]

Ive been playing with NEWT and fired off some emails to see if I can get the secondary I want or whats available and Ill go from there, thanks All :)