Different focal lengths on the same aperture

[endure]

If you have two Newtonians with the same aperture but different focal lengths what difference does it make to what you see?

[Moonshane]

assuming the same magnification, there will in theory be no difference - although the longer focal length will produce a higher magnification with the same eyepiece than the shorter. aperture rules light gathering and the same aperture = the same light gathering. in my experience a slower focal ratio will, however increase contrast and to some extent sharpness on brighter objects like Jupiter, Mars and Saturn. this is probably due to a smaller secondary n the slower scope.

I have used a number of 6" f5 newts and have a 6" f11 newt which is in my experience far superior on double stars and solar system objects.

[alanjgreen]

You will see different "magnification" levels with the same eyepiece used in them

Example

Telescope1 has a focal length of 650mm - you get x65 magnification using your 10mm eyepiece

Telescope2 has a focal length of 300mm - you get x30 magnification using the same 10mm eyepiece

[endure]

I ask because I've been told by one of the telescope dealers that as a complete beginner I would be better off with a 130/900 spherical rather than a 130/650 parabolic. I don't think this was sales patter as the scope he recommended is the cheaper of the two.

[DirkSteele]

I ask because I've been told by one of the telescope dealers that as a complete beginner I would be better off with a 130/900 spherical rather than a 130/650 parabolic. I don't think this was sales patter as the scope he recommended is the cheaper of the two.

I would disregard that as poor advice. The only "positive" I can think of is that collimation needs to be less precise in a longer focal length instrument and as a beginner who may well struggle in the early stages of learning collimation (as many do) it might be an advantage. Now on to the negatives. Spherical mirrors are really something you do not want as light from point sources are not all focused at the same point (light striking near the centre of the mirror focuses further from the mirror than light that strikes near the edge. What does this mean? You will not be able to achieve precise focus, and lose out on fine detail at the eyepiece. Spherical mirrors are easier to make so they are cheaper, but they often include a "correcting relay lens" or some other marketing speak to correct (that should read mask) the problem. This is typically done by including a barlow type lens in the focuser drawtube. Above f/10, this becomes less of a problem, but you don't find too many Newtonian scopes above f/10.

What you should take away from that is parobolic mirror is definitely preferred. Perhaps another SGL member can think of other reasons to recommend the spherical mirror.

Can you actually post a link of the two scopes in question? My guess would be that both scopes have quite short tubes, which would indicate the spherical mirror has a short focal length something like f/4 and a inbuilt barlow lens that increases the focal length to about f/7.

As for "sales patter" perhaps he cannot sell the spherical mirror scopes and he thinks one could be unloaded.

[endure]

These are URLS from FLO (who ISN'T the dealer I was talking to)

Parabolic http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p.html

Spherical http://www.firstlightoptics.com/reflectors/skywatcher-explorer-130.html

[DirkSteele]

Interesting, so not both short tubes as such, so possibly no correcting lens.

Perhaps giving you some numbers might help if you speak to the dealer again. The wavefront error for a spherical mirror is given approximately as

W = 22.55 * D/F^3

Where

W = peak to valley wavefront error

D = Diameter in inches

F = f-ratio

If we consider the diffraction limited condition (which is actually the bare minimum that should be considered acceptable in a telescope, not a gold standard as many manufacturers would have you believe), W has to be equal to or less than 0.25.

For a 5.1” mirror as this scope has, with a f-ratio of 6.92 with a spherical mirror, W would be 0.34. Quite a bit greater than 0.25. This means that spherical aberration will be the greatest limiter on the scope and its performance, rather than diffraction. Not a good situation.

As a demonstration of that rule of thumb, let us consider the same scope with a f-ratio of 10. W = 0.115. This would satisfy our requirement of better than 0.25.

I would hold fire as I am sure some other SGL members will have different views on this topic and you can aggregate all the opinions and make an informed decision, but my two cents would be to for the parabolic, especially as there is only a £30 difference.

[ronin]

I would suggest the parabolic, if it is magnification you were asking for then I would suggest waiting a bit longer and adding about £100 to the budget and get the 150PL on the EQ3-2.

The 150PL has the longer focal length, is parabolic and that bit bigger. Additionally the mount has greater prospects as you can add dual motors to it. Don't normally like saying spend more but to me that is a better overall option.

I can half understand the recommendation of the 130 over the 130P, it will/should give greater magnification and for objects like Saturn is certainly useful. The concern is that the image may not be as good, so you could end up seeing things a bit bigger but not as good.