Effective focal ratio of Newtonian Telescopes

[Allinthehead]

In my search for a Ha filter to use with my Epsilon 160 it got me thinking about my scope and whether it really is an F/3.3. It has a 160mm primary mirror and focal length of 530mm giving a traditional focal ratio of F/3.3. FL/Aperture=FR. Can anyone explain to me how the central obstruction fits in this calculation? Secondary obstruction is 63mm or about 40%.

Regards,

Richard.

[vlaiv]

As far as filters are concerned - it is still F/3.3 scope

There are two ways in which you can use "speed" of telescope:

- maximum angle of converging rays at focal plane

- ratio of aperture to focal length

Second one is used in daytime photography and denotes how long you should expose for to get certain level of signal.

Without getting (again) into whole speed thing, I'll present a case where effective F/ratio can be observed:

Say you have F/5 refractor and F/5 newtonian scope that you'll use with the same camera. Focal length of each scope is irrelevant for this, but we could say that refractor is 100mm and newtonian is 150mm.

If you shoot a nebula with these two scopes and the same camera, for same exposure time - pixels covering that will have the same ADU value.

This is the case where effective aperture comes into effect. If you do that with two refractors - one being 100mm and other being 150mm and both F/5 - that will happen.

If you do that with 150mm newtonian with 30% CO and 91% reflectivity on both mirrors, it will behave as ~ F/5.77 100mm refractor and not as F/5 100mm refractor.

For that reason we can say that effective focal ratio of that scope is ~ F/5.77.

However - max angles of converging rays hitting focal plane do not change. They still hit as F/5 beam. Geometry of the system has not changed.

Interference filters work by having many layers of special coatings that has certain thickness and light going thru those layers gets reflected and either reinforced or cancelled out with itself (wave interference). This happens because of QM nature of the light. Thickness of these layers plays crucial part in which wavelengths get reflected or passed.

When light starts coming in at an angle - it no longer travels shortest distance thru the layers - but longer path. From light's point of view - it looks like layer got thicker and this changes wavelength that this particular layer operates on. A bit like in this diagram:

Since geometry does not change, effects of nominal F/ratio onto filter effectiveness remain the same.

[Allinthehead]

That clears it up for me thanks Vlaiv.

[Spongey]

9 hours ago, vlaiv said:

Since geometry does not change, effects of nominal F/ratio onto filter effectiveness remain the same.

While I agree with everything Vlaiv has said, is it not true that scopes with a central obstruction will be affected worse by filter 'aperture stop' effects due to fast f ratios?

As a higher proportion of waves come in at an angle, more light is lost when compared to a refractor of the same speed. I seem to recall reading an article on this some time ago, I think it was documented over on CN. I'll try to find the link.

Cheers

Edit: found the thread

Edited January 31, 2021 by Spongey

[vlaiv]

10 hours ago, Spongey said:

While I agree with everything Vlaiv has said, is it not true that scopes with a central obstruction will be affected worse by filter 'aperture stop' effects due to fast f ratios?

As a higher proportion of waves come in at an angle, more light is lost when compared to a refractor of the same speed. I seem to recall reading an article on this some time ago, I think it was documented over on CN. I'll try to find the link.

Cheers

Edit: found the thread

It depends if central obstruction is large enough and band pass of filter narrow enough.

In most cases, difference will be negligible, but you are right - central obstruction removes "best" rays in terms of angle with respect to filter - ones closes to optical axis.

[andrew s]

Additionally,  you can get narrow band filters designed for fast optics which correct for the band pass shift.

Regards Andrew 

[Allinthehead]

19 hours ago, Spongey said:

While I agree with everything Vlaiv has said, is it not true that scopes with a central obstruction will be affected worse by filter 'aperture stop' effects due to fast f ratios?

As a higher proportion of waves come in at an angle, more light is lost when compared to a refractor of the same speed. I seem to recall reading an article on this some time ago, I think it was documented over on CN. I'll try to find the link.

Cheers

Edit: found the thread

Thanks for that, funnily enough I had already read that thread a few days ago.

[Allinthehead]

7 hours ago, andrew s said:

Additionally,  you can get narrow band filters designed for fast optics which correct for the band pass shift.

Regards Andrew 

Thanks Andrew, I'm aware of the fast filters too. What I don't like about them is the wide band pass and I'm looking for a filter to image when the moon is about. I have a second hand 3nm on route to me and i'll give it a go. If it proves problematic I'll sell it on and go for one of the high speed ones. Nothing to lose, I had a look on Astrobin too and noticed some great images shot at F/3.3 with Astrodon 3nm.