Newtonian/Dob f/ratio real?

[JD]

I haven't found anything which explains this so perhaps someone here knows.

I used to work in the film industry and I understand t stops. The difference between t stops and f stop is that f stops are theoretical and t stops (transmission stops) are the actual effective ratio. For example, a 50mm f/1.4 will probably be a t 1.5 or 1.6.

So, if a 250mm Dob has a focal length of 1250mm, it has an f ratio of 5. That's simple. But in terms of transmitted light, when compared with a refractor, it couldn't transmit the same amount of light as a refractor of the same diameter.

Yeah, I know a 250mm refractor...dream on. But seriously, the obstruction created by the secondary mirror must reduce the amount of transmitted light. The mirror on a 250mm Dob is about 50,000 square mm but if the light gathering tube has an obstruction of say 5,000 square mm in the middle of it then surely the instrument is less than ideally effective by about 10%.

So is a 254mm f/5 scope really a t 5.55?

Also, how do designers calculate the necessary size for the secondary mirror?

This is not meant to start a flame war about reflectors vs refractors. Of course, the same applies to a cassegrain too. I'm just curious.

[glend (Glen)]

I suggest you have a play with the Newt design app that is freely available on the Net, it does all the calculations for given mirror size, including recommended secondary. Visual newts can get away with a smaller secondary than an imaging newt. The difference lies in the need to fully illuminate a camera sensor as opposed to just an EP and your pupil in a visual scope. So newts do suffer a bit in terms of contrast compared to a similiar sized refractor but the cost of newts is significantly less than refractors. A good newt will be tube baffled, andvthat helps with contrast. Of course newts offer true colour rendition, something that comes at significant cost in refractors.
T-stops really don't get mentioned much in astronomy.

[JD]

What I guess I'm saying is that a newt could not let in the same amount of light as the same size refractor because of the obstruction in the optical path. I was just going though my reasoning.

[raymo]

Since most commercially available scopes larger than 6"diameter are centrally obstructed, they all lose a similar percentage of light, so are not competing with each other for light grasp; hence the fact
that an 8" f/5 Newt loses about 6.25% is not only unimportant, but
visually unnoticeable. Also, the resolution of the scope is unaffected by
the obstruction, as it is purely a function of aperture.
You are obviously right about light grasp reflector versus refractor of same size, but this is usually not an issue because the two scope types
are mostly used for different purposes, so don't usually compete directly for the buyer's dollars.

[JD]

Hey, I'm not worried about it. I'm actually in the market for a 250mm Dob as we speak.

[Atmos (Colin)]

Your average 10" dob is likely to have something along the lines of a 20% obstruction which equates about 50mm which is only a 3.8% amount of light loss or instead of being a 254mm F/5 it is now a 249mm F/5.1. This tiny different fits in the realm of the focal length differences that come from changing some eye pieces

[Kunama]

The F ratio given for a reflector is the focal ratio of the primary mirror not the T ratio of the completed telescope.
As the OP stated, the t ratio will always calculate to 'longer' than the f ratio in an obstructed system.
The obstruction is often expressed in terms of a % of the aperture diameter when in reality it should be a % of the area of the aperture.

Secondary size depends on how far from the primary one wants the secondary to be. To minimize the size of the secondary it is placed as far as possible from the primary while still allowing for the full clear aperture and an allowance for tube currents, focuser height etc.
eg. A 10" F5 will need a minimum UTA diameter of 12" and depending on the focuser used, the focus point will be about 9" from the optical axis of the primary, so the centre point of the secondary will be 41" above the surface of the primary. You can then calculate the minor axis of the secondary.
That's my guess anyways....

[JD]

That's pretty much it. Interesting you mention the "UTA" diameter (I presume you meant OTA) because I hadn't thought about that. Makes good sense.

This was purely an exercise for its own sake and is not part of my selection process for my next scope. It was a case of being theoretical for its own sake. I've had a hankering for a biggish Dob for a while and pretty soon I'm going to get me one.

[Wavytone]

OP, in astronomy the usual measure has been f-stops - focal length/aperture - because there are other variables that may be variable or in-measured:

- transmission losses due to poor antireflective coatings - or none at all, which may be the case on old refractors as well as amateur-made stuff;

- light loss due to the secondary obstruction (newtonians, cassegrains, SCTs and Maks)

- light loss due to reflective surfaces - even new the average mirror coating loses 10% and an old one could lose 40% or more before it is recoated;

- all the additional surfaces before the light reaches your eye - such as star diagonals and eyepieces.

This is one of the reasons why refractors in the range 80-130mm remain popular despite their smallish aperture, you have to step up to a 200-250mm reflector to do significantly better.

And yes there really are big newtonians as fast as f/3.5. They aren't without issues however - field curvature and coma are severe. But for those with aperture fever and don't want high magnification, this is an acceptable compromise.

[Kunama]

Originally Posted by JD

That's pretty much it. Interesting you mention the "UTA" diameter (I presume you meant OTA) because I hadn't thought about that. Makes good sense.

This was purely an exercise for its own sake and is not part of my selection process for my next scope. It was a case of being theoretical for its own sake. I've had a hankering for a biggish Dob for a while and pretty soon I'm going to get me one.

UTA ..... Upper tube assembly also known as secondary cage.....

[Wavytone]

JD,

Ultimately with newtonians most are not concened about moderate light losses - the eye is a logarithmic sensor, not linear, and losses of 20%-30% are barely detectable visually.

The main problems created by secondary mirrors are:

A) the degradation of image quality caused by a large secondary; below 20% of the aperture of the primary mirror it's not much of a problem but when it reaches 30-35% - as on fast dobsonians - it really does cause a noticeable loss of resolution.

B) the supporting vanes, if they are straight, this causes visible diffraction spikes around every bright star or planet.

C) contrast - more the lack of it, in newtonians that are poorly baffled and allow stray light to reach the eyepiece.

For these reasons a smaller refractor can often match the resolution of a much larger reflector, as well providing superior contrast.

While a 250mm dobsonian will give a brighter image at low power on deep sky objects such as galaxies, a 150mm refractor will provide better contrast and easily match it for resolution on the planets and bright objects.