F Ratios and Barlows

[slice of heaven]

A query on F ratios.

I know a faster scope works better on DSOs and a slower one works better on planetry.

Whats the diff between an F5 barlowed 2x and an F10 ?


I need to decide on a guide scope before xmas and need to understand the why,what and how before making a choice.


Slice

[atalas]

Slice I think that its a matter of focal length not focal ratio . The fact of the matter is that its easier to make a good long focal length lense or mirror and harder to make a good short focal length mirror or lense.
The focal ratio is determined by dividing the focal length in mm by the aperture in mm. e.g 2000mm(focal length) divided by 200mm(8"aperture) = f/10 as in a C8 sct. So Slice I think that faster ratio is really only a benefit for imaging, collecting more light quicker .

Now if I only understood what I just said !


Louie

[slice of heaven]

I understand what your saying atalas.
Basically the same way I've understood it.

So maybe I've misinterpreted how some people have stated their F ratio on their stats in imaging using barlows. I've also noticed some small reflectors with built in barlows state the F ratio using the focal length doubled because of the barlow.

Or am I being misled by wrongfull descriptions?

I'm actually after a slow F ratio scope but the length might throw a spanner in the works.

Slice

[trufflehunter (Wayne Robinson)]

Slice that's a good question and one I've wondered about myself quite a bit. I really think the "F" ratio is not as important as the quality of the optics.

As Louie said, it is easier to make a good long focal length lens or mirror than a short focal length one due to the greater amount of curvature required in the latter.

Someone please correct me if my reasoning is wrong but here's my simple theory:

Let's take two 8" mirrors both figured to the same accuracy of one-tenth of a wave. One, however, is F10. The other is half that at F5.

Here comes the theory bit: We assemble telescopes from these mirrors using exactly the same care and quality of components, then use an eyepiece in the F10 scope to give a magnification of, say, 200X and another equivalent eyepiece of half the focal length (or the same eyepiece plus a 2X Barlow) in the F5 scope to give the same magnification. We have to effectively 'stretch' the focal length of the F5 scope by a factor of 2 to give the same magnification. This has the unwanted side effect of halving the benefit of the one-tenth wave error.

So, in order to get an F5 scope to perform as well as an F10 at high power, the F5's primary would have to have an accuracy of one-twentieth of a wave. Extremely difficult to do! Makes sense to me, but the maths could very well be wrong here!

This problem is horribly complicated in an SCT. An SCT has FOUR optical surfaces of some considerable complexity in it's primary light path all of which would have to be figured to an extremely high accuracy to give good combined results. Furthermore, the primary in an SCT is about F2 or even less, so if it was figured to one-tenth of a wave it would be MUCH less than that by the time the convex secondary had its way with it and turned it into a F10, even ignoring the influence of the corrector plate. I think this is why it is so difficult to produce a really outstanding SCT.

[atalas]

Hi Wayne, how is ya! Slice if your changing your focal length by puttting a barlow in place ,that would change the focal ratio as well I think.
If you buy a good fast reflector,it will be easier to increase the focal length that what it would be to shorten it.

Louie

[iceman (Mike)]

Quoting the focal ratio in their images is a bit misleading, it just tells you how much light is let in. The focal length determines the magnification.

SCT's (like the 10" LX200) are good for planetary imaging because their native focal length is 2500mm, where my 10" native focal length is 1250mm, half the length. For me to get the same image scale, I need a 2x barlow.

For deep space imaging, the faster scopes are letting in more light, and don't need exposure times as long. Anyway my DSO imaging understanding is limited.

But for planetary and lunar, it's more important to quote the focal length, because that's what determines the magnification, the image scale.

[ausastronomer (John Bambury)]

Hi Wayne,

You have a couple of errors of optical physics here, which are very easily made

Originally posted by trufflehunter
Someone please correct me if my reasoning is wrong but here's my simple theory:

Let's take two 8" mirrors both figured to the same accuracy of one-tenth of a wave. One, however, is F10. The other is half that at F5. To give a magnification of, say, 200X and another equivalent eyepiece of half the focal length (or the same eyepiece plus a 2X Barlow) in the F5 scope to give the same magnification. We have to effectively 'stretch' the focal length of the F5 scope by a factor of 2 to give the same magnification. This has the unwanted side effect of halving the benefit of the one-tenth wave error.

This is not correct, magnification is solely a function of 2 variables and has nothing to do with the F-ratio of the scope. The 2 variables are FL of the objective and the FL of the eyepiece, the scopes aperture and F-ratio have no effect on magnification. You are not stretching the scopes focal length by using a faster scope, merely using a shorter focal length eyepiece.

Some examples:-

500FLS/10FLE = 50x,
1000FLS/20FLE = 50x,
2000FLS/40FLE=50x,

FLS=Focal Length of scope(objective)
FLE=Focal length of eyepiece

All of the above examples could relate to F5 scopes with different apertures hence they have different focal lengths yet all give 50X magnification with the appropriate eyepiece.

Originally posted by trufflehunter
This problem is horribly complicated in an SCT. An SCT has FOUR optical surfaces of some considerable complexity in it's primary light path all of which would have to be figured to an extremely high accuracy to give good combined results. Furthermore, the primary in an SCT is about F2 or even less, so if it was figured to one-tenth of a wave it would be MUCH less than that by the time the convex secondary had its way with it and turned it into a F10, even ignoring the influence of the corrector plate. I think this is why it is so difficult to produce a really outstanding SCT.

With the Schmidt Cassegrain having a Spherical primary it is not as difficult to produce as a parabolic mirror of the same speed, unfortunately as the primary in a SCT gets faster the field curvature gets worse, for this reason schmidt cameras have the film mounted on a curved plate.

Although there are a number of optical surfaces in a SCT they are not difficult to make per se just time consuming to make very well.
Exceptional quality SCT have been built by skilled opticians, Roland Christen from Astrophysics built a 10" as a one off which Carl Zambuto rated as 1 of the best telescopes he has ever looked through. Unfortunately the cost to make it was in excess of $US 12,000 for a 10" scope, which doesn't exactly fit the mass markets targeted by Meade and Celestron.

The downsides of a SCT is the light loss due to the large number of optical surfaces, the effective light loss due to loss of light gathering area caused by the large central obstruction and the loss of planetary contrast caused by the large central obstruction. The advantages of a SCT are the ability to fit a wide range of accessories, its compactness and ease of mounting.

In answer to Slices original question, Louie went very close to the chocolates, namely that its more difficult to make a fast optic to the same high standard as a slow optic. Noting that a 1/10th wave F5 will give the same quality images at high power as a 1/10th wave F10, your just going to need a shorter FL eyepiece which often has less eye-relief and hence less comfort to get the same power.

A couple of other points to consider:-

A fast scope is a lot more taxing on eyepieces and will require better quality eyepieces to work well.

A fast scope with a barlow may not have quite as high light transmission as a slow scope because of the elements in the barlow but this is likely to be undetectable to the eye.

A fast refractor with a barlow will have a "LOT" more chromatic aberration (false colour) than a slow refactor.

A FAST SCOPE IS A LOT MORE PORTABLE AND TRANSPORTABLE

Let us know what design and aperture of scope your thinking about Slice and I can probably be a bit more specific on what F-ratios you should be thinking about.

CS-John B

[atalas]

Well put John , that was easy to understand cleard some things up in my head as well .


Louie

[slice of heaven]

Specifically I want
1) the best optics I can afford.... So it has to be a newt.

2) aperture around 4"

3) focal length no greater than 1200mm

Slice

[atalas]

Slice you can go bigger than 4" newt with the good quality GSO scopes these days . The prices have dropped dramatically .


Louie

[slice of heaven]

Sorry atalas. Bought a 12" gs for the kids. Have a 12" Parks for myself. I'm after a small scope to fit on an 8" Parks that I'm fitting to an Eq6. So I'm after a small sky piercing planetry guidescope. Something I've never had before. You might call that reverse aperture fever. It'd also give me light , portable grab'n'go scope.

Slice

[atalas]

Hey Slice , I just fell off my chair !

Louie

[ausastronomer (John Bambury)]

Slice,

Do you plan on buying the scope entirely, building the scope from scratch or building the scope from commercially available components.

Don't forget, whatever you do with 4" won't match your 8" as a planetary scope, regardless of sky conditions, it will however make a fine guidescope and be more portable as a grab and go scope.

CS-John B

[trufflehunter (Wayne Robinson)]

Slice, are you looking for a 4" F10 mirror? Is such a thing available? It would certainly make a great guidescope.

John, thanks for making me think about the wave error issue between fast and slow mirrors. I realise now that the results would be the same. The wave error remains the wave error no matter how much it is magnified by the eyepiece! I was thinking up a dead end alley there! The problem, of course, is that it's more difficult to make a 'fast' mirror with the same wave error as a 'slow' one.

PS sorry if I gave you the impression that I believed mag was a function of the F ratio! That was not my intention! Obviously my ramblings above were not very clear!

[slice of heaven]

Aus
It doesnt have to match the 8" on planets as long as it can stand up in it's own rights. I've never had a good quality small ap scope and seeing as I need a guidescope I thought, rather than purchase 'just' a guidescope, why not have a dual purpose scope.
As for building/buying the scope I've 5 months to work it out.
But I'm not grinding no mirror.

Truff
Yes people have them but I dont know about the availability in Oz.

To all
The following was my line of thought so feel free to shoot me down and/or pick holes in it.
An F10/12 4" newt I can fit to a collapsible dob mount and take on trips,camping etc.
I'd rather a newt than a refractor for 2 reasons. Firstly I've really only really viewed through newts and have become 'blind' to their faults and drawbacks. Secondly the cost of quality optics for reflectors is cheaper.
As for the focal length, I can get away with a smaller secondary than I would with a faster scope, so less CO and better contrast. It is only 4" so I'd like to get the most out of it.
Fitted to a dob I wouldn't be bending over so much but it'd still be within reach for little people.
Any input would be appreciated

Slice

[mch62 (Mark)]

I hate to throw a spanner in the works but a 4"f10 Newtonian riding on a Newtonian is going to be subject to differential flexure .
As a guide scope you will want to ensure that there is no movement between it and the main scope or tracking errors will result.
This movement can be in the form of slight twisting as the scope moves from one side to the other and slight mirror shift .
Also thermal contractions and expansions of the tube or trusses.
It can be done but will need to be done well to function as guide scope.

If you want a small CO then it will need to be a truss tube as in a solid tube you require about 25mm clearance for thermals to rise up the tube without causing image problems.
This leads to it's own problem in such a small objective of only 100mm as compared to a much larger mirror in a newt.

If you use a 150mm tube it's going mean a CO larger than 20-25% to maintain a reasonable fully illuminated field.
If you use a truss the clearance issue is reduced by design and you will be able to use a smaller secondary.
I put the figures through Newt2.5 and a small CO is hard to get with the smaller the mirror even at f10.

Personally if you want a guide scope come short grab and go with reasonable quality then one of the 80mm ED's or even one of the Synta Orion Saxon Maksutov-Cassegrains ota's.

The later can be subject to mirror flop again for guiding.

I have gone the first route with the 80mmED as it makes a great guide scope , grab and go , wide field imager and 80mm finder scope with 40mmish eyepieces in one package and it's compact.

I have seen 4"f10 mirrors around in the states for under $100us but there getting rare .

Just some things to be aware of , but if you have your hart set on a newt down load a copy of newt2.5 first and play with the figures to make sure it's viable..

Mark

[slice of heaven]

Thanks Mark for bringing those problems to light. Exactly what I asked for. I'll look into all those points and see what unfolds. I'm not committed to anything yet so everything is flexible.

I'm not keen on refractors or maks as a grab'n'go for various reasons. One of the reasons for both is I dont like tripods .

But if it comes to the crunch I'll just grab a suitable refractor for the guide scope and leave it permanently mounted.

Slice

[ausastronomer (John Bambury)]

Slice,

The points Mark raises are all valid. Flexure issues can be easily addressed "IF" the OTA is properly built and the mounting method takes these factors into consideration.

Its a simple fact of physics that you can use a smaller secondary with a truss design (or open tube) than a full tube design. The reasons as Mark points out are the fact that the OTA diameter for a given mirror size needs to be larger with the tube design to allow tube currents to escape.

As Mark mentions, punch the numbers into Newt and see what you come up with. Depending entirely on the needs of the scope you can get the CO under 20% with a tubed design but the Fully Illuminated Field (FIF) will only be about 4mm and the 75% Illuminated Field about 11mm. This makes the scope unsuited to photography not ideal for DSO work but is more than acceptable for specialist planetary, lunar and double star viewing. I have seen several scopes built using similar parameters.

4"/F10 mirrors are not that common but you may find one by placing a wanted add on Astromart or something. Edmund Scientific in the US make some very good 1/10 wave mirrors that would also be suitable, a 4.25"/F8 parabolic and a 4.25" F10.6 spherical ( easily exceeds Rayleigh Criterion so doesn't need to be parabolic). The Edmund mirrors aren't cheap for small mirrors but are high quality.

IMO its all too difficult really when you could buy an excellent scope like the Orion 80mm ED which will do the job well without any issues. Another very good option is the 90mm/F11 Guan Sheng refractor. This is only about $250 (Andrews) and is a very good scope for the money.

CS-John B

[slice of heaven]

Guys
Can you give me a rundown on illuminated and fully illuminated fields.Whats reasonable and whats not. The numbers I punch in seem to only affect these areas.
But if I dont understand it, it means nothing.


Slice

[ausastronomer (John Bambury)]

Slice,

This depends entirely on what you want to do with the scope. Most of the telescope making books and several websites go into the importance of the size of the Illuminated Field. I will assume you don't intend to pursue photography with the scope because it requires a larger FIF and defeats the purpose of the scope because the secondary size becomes too large.

An old rule of thumb for a general purpose telescope is that the size of the FIF in mm should be 50% of the focal length of your longest regular intended for use eyepiece. In other words if you plan to use a 30mm eyepiece as your normal low power eyepiece you should have a FIF of 15mm. If your longest focal length eyepiece normally used will be 25mm then you need a FIF of 12.5mm. These sizes can be varied, if you intend to use the scope solely for planetary use on a tracking mount with short focal length eyepieces then a FIF of 5mm is fine, as it allows you to reduce the size of the secondary to improve contrast while maintaining image brightness on axis. The off axis images however would vignette and you would notice a little dimming of the images with longer focal length eyepieces at the EOF.

Personally, I wouldn't care for a FIF of less than about 10mm. Kreige and Berry recommend a FIF of about 20mm on their larger Obsession style dobs.

Cs-John B