F12, 4" newt Build.

[bluemaxroe]

Well, I thought I would revive this project as it feels like I haven't worked on this for ages. In fact what happened was that I tested the scope out on Jupiter, and the image was great, nice and crisp. But....I had 3 jupiters, all in a line, like looking through a piece of double glazing at the moon.

This was before the blacking of the tube!

What I deduced along with my friend "valefor" was that the weird little prism secondary was giving me some reflection. Well that's was the only suspect thing in the image train. So I looked around and ended up buying a new tiny secondary from GSO for 20 quid (1/12pv)

Haven't fitted it yet as I needed to make a new spider and secondary holder.

So here is the body of the new holder (below) the mirror part is only 28mm across on the minor axis.

I also found the scope wouldn't come to focus on the dslr sensor. So I will be moving the primary up the tube about an inch too.

That's it for now

[Ags]

4" F12 is an unusual combination... What led you to that choice?

[bluemaxroe]

Well, ro be honest. Its a mirror off ebay. And I have an ed80 so thought I would go for something with more focal length.

On top of that I have an 8" mirror with the same FL to be built after this one. So its kind of a practise run. apart from the 8" will be a surrier truss newt to go on my eq6.

Its a bit of fun really.

[mapstar]

Good to see you back on with it and a nice bit of fault finding there. The new secondary holder looks very well made

Damian

[bluemaxroe]

Points for anyone who can guess what diy household item it was cut from...

I will be impressed if you can get it!

[JamesF]

Very nice, and a little bit different

Can't wait to see the wooden mirror...  Obviously the glass one is just for testing 

James

[Ajohn]

Cotton reel maybe?

John

-

[Soupy]

Points for anyone who can guess what diy household item it was cut from...

I will be impressed if you can get it!

Looks like a curtain pole, or perhaps a chair/table leg!

[bluemaxroe]

Close....it was a stair bannister post. A spare that I had left over....turned out very useful.

[Ags]

Can't help feeling this build is backwards... Shouldn't it be 12" F4? :-)

[Ajohn]

No 4in F12. That will be more or less totally free of coma. I have a 4in F8 kicking around some where with a spherical mirror. They can give surprisingly good views. The only problem really is that the size of the 2ndry limits the field of view. Make that small enough and this sort of scope could compare well with an apo refractor- except for field of view. Make it bigger and the contrast drops - biggest problem with SCT's where it gets up close to 33% or more. Mac's too.

One other mistake I made on my wooden tube was making it hexagonal. Octagonal is a better option when bits and pieces get fitted.

John

-

[bluemaxroe]

Thanks for the insight Ajohn. I like the idea of the F12 4 and 8" reflector. I don't see the long focal length as a problem. I thought I would build it for planetary and for fun.

Not to say I wouldn't mind a 8 or 10 inch F4, that would be amazing too.

I plan to finish this soon so I can get Jupiter and Saturn.

So I will post photos when I get there.

[bluemaxroe]

By the way, and I am not an expert but the secondary is a set size surely? If you make it to big you won't gain anything in terms of a wider field of view. That is determined by the aperture and the focal length right?

I could be wrong.

[Ags]

Bluemaxroe you are mostly right... The field of view is determined by the focal length, but the degree of illumination across the field of view is determined by the size of the secondary - the smaller it is the dimmer the view gets at the edges. This is actually worse at slower focal ratios, so in extreme cases illumination could be very minimal at the edges in a slow scope with a small secondary.

[Ajohn]

 True about dimness after a fashion but imagine the image of an off axis star - if it don't hit the mirror you wont see it.  At the mirror the light from a star is a cone so if only half of it hits the mirror approx 50%  of the light will be lost. Some see that as acceptable and it will happen anyway however big the mirror is.  A star further off axis might loose 66% of it's light etc. There are 2 aspects to the 2ndry mirror. Field coverage and light loss over the field. What ever size of 2ndry is used there will be light loss at the edge of the field it "covers". It's simple trigonometry to work out how big a mirror needs to be to give 100% transmission over a certain field size. One other aspect is when 1 1/4 eyepieces are being used. Not much point having a field over that size. As the eyepiece focal length gets shorter or more correctly it's field stop gets smaller the same applies. 

Texereau uses another method of determining it's size - a limit on radial fanning of stars due to coma. 100um if I remember correctly. He didn't anticipate amateurs using coma correctors but even these leave residues. He also points out that on longer focal length mirrors the rule can result in a very large 2ndry and that this has it's own problems though he doesn't mention what they are. His suggestion in cases where a large 2ndry could be used on his basis for say photography is to have 2 different sizes. 

John

-

[Ags]

You mean vignetting on the primary mirror by the tube itself? I had forgotten to take that into account. I suppose the unvignetted field would be given in degrees by d * 2 / tl * 57.3, where d is the gap between the mirror and the tube, tl is the tube length.

[Ajohn]

No Agnes, vignetting caused by the 2ndry mirror how ever big or small it is but true the tube can vignette the main mirror. That is just going to relate to the required view angle and the tube length from the edge of the mirror. 

TeleVue go through the eyepiece aspects but few other manufacturers if any give field stop diameters

http://www.televue.com/engine/TV3b_page.asp?id=79#.VVWsa-c0YoE

The scope focal length sets what view angle can fit in the field stop as indicated there. Books give other formulae for field size, usually approximations. Wider eyepiece viewing angles allow shorter focal lengths to be used. That allows wider fields to be covered with smaller exit pupils that can still pass through our eyes pupil.  Personally I only use them when they are needed to do exactly that. As I see it if an ordinary eyepiece gives an acceptable exit pupil I see no point in using a wide field type eg twice the angle and 1/2 the focal length. This attitude might not be good for expensive eyepiece sales.

John

-

[Ags]

Ajohn, I'm not following you. It seems I am either missing your point or you are saying the same thing I am.

[Ajohn]

When I mentioned an image of a star not hitting the mirror as quoted below I should have said the secondary mirror.

 True about dimness after a fashion but imagine the image of an off axis star - if it don't hit the mirror you wont see it.  At the mirror the light from a star is a cone so if only half of it hits the mirror approx 50%  of the light will be lost. Some see that as acceptable and it will happen anyway however big the mirror is.  A star further off axis might loose 66% of it's light etc. There are 2 aspects to the 2ndry mirror. Field coverage and light loss over the field. What ever size of 2ndry is used there will be light loss at the edge of the field it "covers". It's simple trigonometry to work out how big a mirror needs to be to give 100% transmission over a certain field size. One other aspect is when 1 1/4 eyepieces are being used. Not much point having a field over that size. As the eyepiece focal length gets shorter or more correctly it's field stop gets smaller the same applies. 

Texereau uses another method of determining it's size - a limit on radial fanning of stars due to coma. 100um if I remember correctly. He didn't anticipate amateurs using coma correctors but even these leave residues. He also points out that on longer focal length mirrors the rule can result in a very large 2ndry and that this has it's own problems though he doesn't mention what they are. His suggestion in cases where a large 2ndry could be used on his basis for say photography is to have 2 different sizes. 

John

-

Any form of secondary mirror has the same characteristic no matter how small or large it is. At some field angle it will only catch part of the cone of light heading for the image plain. An image will still form but will be dimmer because of the vignetting effect the 2ndry mirror has had.

Please excuse my use of don't rather than doesn't. Done on purpose actually.

John

-

[bluemaxroe]

Abs and Ajohn.

Your knowledge is excellent, and thanks for helping me understand better. I don't think I will understand entirely the causes of vignetting, dimming etc. but I am a beginner and am learning as I go.

The biggest thing I want from the build of the scope is a sense of accomplishment, then perhaps on the next build I can strive for excellence as it will be bigger, more expensive and hopefully better !?

[Soupy]

Just remember bigger and more expensive is not always better!

[Ags]

Regarding the secondary, the important number is the distance from the center of the secondary to the focal plane. For a 4" scope that distance might be 50mm for the primary mirror radius, 25mm gap to the tube wall, 10mm for the tube wall, and 25mm for the focusser - 110mm in all. Divide this by the focal ratio and you get the minimum size for the secondary - 110/12=9mm. This is just big enough to fully illuminate the middle of the focal plane, with extreme vignetting around it. To fully illuminate a 20mm field stop, you would need to add ~20mm to the diameter of the secondary.

To put it differently, a small newtonian is disadvantaged because it needs a larger secondary relative to the aperture. Adding 25mm for good field illumination to a 250mm newt with a minimum secondary size of 37mm is not too painful, but doing the same for a 4” newt pushes the secondary from a minimum of 9% to a whopping 34%.

[Ajohn]

It's better to think in terms of the angular field size rather than just say I want a field size of X mm. Then look at the implications of the size of secondary needed which is pretty simple really. 25% by diameter is generally considered ok but 20% is better. Some people who want a planetary scope might try to get smaller than that. 30% etc is more likely to be considered as satisfactory for photography but when it gets to this sort of size there are most definitely contrast penalties. Contrast in real terms relates to resolution.

Focal length sets the image scale. Diameter the degree of light level amplification but on newtonians the off axis aberrations climb rapidly as the diameter goes up for the same focal length. They go down if the focal length of the scope is increased so Texereau starts on that basis by setting just how big the aberrations can be. Pretty sensible really. Borrowing an image the effect is shown here

 

This shows 2 scopes same diameter one has twice the focal length of the other. The aberration level of the longer focal length is 1/4 the size of the shorter one. It's a square law which is why the aberration climbs so rapidly.

There are all sorts of approximations about for determining the field angle that a secondary covers. The borrowed image shows how they work. The dashed horizontal line shows an axial light ray. The vertical one the focal plane. Forget the aberration and assume some angle of incidence as shown in the diagram, the point where the reflected angle hits the focal plane gives the actual dimension this represents on the focal plain. It's easy to sketch this out add the 2ndry mirror and then calculate sizes via say a spread sheet so that the numbers are easy to change to see what happens. A popular angular field size might be sufficient to cover the moon, slightly over 1/2 degree total. If some one is limited to a certain ccd size there is no point really in exceeding what that represents in angular field size. The same idea and basic trigonometry can be used to work it all out including the diameter of the cone of light where the 2ndry has to be placed. For 100% illumination that mirror needs to catch all of it. Past that point it will slowly vignette and eventually not capture any light at all as the off axis angle increases.

Forgetting the aberration might seem strange but another borrowed image can show why

A lens but that doesn't matter coma is coma. This shows the light distribution in it. The blob in the right place is similar to the diffractions spot size of the telescope = small. The light in the wrong place fades slowly so any vignetting will make it fade more quickly. It's pretty small in regions of interest anyway - eg Texereau sets a limit of 0.1mm and then considers the other factor - % secondary diameter of the mirror. People tend to rabbit on about % area which is misleading really as it only accounts for the light loss it causes and not the effect on resolution and contrast.

Compound telescopes, cassegrain types, macs, sct etc sound bad in terms of the size of the secondary mirror often up and over 30% but telescopes are a compromise and as these have more active optical components in them they can reduce things like off axis coma. A mac for instance can be designed to have none at all. All it means in practice is that a smaller scope with a smaller or no obstruction can match them for resolution. This essentially also means higher contrast for the same aperture. In my case I have actually seen this visually comparing a good 8in F10 SCT with a good 5in F9 apo. I'm pretty sure the 5in apo wouldn't compete with a 10in sct resolution wise. It's relatively easy to come up with a good quality F9 5in apo using the more exotic glasses. As with all scopes shorter focal lengths makes things get harder and even more so as the diameter goes up.

John

-

[Ajohn]

As I'm bored and curious I just laid out a 4in F12 newtonian. As I expected it's not far of perfect for a 4in scope over a field angle sufficient to cover the moon which at the size given needs a 22mm diagonal but 25mm is ok. This is the result

It includes the effects of the 2ndry mirror. The mtf plot would be more or less a straight line without any sag in it if that wasn't there. This is a plot of contrast against resolution. 

If I increase the diameter to 8in so it's then F6 this happens

The black circles in the spot diagrams are the size of the diffraction spot so set just what can be resolved but the contrast will be very low at that level. So if looking at Uranus, careful choice of words, the 8in can F6 can clearly do better than the 4in as the field angle needed is tiny and newtonians can be perfect on axis. Over wider fields things aren't so simple as the plots show. The blue line in the 2nd mtf is the perfect one for comparison. The software throws that in if when the plots exceed Rayleighs limit. The plots represent perfect optics - worth bearing in mind too.

John

-

[bluemaxroe]

Well the secondary I have purchased is the smallest gso do at 28mm minor axis....and 1/12 pv.