Drawtube Baffle Questions

[John]

I have been trying out some eyepieces recently which, due to their design, require quite a bit (2 cm) of forward focusser travel compared to more conventional eyepieces (eg: plossls). In my ED100 refractor there are 3 baffles fitted into the drawtube and 3 in the main tube. Obviously the 3 in the drawtube move in and out when the focus is adjusted while the 3 in the tube remain static.

What I'm wondering is, with the wide range of travel that a F9 refractor drawtube has, is there a chance that the baffles in it might cut into the light path (vignetting ?) at the forwardmost focussing positions ? - or would the design of the scope and the apature and positioning of the baffles allow for this ?. How close to the edges of the baffles need to be to the light cone to be effective ?.

The eyepieces in question are the Wide Scan III's.

John,

North Somerset

[FLO]

Ah yes, the optical designers little secret

Aperture masks have been used to improve chromatic abberation (false colour) and contrast for centuries. Using the focus tube as an aperture mask is simply another way of achieving the same result. If you precisely measure the eyepiece exit-pupil, you can use that to calculate the scopes real aperture. It is not uncommon for an 80mm to be stopped down to 65-70mm.

Some will be surprised and the subject might prove controversial but, don't let this trouble you, it is a legitimate way to improve an achromat's performance.

[GordonCopestake]

I'm not sure that the ED100 counts as an achromat. There should be virtually zero false colour to start with so this can't be the reason. I replaced the focuser on my ED80 and never looked back.

[FLO]

I'm not sure that the ED100 counts as an achromat.

Agreed,

I was not being specific and cannot comment on whether the ED100 uses any form of aperture mask.

BTW, have been googling and found the following from Ed Tings website:

So this test quickly settled down to a battle between the Stellarvue and the

Pronto. Based on the star test and the splitting of some doubles, two out

of three of us seemed to be leaning towards the Stellarvue, which at first

seemed to be performing miracles at every turn. There was less false color,

and the sky background was blacker. The images were universally dimmer,

however. If you want one of these scopes, go ahead and buy one now before

the waiting list gets to be too long. I'm telling you, it's OK with me. But...

there is a catch.

On deep sky, the Stellarvue was dimmer than its 70 mm competition - I could

consistently see deeper in the Pronto despite it having less aperture. This

was most apparant while viewing the Ring and M13. There were stars I could

see in the Pronto that were invisible in the Stellarvue. Still, the Stellarvue

held up well, and all of us were impressed by its ability to take high power.

So...you should get one. Really. I'm not kidding. This is not a joke.

But, HEY, DIN'CHA HEAR?! THERE'S A CATCH!!!

All right, all right. Here's the catch. Something was bothering me all

night long. I couldn't put my finger on it, but something just felt wrong

in Mudville tonight.

Compared to the Pronto, the Stellarvue has more aperture, a shorter f/ ratio,

and no special dispersion glasses in its optics. Any one of these factors

should make the scope throw up MORE chromatic aberration. Yet, it has

LESS false color. Something wasn't right. Between the apparent lack of

false color, the near perfect star test, the large airy discs, and the dim

images, I began to suspect the scope had been intentionally stopped down.

This isn't exactly the case, but my guess was close.

We took off the dew shield and measured the objective lens -- 80 mm.

But, looking down the tube, I saw the solution to what had been bothering

me. The focuser drawtube, as indicated before, is really long. Even with

TeleVue eyepieces (which take a lot of out-focus travel) the drawtube

protrudes almost halfway up into the OTA and cuts off a chunk of the

incoming light.

People have been using aperture masks to "improve" their optics for hundreds

of years, so this is nothing new. Measuring the exit pupils with eyepieces

of known focal length yields an approximate equivalent clear aperture of

around 67 mm for this sample.

So the Stellarvue is really something like a 67 mm f/7.16 telescope, not an

80 mm f/6. You're losing 31% of the light grasp and trading it off for image

sharpness. And that's the catch. How much of an issue this will be, however,

depends on the individual. Each potential customer will have to form their own

opinion on this design strategy.

http://scopereviews.com/page1k.html#1

[FLO]

PS: We had a similar/related thread a while back:

http://tinyurl.com/gur4q

[John]

This is all very interesting feedback - thanks.

I guess the real way to find out wouldl be to map out the light cone and then measure the apeture of the baffles with the focusser in the maximim inward focus point and see if any cutoff is occurring. Is there any software available to help map the light path ?.

Mind you, on 2nd thoughts I wonder what the point is - I'm not going to get rid of the scope even there is some apeture masking going on - I like it too much !!.

John,

North Somerset

[FLO]

I guess the real way to find out wouldl be to map out the light cone and then measure the apeture of the baffles with the focusser in the maximim inward focus point and see if any cutoff is occurring.

Not necessary,

Simply put your 24mm eyepiece in and carefully measure the exit-pupil (diamater of the circle of light as it exits the eyepiece). It should be 2.67mm (exit-pupil = aperture in mm / magnification, which in your case is 100 / 37.5). If it is smaller, the aperture must be masked.

Mind you, on 2nd thoughts I wonder what the point is - I'm not going to get rid of the scope even there is some apeture masking going on - I like it too much !!.

Couldn't agree more