23 replies to this topic

[steve t]

Hi,

I ve been trying to find out what the total central obstruction is for the Questar 3.5? So far on the net I've read everything from 25% to 35%. I was wondering if anyone had actually measured it?

Thanks

Steve T 

Edited by steve t, 14 June 2019 - 09:30 AM.

[JamesMStephens]

Primary diameter, 89mm, Center Obscuration, 27.9mm.  So 31.3 %.

 

http://www.company7....questandard.pdf

[Loren Gibson]

I measured the diameter of the disc on the outside surface of the corrector as being 29 mm on my Questar Standard. That implies a 33% obstruction. I gingerly used a caliper to measure it without touching the disk or corrector.

 

I don't have access to my Q right now, but I intend to remeasure this to confirm, when I get a chance.

 

Loren

[steve t]

James and Loren 

Thanks much for the information. 

Regards 

Steve T 

[Loren Gibson]

I just checked my previous measurement: Holding the outside jaws of the caliper over the disk and estimating their position as if they were "gripping" the disk to measure its diameter, I estimate the obstruction to be 28.8 mm, give or take a little.

 

Loren

[agmoonsolns]

Remember, the secondary size is different in Questars made before 1971 due to design differences.

[Kevin Barker]

I just placed a vernier above my 1989 Q field/Duplex scope # 40## odd. I used Loren's method. It is an estimate as I do not want the vernier to scratch the corrector of baffle disc. I get 27.9 mm. I held my head a reasonable distance above vernier to reduce parallax error. And repeated a few times to get the same result.

 

The aperture seems to be around the 89.0 mm mark using the same method. So central obstruction is 27.9/89.0=  31.3%

 

If I put 29 mm on my vernier I can see their is a gap and the disc appears smaller, at 27.9 it appears to hold the disc perfectly.

[steve t]

Again thanks much.

Steve T 

[Kevin Barker]

I would be interested in the baffle diameter of the Q7 classic and Astro.

33 %. ??

[Gregory Gross]

While holding a ruler over (but not in contact with) the corrector, I eyeballed a central obstruction measurement of 31mm on my 1962 Questar, which has the painted-over secondary spot rather than the affixed disk of later models.

[steve t]

Hi Gregory,

Is the secondary spot on your 1962 Q on the inside (primary side) or outside (sky side) of the corrector?

 

Your measurement makes sense since I found a figure, on the Q sight, that shows a MTF for a 34% CO.

 

Please correct me if I'm wrong, but in the over all performance, would a couple of percent difference in CO be noticeable for visual observing? 

 

Steve T (hopefully a future Q owner )

[drprovi57]

on my 2007 Q7 Astro I measured 53mm obstruction with a precise caliper (note that secondary diameter is 46mm) - Questar uses a slightly larger front obstruction to provide cleaner optical image.

 

So for my Q7 the obstruction is 100x53/178 = 30% CO

 

Jason

[Gregory Gross]

Hi Gregory,

Is the secondary spot on your 1962 Q on the inside (primary side) or outside (sky side) of the corrector?

 

Your measurement makes sense since I found a figure, on the Q sight, that shows a MTF for a 34% CO.

 

Please correct me if I'm wrong, but in the over all performance, would a couple of percent difference in CO be noticeable for visual observing? 

 

Steve T (hopefully a future Q owner )

It's on the outside of the corrector. I believe (and I could be wrong -- others: please correct me if I'm wrong) that all Questars have their secondary spot silvered on the outside surface of the corrector, and that spotting is then covered over with either paint (older models like my '62 Q) or a disk (newer models -- I wonder: when did this change happen?).

 

I can't imagine that a difference of a few percentage points would make much difference for visual observing. I would instead think that the difference in optical quality between a Questar in good shape and another Mak of lesser optical quality would have far more of an effect on one's visual experience.

[JamesMStephens]

It's on the outside of the corrector. I believe (and I could be wrong -- others: please correct me if I'm wrong) that all Questars have their secondary spot silvered on the outside surface of the corrector, and that spotting is then covered over with either paint (older models like my '62 Q) or a disk (newer models -- I wonder: when did this change happen?).

 

Not exactly, only the early (R1) Questars had an aluminized spot on the outside surface of the corrector.  In later models (and I think you're right, 1962 is the turning point, though maybe Ben can confirm this) the secondary is an aluminized spot on the inside (R2) surface, with a painted spot on the outer surface acing as a baffle.  After a certain point Questar went to a cemented aluminum disk on the outer surface.

 

Jim

[Kevin Barker]

A friend of mine has an early classic Q7. It has he believes the secondary spot on R1. I recall measuring the painted baffle crudely and getting 33% c.o.
Re a few % difference in c.o. and effects observing. Yes little difference. I think the optical quality is the major factor. Having said that for two scopes of the same quality comparing 33% c.o. to say 27% c.o. then there will be a difference on planetary.

[Optics Patent]

We should be careful not to put too much stock in the percentage of diameter that the obstruction represents. 1/3 of diameter is 1/9 of light gathering and similarly indiscernible on resolution.

[Matt Looby]

We should be careful not to put too much stock in the percentage of diameter that the obstruction represents. 1/3 of diameter is 1/9 of light gathering and similarly indiscernible on resolution.

 

Very true.

There is a difference between  33%, 20% and 0% on effective resolution, namely contrast, but this is beside the point for loving a Questar!

 

Matt

[steve t]

I agree, the Questar is a beautiful piece of engineering and execution. I've wanted one since the first time I saw one back in the early 1970's.

 

Over the years, an empirical rule of thumb I've heard quoted many times is, "with all things being equal, for low contrast object like Jupiter,  an obstructed telescope would perform like an unobstructed telescope per:"

 

clear aperture of obstructed telescope - central obstruction = equivalent unobstructed telescope 

 

Please correct me if I'm wrong, but if I understand it correctly, I can expect the Questar to perform like a perfect 2.5" refractor on low contrast objects and 3.5" scope on high contrast objects like the moon and double stars.

 

Steve T 

[Optics Patent]

Steve, that may be a good rule of thumb as to what to expect but for genrerations Questar seems to have earned the reputation for exceeding expectations.

[Matt Looby]

I agree, the Questar is a beautiful piece of engineering and execution. I've wanted one since the first time I saw one back in the early 1970's.

 

Over the years, an empirical rule of thumb I've heard quoted many times is, "with all things being equal, for low contrast object like Jupiter,  an obstructed telescope would perform like an unobstructed telescope per:"

 

clear aperture of obstructed telescope - central obstruction = equivalent unobstructed telescope 

 

Please correct me if I'm wrong, but if I understand it correctly, I can expect the Questar to perform like a perfect 2.5" refractor on low contrast objects and 3.5" scope on high contrast objects like the moon and double stars.

 

Steve T 

Yes Steve that rule of thumb is about right -

 

I am a Lunar observer the Q  is designed for hard contrast- The Q is a decent planetary scope but the details are softer- 

meaning the Q is producing the same resolution but with less contrast, and under steady moments of seeing  soft images are lovely anyhow.

 

Matt

[steve t]

Steve, that may be a good rule of thumb as to what to expect but for genrerations Questar seems to have earned the reputation for exceeding expectations.

Very true. In my modest experience, with some of the massed produced scopes out there, I believe that the Q doesn't exceed the laws of physics, but is so well executed that it operates at the very limits of physics for a 3.5" scope.  Something that wouldn't be practical for mass produceed scope.

Steve T  

[dougspeterson]

OPTICAL DESIGN ALERT

 

Measuring just the spot diameter on the corrector gives only the minimum. The baffling can intrude further, either the inside corrector baffle of some scopes, whether it flares, etc. Then the primary baffle diameter at the mirror, whether the front edge clips the light cone reducing the CA, or projects deeply into the converging cone blocking more than the secondary obstruction. See Telescope Design, Harrie Ruten, et al for an eye popping expose’.

 

You see all these contributors at the eyepiece where it counts. Measure the pupil with an eye loupe at the eyepiece, both the id and od of the donut.

 

The Questar is pretty tight so 33-34%. Some scopes are way off, with a small silvered spot, implying an illusory 28%, but the baffling produces more like 40%. Example Meade ETX 125.

Edited by dougspeterson, 23 June 2019 - 01:05 AM.

[steve t]

Hi Doug,

Thanks for chiming in. I never considered the impact of the baffling that extends from the mirror. I'll check out Ruten. 

Steve T 

[Kevin Barker]

Taking a look at a Q3.5 through the corrector lens I would be surprised if the central obstruction is greater than the corrector baffle.

 

I appreciate the image is magnified by the corrector but to my eye the Central Obstruction caused by the baffle is the limiting factor.