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The Six-inch 6" B&L Criterion Schmidt-Cassegrain

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#1 dougspeterson

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Posted 20 November 2004 - 11:25 AM

Between the 4 and the 8 is the rarely mentioned six inch. I have one and suprisingly the optics are very good, diffraction-limited. I don't know why, other than it was one of the last ones made apparently. The several 4 inchers I have owned were merely OK, and although I have never owned or looked through the 8 inchers, they certainly do not have a good reputation.

The six inch does have the notorious flaw that, even though the secondary obstruction is nominally 33%, vignetting occurs from the primarly baffle stopping the aperture down to 5 inches with a 40% equivalent obstruction, the same as a Celestron C5. (Perhaps this is one reason the 8s have a bad reputation). This is with a diagonal in place, straight thru you get the full six inches and a hefty chiropractic bill.

#2 Mitrovarr

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Posted 20 November 2004 - 09:48 PM

I took out my Dynamax 8 and checked for vignetting, but I didn't notice any, even with the diagonal in place. That makes sense; it performs like a bad 8" scope, not like a good 6" one. I have a 6" dob with pretty good optics, and while it is far sharper than the Criterion, the Criterion is much brighter.

I think the poor reputation has more to do with the inconsistancy of the optics. My 8" has pretty significant problems; it has roughness, over/under correction (hard to tell which with a SCT), a turned down edge, and zones. They mostly hurt it double star hunting; about the best it can do, full aperture, is a slightly messy split of the two Epsilon Lyrae doubles.

I have an aperture mask, and with it, it does about as well as a good 80-90mm refractor. I've gotten good, sharp views of the Cassini division, which is about as much as you can ask for from this scope, really.

I noticed that these problems got significantly worse when I flocked the tube; I don't think it ever cools down completely anymore.

I've heard about Criterion scopes so bad they are almost totally unusable. I've also heard that the later ones are better, so that may explain yours. Mine is kind of bad, but still usable, so I think it was made pretty close to the end. It's good enough that it is worth using, but bad enough that I wish I had bought something else instead.

#3 Ken Hutchinson

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Posted 03 December 2004 - 04:53 PM

I have a pre-B&L Dynamax 6. I always thought it gave pretty decent images although I do remember that C8's in the club were better. That is to be expected and the night I made this comparison we discovered that the central baffle tube was loose which played havoc with the collimation, not to mention giving image shift the size of Wyoming. I never did have a tripod for it and the portable pier I made for it is no longer serviceable. I will have to see if I can adapt it to my NexStar11 tripod/wedge and compare it to my present scopes. The NS11 ought to beat it bloody but it would be interesting to compare it to my 6" Celestron refractor.

Whether the optics are much good or not I got a lot of pleasure from using it. I wouldn't be in the hobby today if it hadn't been for that Dynamax so it can't be all bad.

Ken

#4 Ken Hutchinson

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Posted 11 December 2004 - 10:06 PM

Here is the beast. It could use a little cleaning. I notice a spider took up residence in there even though I've always kept it capped and in the footlocker it came in. Not surprisingly prey was hard to come by and now he is dead and dry. I would have to pull the corrector to get him out, grrrrrrr.

I used the blue plywood square that formerly mounted it to my home made portable pier as an adapter to mount it on my APT Astro wedge for NexStar GPS scopes. It is extreme overkill for the Dynamax 6 which means it should be very, very stable. Nothing but cloudy weather here though so I haven't had a chance to look through it.

Ken

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  • 278787-dmax6.JPG


#5 dougspeterson

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Posted 12 December 2004 - 12:02 AM

Ken,

Does yours exhibit the vignetting by the primary baffle I mentioned above?

The blue was apparently Criterion's response to the Celestron brown and orange of the era. Then Edmund went with red and all *bleep* broke loose in the marketplace.

My B&L era is of course all black, and I do meet all!

One thing I always liked about mine is the silky smooth motion, much smoother than the Celestrons.

#6 Ken Hutchinson

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Posted 12 December 2004 - 01:17 PM

Ken,

Does yours exhibit the vignetting by the primary baffle I mentioned above?


There is edge of field vignetting with 40 mm and 30 mm Plossls and with a 23 mm Axiom. This is pretty normal for a Cassegrain type instrument. Even the modern big names do this, though at wider fields, perhaps. I have no vignetting with an 18 mm Plossl, 19 mm Panoptic, or 16 mm Konig. I can't think of a way to measure the effect you talk about other than by ray tracing the optical layout. I am seriously considering pulling the corrector to get that dead spider and web out. If I do that I could make the necessary measurements.

Your sources may be correct about the central obstruction. Baffling a Cassegrain is a delicate balancing act between introducing more obsruction and keeping the focal plane from being flooded by unfocused light. Modern SCT's tend toward under baffling to prevent an increase in the CO. If I take the diagonal out of my NS11 I can see right past the very edge of the baffles to the open sky beyond. In the days before small CO's became an obsession baffling may have been biased towards preventing this type of leakage at the expense of a larger CO.

Ken

#7 dougspeterson

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Posted 12 December 2004 - 01:25 PM

The effect I am talking about is illumination on axis, not off. Put in a high powered ocular and look into the corrector. As you move off axis, if the tiny dot of light through the ocular is cut off before your line of site reaches the edge of the corrector, you have vignetting on axis; if not your oculars are seeing the whole clear aperture.

#8 bobbie

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Posted 12 December 2004 - 10:19 PM

Thanks for the pic Ken. The blue makes it look like it's made of plastic! I assume it isn't - and that it has a number of old fashoned and heavy castings (my 8" certainly has). Mine is also very, very black!

#9 Ken Hutchinson

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Posted 12 December 2004 - 11:21 PM

The effect I am talking about is illumination on axis, not off. Put in a high powered ocular and look into the corrector. As you move off axis, if the tiny dot of light through the ocular is cut off before your line of site reaches the edge of the corrector, you have vignetting on axis; if not your oculars are seeing the whole clear aperture.


OK, I will try that tomorrow night if I get the chance. I'm done cleaning, the spider and web are gone and I made some measurements of the optical layout as best I could. Your comments about the baffle are wrong, it does not interfere on axis and if it did it would not cause an aperture reduction, just a CO enlargement. However your comments about on axis vignetting may be true for a slightly different reason: the secondary may be undersized and at best is just barely adequate to field the light cone from the primary on axis. At the extreme forward position of the mirror the secondary needs to be 42 mm in diameter and it is only 40 mm. This would make the secondary the aperture stop and it would have the effect you describe. The extreme forward position is slightly past the infinity focus but the test you describe with the telescope focused on infinity should tell the story.

If anyone is interested the layout is very similar to modern SCT's. The primary is f/1.92 which means the secondary magnification is 5.2. The 10% variation in primary focal ratio from modern practice could simply be due to the focal length tolerance allowed back then.

Ken

#10 Ken Hutchinson

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Posted 12 December 2004 - 11:25 PM

Thanks for the pic Ken. The blue makes it look like it's made of plastic! I assume it isn't - and that it has a number of old fashoned and heavy castings (my 8" certainly has). Mine is also very, very black!


The blue is mostly aluminum castings. The slightly different blue is a piece of plywood. The gray tube is that famous cardboard that is strong enough to fire rockets from!!

If yours is a Criterion then the color change came before B & L. I can remember the sale to B & L but I don't remember the color change at all. I don't remember for sure but I think mine was purchased between 1976 and 1980.

Ken

#11 Ken Hutchinson

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Posted 13 December 2004 - 09:25 AM

The effect I am talking about is illumination on axis, not off. Put in a high powered ocular and look into the corrector. As you move off axis, if the tiny dot of light through the ocular is cut off before your line of site reaches the edge of the corrector, you have vignetting on axis; if not your oculars are seeing the whole clear aperture.


Well you were right after all. I neglected to calculate the required baffle tube diameter. The undersized secondary can result in an aperture reduction to 5.7" but the baffle tube is even worse, 4.99". It is a shame because the 52 mm CO would allow the baffle tube to be big enough to field the light cone, they just didn't take advantage of this. Chopping about 36 mm off the length of the baffle would fix this problem. Would it just reveal a turned edge on the mirror though? It would potentially lead to light leakage past the baffle, though I would think you would rather deal with that than suffer an aperture loss. And here I though the 6's were better than the Dynamax reputation. Boy do I wish I had bought the C5 instead, I came this close!

Ken

#12 dougspeterson

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Posted 17 December 2004 - 11:17 AM

Remember you get the whole six inches without the diagonal, even with the diag the scope thinks it is a C5. I once modified a russian mak with this problem by shortening the baffle tube by about 1/2", dremel tool, aluminum baffle removed. It worked, am thinking about doing this to the 6, already confirmed the wavefront is good in the vignetted anulus.

#13 Guest_**DONOTDELETE**_*

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Posted 27 August 2005 - 03:35 AM

Guys,
I just acquired a Dynamax 6" SCT (pre B&L), and completed the collimation this evening - diffraction limited. It's a beautiful scope, with exceptional machining - controls operate like a swiss timepiece - but the optics were horribly out of alignment when I received it, virtually unviewable - it took me several hours to complete the task. Well worth it, though. Again, it seems that the 8" versions had serious problems, but I have yet to hear bad things about the sixes, and have heard several stories out there (now mine included) about very good ones. I'm quite impressed with the scope - ya gotta know, a friend found this at the DUMP (someone had thrown it away) and passed it along to me. One man's garbage...

#14 dougspeterson

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Posted 28 August 2005 - 06:24 PM

Did you need to collimate the primary? I have one that did. I turns out the four button head screws holding the visual back assembly in place are holding the entire primary mirror assy. They can be loosened, and then shimmed to tilt and collimate the primary if necessary.

#15 Guest_**DONOTDELETE**_*

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Posted 28 August 2005 - 06:29 PM

FYI, here's a picture of my "new" Dynamax 6 SCT... I'll check for vignetting this evening.

Attached Thumbnails

  • 577905-Dynamax 6 tiny.jpg


#16 Guest_**DONOTDELETE**_*

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Posted 28 August 2005 - 06:38 PM

Hi Doug,

I actually got to that, er, experimentationally - I did remove the primary/post screws you mentioned, and checked it out; fortunately, the primary did not need alignment/shimming - I was able to get perfect airy discs at 300x in the final collimation step without adjusting the primary - I simply made sure those screws were evenly tightened, kind of like the lug nuts on my car...!

and BTW, this baby also came with a custom solar filter as well, so I just got back from my first ever sun spot observing session - waay cool! It's kind of like a dream come true.

#17 ForgottenMObject

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Posted 30 August 2005 - 10:23 AM

Amazing that it was in a landfill! Hope it works well!

Makes you wonder how many of the Dynamax's were useless and how many were just terribly out of alignment.


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