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C90 Contrast Issue

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#51 ChristianG

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Posted 30 January 2014 - 03:33 PM

Last thing... On my Apex 90, the corrector cell just unscrews. However, I haven't been able to do so with my C90--should I just keep trying or it's held differently (glue)?

Oh boy... I own too many of those Mak's: Apex 90, C90 Mak, Apex 127, Orion 180 and even a Questar 1978!!!

Plus I bought a Criterion 4000 just for the mount to make a 'poor man's Questar' with it and my Apex 90 just for fun. So you all see why I'd like to improve contrast on it...

--Christian
Ottawa, Ontario

#52 PowellAstro

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Posted 30 January 2014 - 04:20 PM

Late question on the successful baffle mod with black sandpaper that you did, was the improvment also seen at night? I belive the initial contrast test was during daytime, and if it helps in the dark I will do it myself too.


I got my Dad a C90 and have compared mine with the mods to his stock unit at night on Jupiter, Moon and Orion Nebula. I could very easily see the difference on all three objects. To my surprise, M42 shocked me. I could see what seemed to be 1/3 more nebula with the treated scope. Jupiter and the Moon was no contest and the difference made me not even want to look through the stock unit anymore. His will get all the treatment next.

#53 Sarkikos

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Posted 30 January 2014 - 05:54 PM

To those who have constructed a sandpaper or other type of contrast enhancement to fit into the primary baffle of the C90: Did anyone bother to note down the measurements? I assume this would be a rectangle that you fold into a cylinder. What were the dimensions - length and width - of the cylinder that worked best, without overlap and encroaching least into the light cone?

Mike

#54 Sarkikos

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Posted 30 January 2014 - 05:58 PM

Has anyone used the ProtoStar flocking paper for the baffle? I have plenty of that on hand. I don't have the flocking board. I don't think I'll be buying any of the flocking board just for this little project.

Mike

#55 PowellAstro

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Posted 30 January 2014 - 06:00 PM

If the flocking paper is very thin, you should try it and let us know how it works. The sandpaper I used was around .007" thick.

#56 AhBok

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Posted 30 January 2014 - 06:35 PM

I made my sandpaper baffle with a piece 5" long and 1 & 7/8" wide. I cut it evenly with a paper cutter, rolled it into a tube with the edges touching and fixed it with a couple of small strips of clear tape. I then slid it into the baffle. It fit tightly, but not so tight that it would bend or fold. The result was a perfectly cylindrical tube. Mine made a huge difference. The rings from the glare of the moon or nearby stars made my C90 no fun to use at all. Now the rings are gone and the views are nice and dark.

#57 Sarkikos

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Posted 30 January 2014 - 10:09 PM

The rear opening is only 0.63"? That's only 16mm. Is it really that small? Looks like I'll have to get out my digital caliper this weekend.

Mike


I don't usually quote myself, but I had forgotten to measure the rear opening of my C90 until I reread my earlier post tonight.

So I got out the digital calipers this evening to measure the rear opening of my C90. I bought the C90 last year. Unfortunately, the rear opening was set too far back for my calipers to reach. But I measured as best I could by opening the calipers and sighting back at the rear opening of the scope.

Yes, the opening is approximately 16mm. All the other recent C90's should have the same measurement.

Mike

#58 Sarkikos

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Posted 30 January 2014 - 10:19 PM

While I was at it, I also measured the rear openings on my C6 and C5. The C6 is 25.6mm, the C5 is 24.5mm.

Mike

#59 Sarkikos

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Posted 30 January 2014 - 10:45 PM

I also measured the thickness of ScopeStuff flocking that I ordered not long ago. It is 0.32mm thick, according to my calipers. This includes the sticky-paper backing.

There was an earlier measurement for flocking in this thread, saying it was 1.78mm. That seems awfully thick. How could that be? If I set my digital calipers to about 1.78mm, the flocking - even with the paper backing - slips easily through with plenty of room to spare.

The 100 grit sandpaper was said to have a thickness of 0.009", or 0.23mm. That's only 0.09mm less than the ScopeStuff flocking. Will less than 1/10 of a millimeter make that much of a difference? I think I'm going to try the ScopeStuff.

Mike

#60 PowellAstro

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Posted 31 January 2014 - 12:03 AM

It should be just fine. I am interested to see how it works. Some flocking is thicker than others and I had sandpaper on hand. Many years ago I used flocking in a reflector I built and it was almost 1/8" thick. This was almost 20 years ago though and I haven't purchased any for many years. Looks like the new stuff is much thinner.

#61 GlennLeDrew

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Posted 31 January 2014 - 12:31 AM

A mere 16mm rear opening on a C90?! The one I once owned was made to accept a 0.965", or 24.5mm barrel of a diagonal. Is this 16mm referring instead to the primary baffle's font opening? For that seems to be in the ballpark, from my own memory of visually examining it.

#62 PowellAstro

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Posted 31 January 2014 - 12:35 AM

This 16mm is the opening on the front of the primary baffle as well as the opening at the rear where the visual back goes. The primary baffle's ID is 16mm all the way through from front to back.

#63 Sarkikos

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Posted 31 January 2014 - 08:39 AM

A mere 16mm rear opening on a C90?! The one I once owned was made to accept a 0.965", or 24.5mm barrel of a diagonal. Is this 16mm referring instead to the primary baffle's font opening? For that seems to be in the ballpark, from my own memory of visually examining it.


No, I'm not about to open up the C90 just to take a measurement. I'm not that much of a tech-head. ;) As I said, I was measuring the rear opening. I could not get close enough to insert the calipers, so this was an estimate.

I expected it to be wider, also. But 16mm agrees with a measurement that PowellAstro made farther up in this thread.

Mike

#64 Sarkikos

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Posted 31 January 2014 - 08:46 AM

Is this 16mm referring instead to the primary baffle's font opening? For that seems to be in the ballpark, from my own memory of visually examining it.


If the primary baffle's front opening is 16mm, that's farther up on the light cone and should have an even greater impact on reducing the clear aperture than the baffle size on the rear opening. So in terms of CA, I don't see that there is any advantage in the opening being bigger in the back if it's already only 16mm at the front.

Mike

#65 Ed Holland

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Posted 31 January 2014 - 10:44 AM

Interesting thread. The results with the C90 reflect (pun intended :)) those I obtained by adding light trap material to the Orion 127 mm Mak. Daylight viewing contrast under certain conditions was severely affected by stray light. It was easy to demonstrate this using a camera mounted to the scope.

For astronomical viewing I'm not sure there is any benefit, but have not made deliberate comparisons.

I am amused by the adoption of the sandpaper & paint approach to flocking - it was only ever intended as a means to experiment, yet it does work rather well. To anyone using this approach, I would recommend that the paint is allowed to dry very thoroughly (bake in the oven even, say at 250F) to release volatiles that might otherwise deposit inside the scope.

Cheers,

Ed

#66 OrdinaryLight

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Posted 31 January 2014 - 01:47 PM

Ed, the sandpaper works so well and I already had the paper and paint so it was almost instant gratification; just a bit of time to dry then bake the painted paper. I actually did it twice, the first time I was not careful enough about the dimensions and some glare remained. The second time I used 60 grit just for kicks and was more careful about cutting the paper to size.

The return for the time/work involved is so great that if I ever get a bigger MCT (or SCT?) this would be sure to get done to it and has me considering the benefits of fully flocking any dob/newt tube I get in the future.

#67 Ed Holland

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Posted 31 January 2014 - 03:41 PM

Dave, it does work very well, plus it is easily available almost anywhere and simple. I fell on the idea from first principles i.e. what does light trap material have to do, and what do I have "in the shed", but can't claim the invention, as I'm sure it has been done before, and my inspiration may well have been a half forgotten memory of something on CN or elsewhere.

Still I was surprised by the combination of effectiveness, cheapness, lack of delivery time and shipping costs ;)

#68 PowellAstro

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Posted 31 January 2014 - 04:23 PM

I certainly consider it as A Hot Product 2013-2014!!! :cool:

#69 Usquebae

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Posted 31 January 2014 - 06:14 PM

Manish should start selling pre-cut squares of sandpaper like he does with Baader film! :winky:

#70 Ed Holland

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Posted 01 February 2014 - 11:06 AM

We could call it "Sandtrap" :)

#71 PowellAstro

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Posted 01 February 2014 - 12:16 PM

Seems it's as good at stopping light as it is golf balls!!

#72 Dakota1

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Posted 01 February 2014 - 12:49 PM

Ya and cats love it too !!!!!!!!!!!!!

#73 Astrojensen

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Posted 01 February 2014 - 01:23 PM

If it was at night, write it off to being a small telescope with an obstruction. You can flock it up or whatever, but it was, is, and always will be a small scope with an obstruction. Best case, don't look for it to outperform a 65mm refractor.

Just my opinion.. Have owned a C90 in the past, and have a Meade 90mm ETX.

These small scopes simply can't compete with even a good 66mm ED spotter scope.



That is absolutely not my experience. It would at the very least certainly depend on what you're looking at and it most certainly depends on the scope at hand. I've tried one of the first Meade ETX 90mm's and it had absolutely fantastic optics. They really were damn near perfect. On the Moon, the images were dead sharp at 200x (6mm Vixen LV) and reminded me of my 85mm Zeiss apochromat in terms of contrast and lack of glare. Resolution was the same and it had zero issues showing the many rilles around Mare Humorum and Gassendi. On deep-sky, it nicely resolved M13 in many, many stars around the edges. In daylight, I took it to over 500x on artificial stars (the Sun shining in little water droplets in the needles of a nearby pine tree) with no image breakdown. Double stars, such as Epsilon Bootes, were textbook perfect at 200x. First ring was prominent, yes, but it was very, very sharp. No fuzz here. I was most impressed by the optics. The mechanics, not so much...


Clear skies!
Thomas, Denmark

#74 GlennLeDrew

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Posted 01 February 2014 - 05:59 PM

Is this 16mm referring instead to the primary baffle's font opening? For that seems to be in the ballpark, from my own memory of visually examining it.


If the primary baffle's front opening is 16mm, that's farther up on the light cone and should have an even greater impact on reducing the clear aperture than the baffle size on the rear opening. So in terms of CA, I don't see that there is any advantage in the opening being bigger in the back if it's already only 16mm at the front.

Mike


The front baffle opening affects the size of the circle of full illumination. The rear opening controls far off-axis vignetting. One can certainly benefit from a rather larger rear baffle in Casses and Maks. A small-ish baffle fairly far up into the light cone introduces relatively gradual fall-off outside the central zone of full illumination, and so a sizeable field stop is supportable. Hence a large-ish rear baffle.

But again, at least the older C90s had 24.5mm rear openings into which 0.965" diagonals were intended to fit.

#75 PeterR280

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Posted 01 February 2014 - 07:08 PM

What a difference in contrast. I removed the front corrector lens. It unscrews from the body. I then inserted 4 strips of adhesive back flocking in the tube. This was a tedious process. I cut the length to go about 74% into the tube and carefully positioned with tweezers. I then used smaller pieces from the back of the baffle tube to cover the inside.

I also notices that there is a retaining ring behind the corrector lens that is fairly shiny. It's about 1/4 inch in thick ness. I cut strips of flocking and covered the retaining ring and put the corrector back on. The improvement is amazing. It performs like a refractor during the day. I am not noticing any vignetting. I am going to try it on Jupiter tonight.






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