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Something for nothing: Celestron C90

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#126 hardwarezone

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Posted 09 July 2013 - 01:01 PM

Hardwarezone - a CN poster gave me collimation instructions [attached] for his Orion Mak, which looks the same as the C90.

Thanks for the link, I've read that PDF but I am still not sure if the 3 smaller holes are part of the mirror collimation or for factory to secure the whole mechanism to the shell.
It does seem easier if I only have to turn 3 variables instead of 6.

#127 CollinofAlabama

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Posted 09 July 2013 - 11:18 PM

Jag767,

There is NO way a 90mm Mak can be effectively outfitted with a 2" focuser. Not saying it can't be done, physically, but I am saying it is pointless, optically. The entire smaller Cassegrain line of telescopes don't possess a central hole in their primary mirror large enough to support a 2" ep fieldstop until one gets to about 7" or 8". Below this size primary, and you can't possibly collect enough light to satisfy even the smallest 2" eyepiece (something like a Sterling 30mm Plossl). I mean, you can fit a 2" focuser and any 2" ep you want, but you'll be dealing with a woefully vignetted view (significant light thru put drop off at the edges of field). Your images will be bright in the center, then fade outwards, then get downright dark toward the edges. These are the optical facts, and this has to do with the size hole in the primary, and not a function of some 2" focuser someone has jerryrigged to the end of any sub 7" Cassegrain. ALL sub 7" S/MCTs are 1.25" instruments, period, regardless of what one affixes to the end.

#128 jag767

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Posted 10 July 2013 - 04:53 AM

http://www.cloudynig.../Number/4492723 This is what I was going off of as to if it would be worth doing or not. That and the amound of 2 inch gear I have and would like to be able to use.

#129 jag767

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Posted 10 July 2013 - 03:43 PM

Well I got a little creative, took off the focus knob, threw it on a lathe, and took just enough off for a 2 inch visual back to fit. All and all pretty easy to do. Now I don't have the massive extension on the back and has all the function I was looking for. All I need is the weather to improve to give it a good workout. Daytime views with a meade 5k 32mm super plossl look pretty promising.

#130 jag767

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Posted 10 July 2013 - 05:26 PM

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#131 jrbarnett

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Posted 10 July 2013 - 05:31 PM

I agree with Collin; 2" accessories on a 90mm MCT are pretty pointless. It's not just tiny rear baffle diameter, but also the HUGE additional focal length you generate by spacing a 2" diagonal off the rear of the scope. These scopes have variable focal length because the mirror moves when focusing. By moving the eyepiece that far back, you've added considerable 100s of millimters of extra focal length to the system in focus.

Vignetting city. Maybe usable in daylight where you might not notice edge darkening as much, but the amount of vignetting cause by excess back focus coupled with the itsy-bitty rear aperture will show up in a 2" widefield eyepiece at night as an almost "shadow ring" featuring increasingly dimmer stars around a significant percentage of the outer field.

These little guys top out in FOV efficiency with a 32mm Plossl (50-degree AFOV) or 24mm wide field (68-degree AFOV).

Regards,

Jim

#132 jrbarnett

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Posted 10 July 2013 - 05:41 PM

All 6 screws, smalls and larges, are definitely used in collimation. The instructions make that crystal clear, telling you how to identify the **** start with and which way to turn it, and then what order and which direction to turn all 5 additional screws. Three of them push the mirror and three of them pull it, and the total system tension on the back of the mirror must be maintained so you have to tweak them all.

The instructions also eliminate "variables". By knowing which **** must start with and how much and in what direction you must adjust each of the other screws after adjusting the starting screw, there's not much trial an error really. It sounds pretty easy, really, in that it can be done in daylight entirely from the rear of the scope. That's even easier than an SCT where you have to use a star at night and "reach around".

Regards,

Jim

#133 jag767

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Posted 10 July 2013 - 05:53 PM

I agree with Collin; 2" accessories on a 90mm MCT is pretty pointless. It's not just tiny rear baffle diameter, but also the HUGE additional focal length you generate by spacing a 2" diagonal off the rear. Vignetting city. Maybe usable in daylight where you might not notice edge darkening as much, but the amount of vignetting cause by excess back focus coupled with the itsy-bitty rear aperture will show up in a 2" widefield eyepiece at night as an almost "shadow ring" featuring increasingly dimmer stars around a significant percentage of the outer field.

These little guys top out in FOV efficiency with a 32mm Plossl (50-degree AFOV) or 24mm wide field (68-degree AFOV).

Regards,

Jim


Although I'm sure you're correct,

1. This saves me a good amount of money on buying another diagonal and some ep's

2. I kinda like the way it looks (not the first time I've done something pointless just to do it :roflmao:)

#134 jrbarnett

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Posted 10 July 2013 - 10:22 PM

For sure it will be better than the 45-degree prism that came with the scope. Also, when used with 1.25" eyepieces you'll probably not suffer noticeable light falloff off axis.

I'm reminded of the old saying "Never eat anything bigger than your head." I wonder if there's not a variation that goes something like this: "Make sure that the thing in the diagonal isn't bigger than the thing in front of the diagonal." :grin:

- Jim

#135 jag767

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Posted 11 July 2013 - 04:34 AM

For sure it will be better than the 45-degree prism that came with the scope. Also, when used with 1.25" eyepieces you'll probably not suffer noticeable light falloff off axis.

I'm reminded of the old saying "Never eat anything bigger than your head." I wonder if there's not a variation that goes something like this: "Make sure that the thing in the diagonal isn't bigger than the thing in front of the diagonal." :grin:

- Jim


Hahaha well said. Got to use it a little last night and I have to say if there's any decent amount of vignetting well my eyes arent experienced enough to see it.

I did make a very interesting observation though. As brighter object make their way towards where about a 40-45 degree fov would end it became very noticeable the inside of the optical tube is not coated as dark as it could be. I've never flocked a tube before but since I'm already making this my little frankenstein scope I'm thinking I may try it out.

#136 jrbarnett

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Posted 11 July 2013 - 05:26 PM

"...the inside of the optical tube is not coated as dark as it could be."

Amen to that!

If you put a bright star at one edge of the FOV, it will illuminate the interior tube wall on the opposite side, and actually put an ugly arc of scattered light in the field of the eyepiece.

I think flocking the interior walls of the OTA (or at least repainting them ultra-flat black) would help. Other users, though, say the culprit is a shiny lip/missing paint on the edge of the baffle tube and have fixed it by flocking the baffle rather than the whole interior.

For my part, I avoid putting bright objects far off axis. :grin:

On the vignetting question, what 2" eyepiece were you using (i.e., make, model, AFOV and focal length)? It may be that it didn't have sufficient AFOV to suffer much vignetting with that level of extra focal length in back, and that would be handy to know.

- Jim

#137 PeterR280

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Posted 11 July 2013 - 06:13 PM

I just got mine and took some landscape pictures with a DX size camera. It's not bad but some vignetting in the corners. I should try without the extension tube on the visual back they supply to see if there is enough travel.

#138 jag767

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Posted 11 July 2013 - 07:05 PM

"...the inside of the optical tube is not coated as dark as it could be."

Amen to that!

If you put a bright star at one edge of the FOV, it will illuminate the interior tube wall on the opposite side, and actually put an ugly arc of scattered light in the field of the eyepiece.

I think flocking the interior walls of the OTA (or at least repainting them ultra-flat black) would help. Other users, though, say the culprit is a shiny lip/missing paint on the edge of the baffle tube and have fixed it by flocking the baffle rather than the whole interior.

For my part, I avoid putting bright objects far off axis. :grin:

On the vignetting question, what 2" eyepiece were you using (i.e., make, model, AFOV and focal length)? It may be that it didn't have sufficient AFOV to suffer much vignetting with that level of extra focal length in back, and that would be handy to know.

- Jim


I think my solution will be the same as yours, especially since I know nothing about taking it apart and more importantly getting it back together correctly :foreheadslap:

FYI I was using a meade 5k 32mm super plossl 68 degree. I didnt switch to my meade 24mm 82 degree but next good night I get that will be next.

#139 PeterR280

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Posted 15 July 2013 - 11:32 PM

It looks like the rim of the corrector plate is reflecting light. Saturn on the edge of field of view was annoying. Otherwise I was amazed at the views of the moon and Saturn from the little telescope that only cost me $159. I did change the diagonal.

#140 jrbarnett

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Posted 16 July 2013 - 02:18 PM

Good tip, Peter.

Tracking down the problem area would be great. I wonder if flocking *outside* the corrector, along the inner edge of the corrector cell, would be enough to fix the issue. :thinking:

To me the very best thing about these little Synta 90mm MCTs is that if you've ever wondered what a Questar 3.5" would be like, you can find out (mostly) for 1/20th the cost. :grin:

- Jim

#141 PeterR280

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Posted 16 July 2013 - 03:06 PM

The corrector may be backing up against unpainted aluminum or it could be internal reflection from the glass.

#142 pdxmoon

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Posted 16 July 2013 - 05:22 PM

Jim, this scope sounds like a great way to wiggle my toys into the Mak world, whilst providing me with a don't worry about it star party scope. $149 on Amazon for the deal with one eyepiece and no try, up to $184 on Optics with the tripod deal.

#143 jrbarnett

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Posted 16 July 2013 - 06:28 PM

The scope is great. The tripod is not. Not even solid enough for point and shoot photography with a compact IMO. Save the extra money and skip the tripod. It is indeed a great way to test the MCT waters and get a daytime spotter you don't mind using and abusing as well.

I'll be taking mine to Chaco Canyon at the end of next month to use to show my hiking mates several of the Great House line-of-sight alignments from the mesa tops. It probably won't be used at all at night, and I'm totally okay with that. It's so useful, everyone in the world needs one. :lol:

Regards,

Jim

#144 pdxmoon

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Posted 16 July 2013 - 07:58 PM

Jim, what do you use as a grab tripod for this? Porta? I heard there are issues with mounting--do I avoid them by using rings and the dove bar?

#145 tnakazon

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Posted 16 July 2013 - 09:37 PM

I had a problem using this scope with mounts having a side dovetail saddle (e.g. Vixen Mini-Porta II, Orion Versago II) because the finderscope would be in an awkward position (at 7-8 o'clock). Will definitely not use a photo/video tripod for this scope.

Need to use a mount with the dovetail saddle on the bottom of the tripod (e.g. Celestron Astromaster CG2 & CG3 mounts) if you don't want to use mounting rings for this scope.

#146 jrbarnett

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Posted 16 July 2013 - 10:54 PM

The C90's integrated doevtail is tapped with a 1/4-20 tapping (small tripod standard). You can get L-brackets with a Vixen type dovetail on one leg and a plate with 1/4-20 mounting stud on the other leg that will thread directly into the tappings on the bottom of the fixed dovetail, placing everything in its correct position on a mount like a Porta II. Here's an example:

http://www.telescope...Orion-Doveta...

Now what you might have to do with a Porta II is remove and rotate the saddle 90 degrees (if you can) in order to keep the slo-mo knobs in good position. But maybe not.

Regards,

Jim

#147 tnakazon

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Posted 16 July 2013 - 11:36 PM

Thanks - I'll look into this to see if I want to use my C90 on my Vixen Mini-Porta.

#148 jag767

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Posted 17 July 2013 - 12:28 AM

I have mine on an ioptron ball head and have quite enjoyed it. Just my 2 cents.

#149 Binojunky

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Posted 17 July 2013 - 10:35 AM

Mine lives on a Porta with a right angle bracket giving the scope an upright position, you can also mount it on a left sided mount that will still put the finder at a top position,however mount on the right and as mentioned the finder is underneath.
Regarding right angle brackets, if you are handy its not to difficult to make your own, or get the one from Orion ($30) that fits the various marks of the Versago, buy the Vixen unit and its close to $100, way over priced for what it is,DA.

#150 caheaton

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Posted 17 July 2013 - 11:46 AM

I have mine mounted on a Meade AZ3 mount. I suspect any similar mount would work fine. Even pointing at vertical it holds position well.

Here's a link that shows the mount:
http://www.meade.com.../az_series.html




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