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Classic Cassegrain Popularity?

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#101 TG

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Posted 24 July 2015 - 07:33 PM

OK, here's a spreadsheet that I cobbled together to show the various parameters of a Classical Cassegrain. Hopefully, I've entered the formulas correctly (rows I thru J are from the Wikipedia page for Cassegrains and L thru N for calculating field curvature from telescope-optics.net; both links are in the spreadsheet).

 

http://1drv.ms/1HP8wYh

As you can see, as the primary becomes slower, the field becomes flatter but the secondary conic increases which presumably make it harder to make. As the primary becomes slower, the minimum secondary size increases as well.
 
You can change the numbers in the highlighted fields for different primary-size/system f-ratio/back-focus combinations to see how everything else changes. 

 

Note: click the 'edit workbook' button to enable editing of the spreadsheet.
 
Tanveer.


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#102 vahe

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Posted 27 July 2015 - 08:56 PM

Does anyone own or has looked through 300mmF/20CC ? I am just curious, the website does not give the weight of the scope and or the central obstruction size. Also what exactly is the Supermax33 substrate? is it something similar to pyrex meaning low expansion but not necessarily zero expansion ?

What kind of average Strehl can be expected from the CC series?

 

Vahe



#103 Catalin Fus

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Posted 28 July 2015 - 10:30 AM

With the weight I can help here : 17kgs (depending a bit on accessories)

CO = 24% for the f3.7 primary; a bit less for the f3 primary and shorter OTA overall

 

Supremax33 is the name for a borosilicate glass delivered by Schott, which has same characteristics as Corning's Pyrex. Just a trade mark, same glass properties.

I'd recommend for these telescopes quartz as a better alternative for the simple motif that it comes stress free. We've just had an experience with a blank that was not annealed correctly and Schott started lately to have quality issues.

The thing is that if the blank has some internal tension left, you simply can't do a good figure on the mirror like on a substrate which comes stress free (quartz or clearceram). For planetary details this is crucial in my opinion as you need the highest quality surface possible.

 

We guarantee .94 Strehl at the eyepiece for the reflector line and delivered only from .95 upwards. RCs are in the .94 - .98 range,  Cassegrains are usually in the .96-.98 range.

 

Hope this info helps.


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#104 Guest_djhanson_*

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Posted 29 July 2015 - 02:19 PM

Hi Vahe, I believe Phil owns a CFF300CC (he posts regularly here).  And to add to Catalin's comments, my CFF350CC tested out at a 0.98 Strehl.  cheers, DJ



#105 vahe

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Posted 29 July 2015 - 02:49 PM

 I believe Phil owns a CFF300CC (he posts regularly here).  And to add to Catalin's comments, my CFF350CC tested out at a 0.98 Strehl.  cheers, DJ

 

Like to hear about CFF300CC from anyone who has one.
Have you tried your CFF350CC on Saturn yet? I bet yours has the aperture to resolve the Encke Gap on a good night.
I have always liked F/20 scopes of various designs particularly these days when I do mostly planetary observations and do not travel much.

 

Vahe



#106 Guest_djhanson_*

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Posted 29 July 2015 - 07:41 PM

 

Have you tried your CFF350CC on Saturn yet? I bet yours has the aperture to resolve the Encke Gap on a good night.

 

Vahe

 

Hi Vahe, below is my latest Saturn image taken in Arizona (low desert near Phoenix).  Seeing was average to slightly above average that night (maybe 6/10).  I've definitely had some better seeing nights than this one this summer.  

 

First impressions for me is that the CFF350 produces slightly more contrast than my previous C14HD (which was already very good).  This may make sense since the CO has dropped from 32% to 22.5%.  cheers, DJ

 

http://www.cloudynig...cff350-arizona/



#107 PhilCo126

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Posted 30 July 2015 - 01:40 AM

Looks like the f/17 CFF 350 mm is a bit shorter than the f/20 CFF 300 mm… a bit heavier but easier to handle.
Did the graduation marks for the collimation screws come as “standard” issue ?


Edited by PhilCo126, 30 July 2015 - 01:42 AM.


#108 Catalin Fus

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Posted 30 July 2015 - 02:11 AM

Phil, your scope has a f3.7 primary while Dwight's has f3 primary. This is a roughly 60mm difference.

 

Engraved graduations will be standard for all new OTAs delivered. 


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#109 PhilCo126

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Posted 30 July 2015 - 03:02 AM

Another proof CFF produces superb telescopes :waytogo: 



#110 deefree49

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Posted 04 August 2015 - 09:24 AM

That seems to be an awesome scope!

 

Worth the wait, I'm sure. It's good to see that there are scope makers who want to lead and innovate. You can see the quality and care that went into the design and build and it looks very impressive.  

 

 

 


Edited by deefree49, 06 August 2015 - 01:15 PM.


#111 PhilCo126

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Posted 11 August 2015 - 01:02 PM

About time someone loads up a Classic Cassegrain Collimating video on YouTube  ... :idea:



#112 Guest_djhanson_*

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Posted 12 August 2015 - 01:10 PM

any link? :)  I'd be curious to see (seen a few RC ones there).

 

I must say, using the Tak collimator, HG laser then star collimation has proven effective for me.  And on the CFF having the collimation knobs at the primary makes doing final star collimation pretty simple (compared to reaching over an SCT secondary and adjusting).  dj


Edited by djhanson, 12 August 2015 - 01:12 PM.


#113 Jeff B1

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Posted 13 August 2015 - 06:22 PM

Never had any problem collimating a Classic Cassegrain.  My 12.5” f/30 was not portable so it did not get knocked about, but even still if the telescope is constructed correctly that should never be a problem either.  For routine collimation checks and minor adjusting one can use a Cheshire type collimating gadget, then a star to fine tune the mirrors. 

 

When I first made my 12.5” Cass I left out the secondary and using a homemade eyepiece gadget without lenses and with cross hairs to center the spider and make sure the focuser was mounted correctly on the back plate.  Then I installed the primary baffle that was mounted to the back plate with three “collimating” screws.  Then installing the secondary and collimating it true to the focuser eyepiece entrance.  These are mechanical alignments and insured the mirror holders and cells are square with the back plate and telescope tube.

 

The primary goes in, then collimating it with all the shadows and stuff comes next.  Once the cells, back plate, tube, baffle and secondary are all course aligned then fine tuning can be done with a star using various eyepieces; staring out with a reasonable magnification and ending with a ridiculously high mag.  I always found collimating a Cassegrain easier than a Newtonian.


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#114 Guest_djhanson_*

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Posted 16 August 2015 - 01:12 PM

Jeff, that's good info to know (myself being a new CC owner).  cheers, dj



#115 Sunspot

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Posted 17 August 2015 - 09:21 AM

dj,

 

Since your 14 is an open tube, how do you protect it from the weather when you aren't using it?

 

Paul



#116 Guest_djhanson_*

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Posted 22 August 2015 - 11:34 AM

Hi Paul, I store the CFF 350 inside my home, actually just mounted on the G11.  I protect it from dust using the pantyhose shroud, nylon cover over spider and secondary, and there is another cover that can be placed over the carbon tube.  One thing I've noticed with an open tube truss is a bit more dust does collect on the primary, not surprisingly, but I'm not concerned and will eventually probably get a can of pressurized air to blow any dust off.  I'll take a little dust on the primary over thermal cooling issues:)  I am finding that it does come to thermal equilibrium fast (or at least much faster than my previous C14 did).  -dj



#117 PhilCo126

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Posted 06 September 2015 - 06:53 AM

The CFF telescopes come with lycra covers but the mirror is still prone to collect dust, so cover Your telescope with a thermal cloak by Orion or put it in a large Geoptik bag -_-



#118 Guest_djhanson_*

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Posted 06 September 2015 - 01:57 PM

Hi Phil, I would agree.  The lycra does help some, but it's somewhat porous to maybe smaller dust particles.  With that said, Catalin is fabricating a nice primary mirror dust cover (to cover the opening at the carbon tube/baffle).  This should also help to prevent dust accumulation.  

 

But knowing it's an open truss design, I had anticipated some dust collection on the primary over time (like RC or Dob open truss designs).  I get this with my camera lenses and actually never clean them, but occasionally I will use a safe bulb blower and blow any dust off.  cheers, DJ



#119 Max Power

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Posted 15 September 2015 - 08:14 AM

You guys make it sound like cassegraines are so hard to collimate, guess that's why no one bid higher on ebay for a 12.5" Coulter mirror set.  Came with mirror cells and the 35 year old coatings looked pretty good.  Mirror spacing was carved on one side of mirror.

 

Not knowing what I was getting into, put the mirrors in a 14" cardboard tube, bolted the focuser to the back of mirror cell to leave the tube open for cooling.

 

Hardest part was making the baffle tube, not included.  Used pvc pipe with masking tape to build up for a snug fit in central mirror hole.  Made baffles on each end to hug light cone.

 

For collimation used a Howie Glatter laser, the one with the removable targets.  Used all four- dot, square grid, concentric circles and circle with cross target, projected on wall.

 

Finished yesterday, tested last nite and IT'S GREAT!!!  Stars are nice and round, image looks same either side of focus.  Just wish there were some planets besides Neptune and Uranus to look at to really jack up the power and see how good/bad the optics really are.


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#120 Bomber Bob

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Posted 15 September 2015 - 08:23 AM

You guys make it sound like cassegraines are so hard to collimate

 

I managed to get mine close by eyeballing indoors, and tight on its first star test.  It's fussy, but not unreasonable.

 

Would like to see pictures of your Coulter 12.5" scope!



#121 PhilCo126

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Posted 15 September 2015 - 09:01 AM

Well it might sound "hard to collimate" for someone who has been using a refractor for 40 years...

Aperture fever and astrophotography pushed me towards a Cassegrain... :smirk:



#122 jrcrilly

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Posted 15 September 2015 - 09:17 AM

Well it might sound "hard to collimate" for someone who has been using a refractor for 40 years...

 

...or a Newt, or an SCT, or a Mak, or a CDK. Having two mirrors that both have optical centers (and which don't necessarily agree with their physical centers) bringsthe complexity up a notch compared to designs using only one mirror with an optical center. It can be done (I've owned and used five Cass reflectors) but it is more difficult.


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#123 macdonjh

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Posted 15 September 2015 - 06:15 PM

You guys make it sound like cassegraines are so hard to collimate, guess that's why no one bid higher on ebay for a 12.5" Coulter mirror set.  Came with mirror cells and the 35 year old coatings looked pretty good.  Mirror spacing was carved on one side of mirror.

 

 

 

Congratulations on your new scope.  I'm glad you are enjoying it.  Even more than any collimation difficulties, I think what modern amateurs don't like about classical Cassegrains is the long focal length.  Good for me, and for you.  We can get out of favor equipment at good prices.


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#124 Max Power

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Posted 15 September 2015 - 08:44 PM

12.5" f/15= 187.5"=4763mm focal length.

 

My eyepiece set is all wrong for this.  Gonna see if Explore Scientific 68's which are good at low powers, are worth a darn at hi-powers.

24mm=200x        20mm=240x      16mm=300x

 

Got a Delos 14mm for 340x and a Nagler 12mm for 400x, these will be good.

The eyepiece change over might cost more than the scope.  Didn't think about this.



#125 Stephen Kennedy

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Posted 15 September 2015 - 09:51 PM

Even though you purchased a classical Cassegrain and presumably do not plan on trying to make one, you might want to get a copy of "How to Make a Telescope" by Jean Texereau.  He devotes several chapter to making a Cassegrain and the book contains a wealth of information about them.  When he discusses making the optics he presents examples that use a 75 mm eyepiece to get a minimum magnification of 100X. 

 

If you go to the Eyepiece Forum you will find a thread describing an 80 mm eyepiece made by Masuyama of Japan which is currently being marketed.


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