Third Planet Optics (TPO) 8" f/8 Ritchey-Chretien Astrograph OTA…
In 1910, American optician & astronomer George Willis Ritchey & French astronomer Henri Chretien designed a specialized Cassegrain that would later become the telescope of choice for many observatories and professionals around the world. The Ritchey-Chretien astrograph has many benefits that make this design appealing to anyone who is serious about astro-photography or imaging. Here are a few of those benefits:
- Good-bye Coma: An RC has virtually no coma (stars look like little comets around the edges of the field), which means there will be greater image quality across a wider field of view.
- No Chromatic Aberration: Because a Ritchey-Chretien does not use lenses or corrector plates, the design does not suffer from chromatic aberrations, or false color. If you've ever looked through an achromatic refractor (non-APO), you will have seen chromatic aberration.
- No Spherical Aberration: The use of hyperbolic mirrors for both the primary and secondary removes the problem of spherical aberration from this optical system, an optical effect caused when light rays do not all come to focus at the same point, resulting in an image that is not in perfect focus.
8" f/8 Ritchey-Chretien Astrograph Highlights…
Optical Highlights: This TPO Ritchey Chretien telescope has 8" (200mm) of aperture and a focal length of 1600mm. The concave hyperbolic primary and convex hyperbolic secondary are made from low expansion quartz, and finished with a scratch-resistant highly reflective 99% dielectric coating for great contrast. The primary mirror is fixed in place in a metal mirror cell, and the secondary resides in a metal housing that can be collimated.
Multiple Knife-Edge Baffle System: The computer designed and optimized baffle system in the TPO Ritchey-Chretien works wonders at keeping stray light at bay. Inside the tube are ten light baffles, and the primary and secondary mirrors are baffled as well.
2" Dual Speed Crayford Focuser You'll love the 2" 1:10 Dual Speed Crayford focuser that comes standard on this RC. The dual knobs allow you choose the speed with which you focus. One turn of the larger knob equals ten turns of the smaller knob, so minute adjustments…when you are "almost there"…are easy to do with this high quality focuser. A 1.25" compression ring adapter is also included so you can use both 2" & 1.25" eyepieces. Two spacers are included as well, which allows you to adjust the focus position for different cameras with various back focus requirements.
A Fixed Primary Eliminates Image Shift Schmidt-Cassegrain & Mak-Cassegrain telescopes achieve focus by moving the primary mirror back and forth inside the optical tube assembly, and this movement can cause image shift. While manufacturers have done a pretty good job of minimizing image shift on their telescopes, a moveable mirror makes it almost impossible to eliminate it completely. The Ritchey-Chretien has a primary that is fixed in place, removing the possibility of image shift and also the job of collimating the primary.
Two Dovetails & Finderscope Base Included Talk about versatility! One Vixen-style dovetail is attached to the bottom of the OTA, while another Vixen-style dovetail rail is attached to the top for quick and easy attachment a variety of mounts. The rails can be removed if you wish to use mounting rings (sold separately) instead. While a finderscope is not included, the OTA comes standard with a finderscope base that will accept Vixen-style brackets (if you want an optical finder) or most red dot finders.
Long Back Focus of 250mm allows you to mount lots of imaging gear!
Please Note: The image shows the carbon fiber version of this telescope. The rolled steel version is white with black metal trim. All other accessories are the same.
- Additional Information
SKU OS-8RC-M Manufacturer TPO Telescope Series TPO Ritchey-Chretien Optical Design Ritchey-Chretien Mount Type None - Optical Tube Only Warranty 2 Year Warranty Telescope Aperture 8" Telescope Focal Length 1600mm Telescope Focal Ratio f/8 Length of Optical Tube 21.8" Optical Tube Weight 15.8 lb. Optical Coatings 99% Dielectric Tube Color or Finish Gloss White Limiting Stellar Magnitude 14 Highest Useful Magnification 300x Diagonal Included? No - Sold Separately Finder Included None OTA Mount Type Vixen-Style Dovetail
Customer Reviews 2 item(s)
- Good scope hard to collimate
I have purchased one of the first units of this scope imported by OPT, but it is a proven design that has been sold under various brand names for a while now. This telescope works very well, but collimating it is difficult. Sooner or later your primary will need adjustment and the fun begins from there (if you call spending hours collimating a scope fun). Once you understand how it is done, it gets easier and easier, but it's not as simple as an SCT to collimate.
On the other hand, the scope is faster than a typical SCT, it cools down faster due to the open tube design and works pretty much as advertised. People complain about the focuser being attached to the primary mirror causing the primary to tilt and loose collimation. I have not experienced this with the 8" version, maybe it is an issue with the larger versions. However, you still need to buy a tilt plate so you can adjust focuser tilt. I have had to significantly tilt my focuser to reach good collimation.
The scope has 2 miles of back focus, so you can install a good focuser, a rotator, a filter wheel and a coffee machine.
There is also no reducer specifically designed for this scope and most of the reducers people use have one form of compromise or another when used with this scope.
Is it perfect? No. Would I recommend it? Yes... very few scopes at this price point offer what this scope does. So unless you have 3 to 4 times the budget, it is hard to get a better scope.
1= Houston, We Have a Problem & 5= It's Out of This World
- The TPO 8" RC telescope is a great buy.
The TPO 8" RC telescope is a great buy. There are a few differences on the included items for the better. It states two vixen rails but bottom rail is a losmandy. Since I have a G-11 this is great. Also it came with three screw on back focus adapters to deal with the larger back focus this scope has. You just unscrew the focuser from scope and insert how ever many adapters you need to get focuser in right range. No slope in optical path that way. I did an initial out of focus star test and it was not very far off from perfect. I cant wait for the next new moon to come to due some imaging with it. If you are looking forward to buying a RC telescope this is a good scope to start with and while they have there 15% discount you cant go wrong.
1= Houston, We Have a Problem & 5= It's Out of This World
- Included Items
- TPO 8" f/8 RC Steel Tube OTA
- 2" Dual-Speed Crayford Focuser
- (2) "V" Dovetails
- 2" and 1.25" Compression Rings
- Finderscope Mounting Shoe
- Questions & Answers
Product QuestionsQuestion by: JustStarting on Jul 13, 2014 6:04:28 AMDawes' Limit says that the maximum resolution is 116 divided by the aperture in mm or 4.56 divided by aperture in inches. A 6" scope has a maximum resolution of 0.76 arcsec and the 8" has 0.57 arcsec. Typically, seeing and tracking errors will prevent you from seeing all the way down to this threshold, but it is the goal for high resolution imaging when determining a good pixel sampling resolution.Answer by: Chris (Admin) on Jul 15, 2014 12:56:07 PMQuestion by: Rick on Aug 17, 2016 8:22:00 PMHello,
The CGEM DX will not hold the Vixen style dovetail, so I would recommend retrofitting this onto the CGEM: https://www.optcorp.com/adm-dualcgemsado-celestron-cgem-saddle-upgrade-kit-orange.html
This will hold both Losmandy and Vixen style dovetail bars.
TonyAnswer by: Tony Tardich (Admin) on Aug 18, 2016 1:33:00 PMQuestion by: SoftwareRancher on Sep 15, 2014 9:28:35 AMRitchey-Chretien Astrographs are really designed primarily for photography, although they can be used visually. The secondary obstruction is very large (to provide better illumination to the image focal plane), so the scope will have lower visual contrast than a telescope of the same aperture designed for visual use.Answer by: Chris (Admin) on Sep 17, 2014 8:08:44 AMIs an OAG required/needed for guiding with this scope? If so which one would be suggested when using a DSLR for imaging? ThanksQuestion by: tim57064 on Oct 25, 2014 2:20:29 AMHello Tim,
Thank you for your question. You could use a guidescope to guide with this telescope. You would have less flexture issues if you used an OAG versus a guidescope however, there would be a narrower field of view to pick stars from with an OAG.
For the combination that you're referencing a good OAG to use would be the Teleskop Service Thin Off Axis guider M48/EOS. You will need to get one of the 48mm extension tubes from Teleskop Service in order to get this OAG into the 2" focuser for this telescope.Answer by: Roderick (Admin) on Nov 3, 2014 8:59:57 AMHi; just need to know what 'scheduled delivery' means, and is there a price difference between carbon fiber tube and rolled steel tube, if so, what is it? Thanks, JeffQuestion by: Jeff on Mar 10, 2014 4:57:54 AMScheduled delivery means this item is currently out of stock and while they are items we order regularly, we do require a 10% deposit as these items may be more difficult to obtain from the manufacturer and/ or require prepayment from OPT. The balance is due when it arrives. You may cancel these orders at any time.
Here is a link to all of the stock statuses on the OPT website and what they mean for your reference: http://www.optcorp.com/ajax/category-description.php?store_code=default&block_id=stock_status
The Carbon Fiber version of the 8" RC is $1395: http://www.optcorp.com/tpo-8in-f-8-rc-ritchey-chretien-carbon-fiber-ota.htmlAnswer by: Chris (Admin) on Mar 11, 2014 8:31:45 AMI have concerns with the amount of weight the stock focuser can handle. Would an Atik 383 w/ EFW filter wheel, offaxis guider and Starshoot guide camera be to much for the unit to handle?Question by: John Massey on Dec 6, 2016 12:38:00 PMDoes this scope come with two vixen style divetails, or one vixen and one losmandy? I am considering this scope for our observatory, and we already have a G-11. Thanks!Question by: Fabiano on Nov 8, 2014 1:14:18 AMHello Fabiano,
Thank you for your question. This scope comes with two vixen dovetails so you will need to upgrade the dovetail saddle on your G-11 in order for this telescope to be mounted.
One such saddle is the ADM Dual Saddle Plate- Vixen/ Losmandy Combo- For Losmandy G11 Mounts. This replaces the existing dovetail saddle on the G-11 and this saddle has both Losmandy & Vixen style saddles.Answer by: Roderick (Admin) on Nov 17, 2014 8:47:06 AMQuestion by: Greg on Oct 3, 2015 3:16:00 PMNot natively. The standard image circle is ~28mm - enough for APS-C. You can add an optional corrector from Teleskop-Service (the TS-RCKORREKTOR - $309) to increase the image circle to ~45mm, which will then allow full frame sensor cameras to be used, but the backfocus of 109mm from corrector to sensor would be very critical.Answer by: Chris Hendren on Oct 4, 2015 1:08:00 PMHow would you clean the primary mirror? I've seem reports that it would never need cleaning bit suspect thats optimisticQuestion by: PeterO on Aug 20, 2014 1:12:39 AMTypically, cleaning would only need to be done every couple years if the scope is properly capped when not in use and not used or stored in a dusty environment.
To access the primary mirror, you would need to remove the dovetails and then remove the screws holding the rear mirror cell to the main tube and pull the primary mirror and cell out the back. Make sure to mark the tube and rear cell with painters tape or some other temporary method so that you re-attach it the same way when you are done cleaning. When done, reverse the steps to put the scope back together.
Removing and re-attaching the cell will affect collimation and you will need to recollimate the scope after re-assembly.Answer by: Chris (Admin) on Aug 20, 2014 8:57:25 AM
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