Astro-Tech Photo-visual Coma Corrector & field flattener for fast focal ratio Newtonian reflectors


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The Astro-Tech Coma Corrector is a photo/visual auxiliary lens system that fits into the 2" focusers of all f/3.5 to f/6 Newtonian reflectors to drastically reduce coma and curvature of field in astrophotography and visual observing . . .
Our Product #: ATCC

Product Description

All Newtonian reflectors exhibit coma, an optical defect that turns normally point-like stars into annoying comet-shaped flares the further you get from the center of the field. The faster the scope's focal ratio, the closer to the center of the field the coma starts to become apparent.

The photo/visual Astro-Tech Coma Corrector is a three-part 2" auxiliary lens system that drastically reduces the coma and the curvature of field in a fast focal ratio Newtonian reflector during visual observing and astrophotography. It essentially makes an f/4 telescope perform like an f/8 in terms of coma. While the Coma Corrector has been optimized for use with an f/4.5 reflector, it will reduce the coma of all carefully-collimated f/3 to f/6 reflectors.

The Astro-Tech Newtonian Coma Corrector was designed for Astro-Tech by Roger Ceragioli, the well-known optician and optics designer who was formerly the Senior Optician of the Small Optics Group at the world-class University of Arizona Steward Observatory Mirror Lab in Tucson, AZ. The "small" in Small Optics Group may be somewhat of a misnomer, as Roger's "small" group specialized in mirrors and lenses with diameters up to a meter in size. The majority of backyard astronomers would probably consider a one meter scope to be a step up from "small," but it's small only in relation to the 6 to 9 meter spun mirrors the Mirror Lab is famous for producing for professional observatories.

Stars at the edge of the field change from comet-shaped flares to points of light with the Astro-Tech Coma Corrector, so that stars look virtually the same anywhere in the field photographically or with a good wide-field eyepiece. Faint stars at the edge of the field do not fuzz out to invisibility due to coma and field curvature, but show as tightly focused points as much as six times smaller than their size without the Coma Corrector. This not only is pleasing visually, but reduces significantly the need to constantly nudge a Dobsonian's optical tube to keep an object centered in the scope's "sweet spot." In effect, the entire field of a fast focal ratio Dob becomes the sweet spot with the Astro-Tech Coma Corrector in use.

The photo/visual Astro-Tech Coma Corrector does an exceptional job photographically with fast focal ratio imaging Newtonians, like the 8" f/4 Astro-Tech AT8IN system. It will increase the focal ratio by a small amount, about 10%, which is less than the 15% typical of premium coma correctors, some of which cannot be used both visually and photographically.

The economical Astro-Tech Coma Corrector has three parts. One is the 2" barrel that contains the fully-multicoated four lens/two group optical system. This 2" barrel slips directly into your scope's 2" focuser drawtube. The bottom of the 2" optical system barrel is threaded to accept standard 2" filters.

The second part is a removable 2" compression ring eyepiece holder that threads onto the optical system barrel for visual use. For extra security and a very solid accessory connection, two thumbscrews are provided to tighten this compression ring.

The 2" eyepiece holder unscrews from the Coma Corrector's lens assembly to allow a supplied male T-thread adapter to be installed directly onto the lens assembly. This allows the Coma Corrector to be connected directly to the female T-thread mounting on the front plate of most CCD cameras that is revealed when the CCD nosepiece is unthreaded.

The back focus from the last Coma Corrector lens surface to the image plane is 75mm, although this spacing is not a tight tolerance. If it is kept within 2-3mm (+/-0.1") of the 80mm point, there should be no notable change in visual or imaging performance. This assumes that the system is refocused as a whole for the best focus once the Coma Corrector is in place. That refocusing might shift the Coma Corrector inward or outward from the nominal position by as much as ~5-10mm (~0.25-0.5").

Optional third part spacers may be needed to place your DSLR or CCD at the optimum position behind the Coma Corrector, depending on the length of your imaging equipment accessory train. Some experimentation will be needed to optimize the position for your particular combination of camera, filter wheel, etc. Exact spacer thicknesses are not as important as precise collimation of the telescope and refocusing for best focus with the Coma Corrector in place.

Tech Details

Weight 11.2 oz.
Warranty 1 year


Review by:
On a 10 inch f/5 visually good for wide field, but for high power take it out. Photos with full frame Canon 6D show that while stars are round nearly to the long edge, the whole field (including the center) is slightly blurrier than the center of the field with no corrector. If you want sharp focus in imaging, this coma corrector is not recommended, a 3 or 4 element will be required. (Posted on 2/27/2021)
Review by:
This was a perfect addition to my AP kit for my DSLR. Provided crisp stars out to the edge. I moved to a Skywatcher Quattro when I moved from DSLR to a cooled CMOS because the long backfocus left the right out of balance on my CEM25. (Posted on 10/17/2019)
Review by:
The coma corrector appears to be really well made, but it is not accessorized properly as sold. It should come with various adapters to space it properly for typical eyepieces. I was only able to get it spaced right for 2 of my eyepieces and it is not the one that I want to use it with (56mm plossl for an F/4.5 16"--too large of an exit pupil and my 9mm Nagler--not necessary). My shorter focal length 38mm would not reach focus no matter what I tried. There should be some adapters included in the kit. So as of right now my coma corrector is just sitting on the shelf. Kind of a drag... (Posted on 7/22/2019)
Review by:
The AT coma corrector was a godsend for imaging with my stock Canon DSLR. The ~78mm of backfocus needed was not an issue while working with this corrector and the stars were sharp all the way to the corners. However, with my cooled CCD, that extra 23 mm of back focus made things a little ungainly for balancing. Overall a great starter corrector! (Posted on 7/22/2019)
Review by:
I've used this Astro Tech coma corrector for the past 3 years and it is a god-send! I was never happy with my deep space images until I ponied up the money and bought this bad boy. Works well with my Astro-Tech 8" Newt and my Nikon D5300. Flat images to the edges, no more eggy stars! A must have for those fast newts. (Posted on 1/14/2019)
Review by:
I got this for my f5 newt, it works wonders after I got the spacing down for my camera and eyepieces. I recommend for anyone needing a cc for visual or photography. (Posted on 1/7/2019)
Review by:
I've been using the Astro Tech Coma Corrector for a little over a year now with my Meade 16" f/4.5 Lightbridge and it pretty much lives in that telescopes focuser now. This corrector paired with my 24mm Panoptic in my scope gives a very flat field of view out to the very edge, very aesthetically pleasing. Without the corrector, the coma is very apparent in the 24mm on the outer 1/3rd of the field of view. (Posted on 12/1/2018)
Review by:
Use it on a Meade 12 inch Lightbridge. With a good setup, it does a good job of correcting the views. (Posted on 11/22/2018)
Review by:
If you only use Plössls, you don't need this. It was after using 34mm and 20mm wide field eyepieces on my 10" dob that I decided to read and learn about coma, which led me to wanted more photons "hitting the eyepiece where they should", and ended up getting this coma corrector to fix it. The big issue with this coma corrector is that it requires extra spacing, but it does not come with the spacers (nor even documents the length needed). I would not have been able to set it up if it wasn't for the CloudyNights thread about "setting up the GSO coma corrector", but once I had a 20mm M48 spacer and knew the procedure, I was able to tune my eyepieces and get a much cleaner and gorgeous field of view. I really like looking at open clusters, and the way this coma corrector "cleans" the look of stars is worth it.
Caveats: Setup is complex (the price you pay for saving money compared to Televue's coma corrector), and it requires collimation to be spot on (so you should get good collimation tools before you consider any coma corrector). Finally, make sure you get well corrected eyepieces: The fast <F5 optics that show coma from a primary mirror also induce astigmatism on inexpensive eyepieces, so make sure your widefield eyepieces are well corrected for astigmatism (Meade SWA/UWA, ES68/82, Lunt, Pentax, Televue or similar) since the coma corrector will not fix eyepiece astigmatism. (Posted on 11/15/2018)
Review by:
So I had purchased an Orion 8inch f/3.9 Newtonian Astrograph, and from what reviews I read, when doing astrophotography, a coma corrector is a must, due primarily to the fast optics in the astrograph. Without a CC stars around the FOV are comet shaped, with the comet heads pointing inwards towards the center, but I purchased a CC (which this among many others are made by a company called GSO) and although sometimes I have trouble getting a good focus this fixes the comatic aberration pretty well. It’s a great starter CC but i do plan on upgrading in the future, either way I do not regret this purchase! (Posted on 10/31/2018)
Review by:
Using an f/5 Newtonian reflector, with several lenses, great color and flat fields. AFOV was superb! Thanks, Astonomics for great advice, great products and Cloudy Nights. Makes astronomy fun and educational. Looking forward to upgrading my mount and ota with you next year. I have purchased many products and accessories from you over the years. Since my retirement, astonomy is my passion-viewing the glories of creation!! Thanks again! (Posted on 10/14/2018)
Review by:
I ordered this photo/visual comma corrector to replace my Televue comma corrector. The Televue is considerably heavier than the Astro-Tech corrector. I did have to buy a 10mm spacer to make it work correctly with my 20D. but that is minimal. I am very satisfied with the results it produces for the price point. Plus, its lighter and makes balancing easier.

I would recommend to anyone with a fast focal length newtonian. (Posted on 8/4/2017)
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