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Thread: DIY 1.25" Coma Corrector

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    Default DIY 1.25" Coma Corrector



    There seems to be a lack of 1.25" Coma Correctors on the market to help with coma on my f5 Newtonian, so I decided to make one using a design from a 1991 magazine. All parts are stock items (eyepiece barrel, filter holders and a filter extension tube) so it is easy to screw together. The CC is held inside the focus tube by eyepiece screws, allowing the distance to the camera focal plane to be adjusted.

    Last night, although almost a full moon, the clouds cleared, allowing the CC to be tested. There was a bit more vignetting and a slightly darker image, but this simple design seemed to do a reasonable job of correcting coma. See the attached images (if the resolution is good enough).

    members/bobro-albums-diy-1-25-coma-corrector-picture152180-ccimage-t.JPG members/bobro-albums-diy-1-25-coma-corrector-picture152181-cc1.jpg members/bobro-albums-diy-1-25-coma-corrector-picture152182-withoutcc.jpg members/bobro-albums-diy-1-25-coma-corrector-picture152183-withcc.jpg

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    Default Re: DIY 1.25" Coma Corrector

    The DIY CC appears to have done a nice job? Can you please provide details on the magazine article?
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    David aka Shelldrake
    Equipment: Sky-Watcher 305mm Dobsonian (f/5), 120mm refractor (f/8.3, HEQ5 mount); Orion StarBlast 4.5 (f/4), Celestron 80mm refractor (f/11, AZ3 mount); Orion ST80A refractor (80mm f/5, NexStar GTL GoTo mount), Jason 313 60mm refractor (f/15.2, EQ1 mount), Celestron 10x50, 8x56, 15x70 binoculars; Eyepieces: SWA 32mm (2"); 32mm Plossl; 20mm, 9mm Expanse; 25mm, 18mm, 12mm, 9mm X-Cel LX; 6.7mm ES 82; 5mm X-Cel; Meade 8-24mm Zoom; Orion Shorty 2X Barlow, Antares 3X Barlow; Camera: ASI120MC-S.

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    Default Re: DIY 1.25" Coma Corrector

    Quote Originally Posted by Shelldrake View Post
    The DIY CC appears to have done a nice job? Can you please provide details on the magazine article?
    Certainly. I can't show the article due to copyright, but it can be downloaded from here :
    https://www.myscienceshop.com/produc...load/tmkpdf045
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    Default Re: DIY 1.25" Coma Corrector

    Looks like it's doing a great job. It does look like the scope is really out of collimation though. Could look even better I bet. Thanks for this info!
    My eyes
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    Default Re: DIY 1.25" Coma Corrector

    I would be interested to see the test once again, after you get the scope collimated (assuming there was a need to do that).

    Which scope were you using?

    Clears,
    Joe
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    Default Re: DIY 1.25" Coma Corrector

    Quote Originally Posted by BABOafrica View Post
    I would be interested to see the test once again, after you get the scope collimated (assuming there was a need to do that).

    Which scope were you using?

    Clears,
    Joe
    There have been a few comments about the collimation of the scope. I did collimate the scope before the CC test and, as far as I could tell, the resulting collimation was ok. I'm fairly sure I understand how to collimate the scope - centralising the secondary, aligning the secondary with the primary and then aligning the primary. However, I always seem to end up with a similar result. Of course, I may not be collimating it correctly.

    Looking at the shape of the stars on the upper left of images, I wonder if the shape could be down to astigmatism? The scope is a Meade Polaris 130 - not an expensive item and likely not designed with imaging in mind.

    It wasn't until having the scope for a while that I noticed a dent in the OTA at one side the focus tube. I've pressed this out but perhaps it is having an effect.

    So I'm puzzled. Perhaps I'll take the scope to my local astronomy club to see if someone there can help/advise. I haven't done a star test of the collimation, so will try this to see if it helps with better adjustment.

    In the meantime, any advice greatly appreciated.
    Bob

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    Default Re: DIY 1.25" Coma Corrector

    Here are a couple of recent images, taken with the gap reduced between the 2 lenses to a couple of mm. Sorry about the quality - flats didn't work due to movement of the focus tube.Stars3s.jpgLeo_triplet4s.jpg
    KathyNS likes this.
    650/130 reflector, EQ2/CG5. Guiding : PHD2, Orion 50mm mini guide scope, AR0130 camera module, AstroEQ. Canon 1000D (modified).

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