Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

  • Announcements



Coma corrector backfocus vs camera backfocus

Recommended Posts

Bikeman55    7

I have an Orion 8" astrograph f/3.9. I'm looking to purchase a Baader 2" Multi Purpose Coma Corrector - MK III, I'm currently imaging with a DSLR which the backfocus should be fine. My question pertains to looking into the future. If I purchase this coma corrector now, which says it has a backfocus of 55mm, and later upgrade to a different camera, say a ZWO ASI178MM, will I still be able to use this coma corrector with that camera when the ZWO camera says it needs a backfocus of 17.5mm.

How does the backfocus of the coma corrector relate to what the camera wants for backfocus.

Hopefully I've worded my question well enough. Any help would be appreciated.

Thanks in advance.

Share this post

Link to post
Share on other sites
symmetal    79

The term backfocus is very misleading and means different things to different people.

The 55mm 'backfocus' of your CC is the distance required between the CC back plate and the image sensor. A Canon DSLR has a 'backfocus' distance of 44.1mm between the sensor and the front of the lens flange. The T adapter has a depth of nominally 11mm. 

The 44.1 + 11 is about 55, the distance of your CC 'backfocus'. Hurrah!

Your 17.5mm 'backfocus' of the ZWO is the distance between the sensor and the front plate of the camera. This means you would need a T extension of 55 - 17.5 = 37.5mm to achieve the CC 'backfocus' distance.

The small chip size of the ZWO compared to your DSLR means that the CC probably isn't needed as only an area  equivalent to a centre cut-out of your DSLR is being used so the coma error will be much reduced.


Edited by symmetal
  • Like 3

Share this post

Link to post
Share on other sites
Bikeman55    7

Fantastic. Thanks so much for the response. That perfectly answered my question.

Question seems stupid now that I know the answer, I should have known that but thank you for the quick response. Much appreciated.

Share this post

Link to post
Share on other sites
symmetal    79

Glad to help.:smile: The question isn't stupid, but having the term backfocus to mean how far the sensor is mounted from the front of the camera is. :wink2:


Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By bobro
      650mm fl /130mm aperture  reflector scopes are great for imaging with the fast focal ratio of 5, but they do suffer from coma aberration due to this.
      My scope is no exception and seems to have especially bad coma. As coma should reduce with increasing focal ratio, as an experiment I cut a 108mm circlular hole in a piece of card and placed it centrally across the front of the OTA, effectively changing the focal ratio to 6 with the reduction of the aperture from 130 to 108mm.
      Below are two single subs : one without the card(f5 at 120secs) and one with the card (f6 at 180secs). I was surprised by the great reduction in coma in the f6 image. (Almost full Moon so rather bright subs.)
      Of course the reduction in aperture means an approximate 50% increase in exposure time, but it could be useful for brighter targets.

    • By Bikeman55
      I have just gotten set up for the first time with astrophotography and have taken my first couple images. I have an Orion 8" Newtonian Astrograph F3.9. To the best of my knowledge I am collimated extremely well, I am still learning everything but I feel like my collimation is right on.
      The problem I am having is with the coma I am getting in my images. I am using a HighPoint Scientific coma corrector but it doesn't seem to be doing much, I wanted to take some photos without the coma corrector to see how it would compare but I don't have the proper adapters to attach my camera and get it to focus without the coma corrector.
      My question is, is my coma corrector insufficient and should I get a new one? My HighPoint Scientific coma corrector costs about $120, the description says that it is for newtonians with a focal range of F3 to F6 but optimized for F4.5. My telescope is definitely toward the lower end of that range and below what it is optimized for. 
      I have attached a slightly edited image to give you an example of the issues I am having for your thoughts. You can see around the edges, and even when zoomed in on the stars in the center, they exhibit pretty bad coma. 
      Just curious on what your thoughts might be. 

    • By The-MathMog
      I am looking into getting a coma corrector at some point. It might be after I get a new OTA, as that one will likely have more coma, and that coma isn't the biggest of issues right now. BUT, I've been surfing around, trying to figure out how coma correctors actually work and what other effects they might have, and it has been a bit confusing to me, especially concerning focus.
      I've read several places that some CC's will either require in-focus or even out-focus, even out-focus in such a case where prime-focus, which wasn't possible before, actually can become possible. I'll quote what I read, but I'd like people to give some comments on how that would work out:

          "I've found that the GSO requires about 10mm of focuser in-travel when using the supplied eyepiece holder and an additional 25mm of spacing between that holder and the optics unit.  For every additional 4mm of spacing, you save about 1mm of in-travel required.  So I guess if you put 110mm of spacing in there, you wouldn't change the focus point at all.  Of course, the correction would be awful.

      If you screw the same 70-75mm of spacing onto the back of your 2"-1.25" adapter, you'll need about 1.5 inches of out-travel to reach focus.  Since my focuser won't go that far, I just pull the whole assembly up and out of the focuser about and inch or so.  Thus, you'll always be able to reach focus with 1.25" eyepieces and possibly even with cameras setup for prime focus photography via the T-mount adapter that wouldn't be able to reach focus otherwise." Louis D -

      This was interesting to me, as I am actually not able to achieve prime-focus as of right now with my current OTA (Celestron 130SLT), and was wondering if acquiring a CC now instead of later, would be able to fix this for me? And if so, does the focal-length attributes of the CC determine this? Like some CC's acting as a 0.9x reducer or a 1.15x. And what CC's would you recommend that I eventually get? My current plan for the new OTA, is a Skywatcher 8'' Quattro CF, so I'll want a CC for it eventually, but if the CC can also "fix" focus issues with my current OTA, that would kill two birds with one stone as I currently have to use a barlow to focus my Nikon D5200.

      So any insight into this or general advice would be very welcome!

      -Mathias M. M.
    • By moise212
      Hello all,
      I bought relatively recent a SCT 0.63 reducer/flattener for my C6. I'm planning to use it with the ASI1600 camera, but it seems that I can't figure out the proper spacing distance between the reducer and the sensor. That or there's another problem with the scope that I can't figure out.
      The first distance that I tried was ~105mm. I achieved this by using a SCT adapter to T2 which is 50mm + a Baader VariLock set to ~28mm + the filterwheel 20mm + 6.5mm the distance from the sensor to the camera thread. + the filter width, it should be ~105mm.
      I understood that the closer the sensor is placed to the flattener, the better the field is flattened, but the reduction is weaker. I didn't mind that. I then tried to use an M48 (has also a T2 thread inside and deeper) to T2 adapter which was about 15-20mm.
      In both cases the stars were not in focus away from the center, with an out of focus elongation towards the edges (I don't know which word describes this).
      The following M13 is with the Baader VariLock and the Crescent was with the shorter adapter. I didn't have time to test without adapter at all and now it's going to stay cloudy for a while.
      Don't mind the quality, the M13 is unguided with ~20min (6s subs), the Crescent is ~20min (1min subs), guided. Very bad seeing.
      Did anyone use a C6 with a 0.63 reducer/flattener and APS-C or 4/3 sensor with success? Or does somebody know which is the correct backfocus distance and how to get rid of the distorted stars towards the edges, but still using the same scope + flattener?
      Many thanks and clear skies,

    • By Gerry Casa Christiana
      Hello all
      Im just started in imaging and I have to say I'm pleased with the first result BUT on closer inspection I see I have slightly oblong stars. So people know what I've got and done here it is  
      I have a baader coma corrector that I'm using set to 55mm from canon sensor and I have tried short exposures and I notice that it's still there in short exposures. Could it be collimation? It's the only thing I didn't do that evening. 
      I would appreciate your input!