I've not yet had a good enough imaging night to really test it out, but my first few runs have shown a reduction in coma, but a decent increase in the curvature,
Reading around these coma correctors seem to prefer a slight increase in distance over the quoted 55mm, I'm curious if anyone has shimmed theres and can give some ideas on how far you had to shim it to resolve the error.
What I have managed to dig up so far in various threads across the internet, with 8" and larger being calculated by someone simulating the mirror conic, is the following rough outline, which to me implies its more to do with the F stop than the size of the scope.
I’ve used a Baader Mk3 cc in both my Canon 600D and cooled OSC 2600MC for my variety of newts and never noticed any curvature issues or Star shape issues
Only bad collimation, camera tilt , bad tracking and guiding have caused issues with star shape
First time I’ve heard a Baader Mk3 cc causing issues ??
I’ve used a Baader Mk3 cc in both my Canon 600D and cooled OSC 2600MC for my variety of newts and never noticed any curvature issues or Star shape issues
Only bad collimation, camera tilt , bad tracking and guiding have caused issues with star shape
First time I’ve heard a Baader Mk3 cc causing issues ??
I also use a variety of spacers with my 2600MC camera with the Baader Mk 3 CC to achieve focus in my newts ( also Duoband filters in the mix as well )
Interesting topic!
Can you post some images comparing with and without the MPCC? I'd be keen to see the curvature that you mention. I'm at a touch over 55mm with an 8" F5 and the field of my relatively small sensor seems well corrected to me.
Bilden that is part of what led me down this path, your not the only one that noticed this quirk, however I wanted to go furthur and track down the why
This thread is the main point I found towards how to resolve it, and there are about 18 other threads along similar lines, usually for smaller newtonions, with so many "I fixed it" end points after being told to buy a shim kit, and never describing the added distance https://www.cloudynights.com/topic/5...oma-corrector/
I should add, while digging, it seems the paracorr and GSO have different offsets for different situations, the GSO seems to need to change by 1mm per 100mm of focal length delta from 850mm, which I think could probably be mapped back to F/stop, (55mm at F/6), (54mm at F/5) etc, the paracorr seems to have a tolerance about 0.7mm from some threads to completly flatten it.
Due to how small the images are from maxpilote's its not much to show, but without its saying about 12% curvature, with about 26%,
Right now I'm at 54.6mm spacing, based on my F/5 ratio, I expect I will need to add 2.5-3mm to zero out this curvature, which I was hoping to cross reference a little with this thread, the more data points, the higher the confidence I can have, and hopefully help others get better images out of this device (the conic calcs seem that once shimmed is one of the better coma correcters)
Startrek would you happen to know the total distance of your setup, (filter thickness aswell for compensation)?
Jahnpahwa I'll try and get some subs next clear night, (dont have with / without from same night), in return could you try and arrange a similar pair, and how far over 55 you are, the difference between apeture I am not yet clear on. only a clear trend on F ratio.
I'm at 48.8mm from end of last spacer to the camera front edge, another 6.5mm to sensor plane, 55.3mm total. My sensor is the zwo1600, so only 21.9mm diagonal sensor. My thinking is that my sensor size might sit inside the well flattened area as shown by the curvature mapping on the stargazers lounge link (which is APS-C at 26.7mm diagonal).
And yep, I'll grab some with-withouts next time I have the newt out
Bilden that is part of what led me down this path, your not the only one that noticed this quirk, however I wanted to go furthur and track down the why
This thread is the main point I found towards how to resolve it, and there are about 18 other threads along similar lines, usually for smaller newtonions, with so many "I fixed it" end points after being told to buy a shim kit, and never describing the added distance https://www.cloudynights.com/topic/5...oma-corrector/
I should add, while digging, it seems the paracorr and GSO have different offsets for different situations, the GSO seems to need to change by 1mm per 100mm of focal length delta from 850mm, which I think could probably be mapped back to F/stop, (55mm at F/6), (54mm at F/5) etc, the paracorr seems to have a tolerance about 0.7mm from some threads to completly flatten it.
Due to how small the images are from maxpilote's its not much to show, but without its saying about 12% curvature, with about 26%,
Right now I'm at 54.6mm spacing, based on my F/5 ratio, I expect I will need to add 2.5-3mm to zero out this curvature, which I was hoping to cross reference a little with this thread, the more data points, the higher the confidence I can have, and hopefully help others get better images out of this device (the conic calcs seem that once shimmed is one of the better coma correcters)
Startrek would you happen to know the total distance of your setup, (filter thickness aswell for compensation)?
Jahnpahwa I'll try and get some subs next clear night, (dont have with / without from same night), in return could you try and arrange a similar pair, and how far over 55 you are, the difference between apeture I am not yet clear on. only a clear trend on F ratio.
Distance from the sensor protection glass of my 2600MC to the ridge or lip of the Baader Mk3 coma corrector or where it stops into the focuser is approx 62mm
My 2” ZWO Duoband filter is screwed into the end of the Baader Mk3 coma corrector.
I use this set up in my 8”f5 newt , 6”f5 Newt and 6”f6 newt and to be honest I can’t notice any discernible curvature across the field , all my images look fairly flat and if there is any curvature it is negligible
Hope the above is of some assistance
Martin
Here’s some of my images captured in the past 6 months at two different locations ( Sydney Bortle 8 light polluted skies and South Coast NSW Bortle 3 dark skies ) using my 2600MC camera , Baader Mk3 coma corrector , with and without Duoband filter and both 8” and 6” newts
I can’t see any field curvature across the Star field in any of them and if there is it’s almost negligible
I’m more concerned about good tracking, good guiding and no camera tilt ( nice round stars ) rather than negligible field curvature from my Baader coma corrector
Newts don’t suffer to much from field curvature, their fields are reasonably flat , it’s refractors that suffer from field curvature !!
Cheers
Martin
Saying your collumation is spot on, your tilt is a little weird, and your curvature is less than 8% on average, so until we get some more data points, seems going over hurts less than being under, though I have not done the math to correct for differences in sensor size
Saying your collumation is spot on, your tilt is a little weird, and your curvature is less than 8% on average, so until we get some more data points, seems going over hurts less than being under, though I have not done the math to correct for differences in sensor size
I have nothing further to say on this post other than -
If my attached images don’t convince you that my Baader Mk3 Coma corrector does what is was specifically designed to achieve ie: eliminate coma in the field then my advice to you is choose another hobby, maybe spend your time doing a science degree in optics
By the way Collumation is spelt “Collimation”
Good luck !
Yeah, seem that there are two different issues, or two lines of sight on the same issue.
1) how to generate a flat field as assessed by software, and
2) how to generate a field that appears flat/well corrected in images.
For me, with my relatively lazy/economical attitude to the hobby, there are enough things to get "right enough" to make images look "decent" (especially as a beginner) so I wouldn't chase any imperfections that only software can detect.
I read a post by Daniel Mounsey the other day discussing similar, that we as beginners often get hung up on numbers and hyper sensitive metrics, where as in reality it doesn't make much material difference.
That said, I applaud your effort here, and if you do find a magic formula for spacing, I'd apply it for sure (oh, and I will supply some single star field subs as soon as the weather clears)
I’ll chime in here. I have an 8” f/4 Newtonian. I have just replaced my old MPCC with an MPCC3. The original was good down to f/4.5 where as the mk3 is claimed to be good to f/3.5. I still haven’t got the spacing right as I still have coma at about 80% of the field and I am quite a way beyond the 55mm spacing. To throw something else into the mix though, I have a filter draw in between it and the sensor. Obviously the glass is going to have an effect on the spacing too. Once I get it “ right “ at least visually then I will post my spacing.
Here's a single Cent A sub in blue from the other night, with MPCC at 55.3mm including a ZWO blue 1.25" mounted filter.
Still no luck on a clear night (or even just a clear moment!) here
Lower left looks pretty bad to me, upper left marginal, but right looks decent with the eyeball. I just use the stock, 2-screw eyepiece holder to hold the nose of the camera in the focuser and this is something on my list to address, with a baader clicklock.
So these are bottom left crops of my field. Marked accordingly, one is with a 55mm standard spacing. The next was at 58.5. 3.5mm extra has made this much better but I still don’t think it’s quite enough. So the image train is Camera (17.5mm deep sensor) > 20mm spacer > 21mm filter draw > MPCC.
This isn’t the guise I’ll keep it in once I’ve got the spacing correct but it will do for now. I’d rather have the filter draw next to the camera.
