Wonder if you could use these coma correctors to convert a newtonian into something akin to a cassegrain. I mean most cassegrain designs use parabolic secondaries of some sort, or something in the light path to correct for coma/astigmatism. With a parabolic primary and a flat secondary that reflects light back into the primary out through the rear hole, you'd have to worry about coma a lot, especially if the focal ratio was fast.

Coma Corrector Comparison
#51
Posted 30 August 2018 - 12:28 PM
#52
Posted 30 August 2018 - 04:21 PM
In a Cassegrain, the secondary mirror is a convex surface.
That might require a different configuration of coma corrector,
Celestron seems to have managed it in the Edge HD scopes.
#53
Posted 30 August 2018 - 04:48 PM
So what is your recommendation for the Newtonian as a coma corrector?
#54
Posted 30 August 2018 - 05:18 PM
In a Cassegrain, the secondary mirror is a convex surface.
That might require a different configuration of coma corrector,
Celestron seems to have managed it in the Edge HD scopes.
And if you investigate places like Astro Systems Austria and Officina Stellare you will find correctors (and reducers and extenders) for Classical Cassegrainians, Ritchey Chretien Cassegrainians, and for Newtonians, each with their own prescription based on the kind of scope it is going into.
And when push comes to shove, the Corrector for a Ritchey Chretien Cassegrainian at F/10 is different than the corrector for a RCC at F/8 even when both have the same back focal distance.
So what is your recommendation for the Newtonian as a coma corrector?
Paracorr 2. Yes, bite the bullet, and simply get the best.
#55
Posted 30 August 2018 - 11:36 PM
I don't know if human eyes though can tell whether coma is inside the airy disk when the eyepiece is long. People on here can tell the difference though.
Edited by stargazer193857, 30 August 2018 - 11:36 PM.
#56
Posted 31 August 2018 - 02:34 AM
I see, then P2 it is, thank you very much!
#57
Posted 31 August 2018 - 02:47 AM
Just for my curiosity, is Paracorr really better than ASA 2"/3"/4" correctors?
#58
Posted 31 August 2018 - 02:49 AM
I see, then P2 it is, thank you very much!
Of course you also need to consider the image scale - the difference between the P2 and the ASA 2korr (0.73x) image scales is 57% - ie quite a lot. The latter works brilliantly for me. Its not a matter of P2 or nothing.
I think that visually the P2 is probably the clear choice, but for imaging its not remotely so simple.
#59
Posted 31 August 2018 - 03:08 AM
Of course you also need to consider the image scale - the difference between the P2 and the ASA 2korr (0.73x) image scales is 57% - ie quite a lot. The latter works brilliantly for me. Its not a matter of P2 or nothing.
I think that visually the P2 is probably the clear choice, but for imaging its not remotely so simple.
Your post need more explanation because my main purpose is "IMAGING", so visual part isn't my plan.
#60
Posted 31 August 2018 - 03:32 AM
Your post need more explanation because my main purpose is "IMAGING", so visual part isn't my plan.
Not sure how much more I can explain. Your choice of CC will determine your image scale, ie arcseconds/pixel, and FOV.
For me, the ASA 2korr led to a scale of 0.65"/pix, which was pretty much exactly what I wanted, so I got one. A P2 would leave me considerably oversampled, and with un unnecessarily small FOV, so a poor choice.
Also - and this is worth noting - this side of the Atlantic, people arent nearly so awestruck by all things Televue anyway.
I waited for my 2korr to turn up second hand though!
#61
Posted 31 August 2018 - 07:32 AM
Not sure how much more I can explain. Your choice of CC will determine your image scale, ie arcseconds/pixel, and FOV.
For me, the ASA 2korr led to a scale of 0.65"/pix, which was pretty much exactly what I wanted, so I got one. A P2 would leave me considerably oversampled, and with un unnecessarily small FOV, so a poor choice.
Also - and this is worth noting - this side of the Atlantic, people arent nearly so awestruck by all things Televue anyway.
I waited for my 2korr to turn up second hand though!
I still or really don't know about arcseconds/pixel at all, someone gave me a short or quick explanation while i was asking about planetary cameras, but i still don't know why or how this can determine the choices of items anyway? i though if i buy any camera astro and any scope and any accessories such as corrector or flattener and filters then it will work just fine without worrying much about what you call it arcseconds/pixel, sounds it is something important to read about.
Now should i decide on which corrector first then i choose which scope to match it or i can buy the scope first then i search which corrector can match it? mostly i choose the scope first then i buy whatever it needs later.
#62
Posted 31 August 2018 - 08:14 AM
I still or really don't know about arcseconds/pixel at all, someone gave me a short or quick explanation while i was asking about planetary cameras, but i still don't know why or how this can determine the choices of items anyway? i though if i buy any camera astro and any scope and any accessories such as corrector or flattener and filters then it will work just fine without worrying much about what you call it arcseconds/pixel, sounds it is something important to read about.
Now should i decide on which corrector first then i choose which scope to match it or i can buy the scope first then i search which corrector can match it? mostly i choose the scope first then i buy whatever it needs later.
Sounds like you still have a thousand interdependent options to choose from in terms of mount/scope/CC/imager! Not easy.
arcseconds/pixel is important, but so are many other things. Perhaps you shouldnt worry about it too much right now!
And as you suggest, anything can be made to work, just not optimally. Anything is better than nothing.
#63
Posted 31 August 2018 - 08:52 AM
I still or really don't know about arcseconds/pixel at all, someone gave me a short or quick explanation while i was asking about planetary cameras, but i still don't know why or how this can determine the choices of items anyway? i though if i buy any camera astro and any scope and any accessories such as corrector or flattener and filters then it will work just fine without worrying much about what you call it arcseconds/pixel, sounds it is something important to read about.
Now should i decide on which corrector first then i choose which scope to match it or i can buy the scope first then i search which corrector can match it? mostly i choose the scope first then i buy whatever it needs later.
If your mount keeps the planet image in the center, where there is no coma, there is also no need for a coma corrector for imaging of planets.
But the resolution in your shot will be determined by the focal length of the scope--a longer focal length means a larger image scale.
That may be not optimum for your camera, depending on the pixel size of your camera.
For instance, if your telescope resolves to 1" and each pixel represents 4" of image you won't see the resolution in your images you would if the resolution of your scope matched
the pixel size.
Knowing the resolution of your camera and image scale of your scope can tell you whether you need reduction in image scale or expansion.
The Paracorr is an excellent coma corrector, but it will expand the image scale by 15%. Other coma correctors may shrink the image scale, or not change it at all.
If you are imaging planets, you don't really need a coma corrector.
But if you are imaging anything larger, you do.
Your last line is how most people choose to do it.
#64
Posted 31 August 2018 - 09:45 AM
Sounds like you still have a thousand interdependent options to choose from in terms of mount/scope/CC/imager! Not easy.
arcseconds/pixel is important, but so are many other things. Perhaps you shouldnt worry about it too much right now!
And as you suggest, anything can be made to work, just not optimally. Anything is better than nothing.
I hope so, at the end i have to start somewhere or get something, for now i got a Newtonian and F4 corrector, hope it will show me what else i need later if i want to buy more, it is just i don't know if things should match properly for optimal results as you said, i asked a lot since last year and i feel i won't do anything if i keep worry always about calculations or numbers.
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#65
Posted 31 August 2018 - 09:57 AM
If your mount keeps the planet image in the center, where there is no coma, there is also no need for a coma corrector for imaging of planets.
But the resolution in your shot will be determined by the focal length of the scope--a longer focal length means a larger image scale.
That may be not optimum for your camera, depending on the pixel size of your camera.
For instance, if your telescope resolves to 1" and each pixel represents 4" of image you won't see the resolution in your images you would if the resolution of your scope matched
the pixel size.
Knowing the resolution of your camera and image scale of your scope can tell you whether you need reduction in image scale or expansion.
The Paracorr is an excellent coma corrector, but it will expand the image scale by 15%. Other coma correctors may shrink the image scale, or not change it at all.
If you are imaging planets, you don't really need a coma corrector.
But if you are imaging anything larger, you do.
Your last line is how most people choose to do it.
I didn't know much about scopes at all, and then later i didn't know much about Newtonian until i read more and more, and i thought i can use Newtonian only for DSO imaging but it is likely i can use it for planetary imaging if i know how to keep planets on the view/frame.
I am thinking to use my Newtonian for everything possible, my 8" F5 is mainly for DSO specifically far small galaxies or even small targets, it put 1000mm focal length, so i will choose targets that serve that focal length, and i use QHY163M camera which is same as ASI1600mm pixel size, i saw nice images from this camera and Newtonian of this size, so i can assume it won't be a problem, unless those results are not good to your standard, but it is amazing nice to my standard, almost i saw 2 Newtonian results using same as or similar to this F4 corrector i have, but i still think i can have better corrector if possible, or maybe not necessary if the scope isn't good enough, a Paracorr or ASA corrector maybe need high quality mirrors or Newtonian, yes or not?!!!
I was thinking about ASA reducer corrector to bring my Newtonian little bit wider than 1000mm, but i can't tell what else i may face if i use a reducer, maybe more issues will happen, until now i didn't use my Newtonian at all for some reasons, but hope i can be ready once i decide to start using it, only have a cheap Cheshire collimator eyepiece type, have to decide if it is enough or must get something else before i use any corrector or start using a Newtonian, and with reducer it may ask for better collimating, but i want to know if the issue of coma is more in center or edges, and if i will crop little then does it matter how good is the corrector i am using? I asked about Paracorr vs. ASA corrector, there are 4 ASA correctors, one with 1.0x and the rest are not 0 changing as you know, so are all of those also not that good as i saw that test of different correctors but they didn't include all ASA correctors, and in another test old i saw it shows ASA corrector as the best and PC2 was second.
#66
Posted 31 August 2018 - 10:31 AM
An 8" f/5 has an area in the center of the field that is completely without coma that is 2.22mm wide.
The image scale of an 8" f/5 on its focal plane is 3.38' per millimeter.
So the coma-free zone, where coma is smaller than the Airy Disc, is 7.5'
Inside that zone, a coma corrector will make no visible difference.
Jupiter is around 3/4' and Mars is < 1/2' and Saturn is about 3/4' and Venus only gets up to 1'.
So for any of these, a coma corrector isn't needed.
For any object larger than 7.5', or where you want good star images in the shot, you should use a coma corrector.
An 8" scope can resolve double stars down to about 0.56"
At the image scale of your scope, that is 0.00276mm, or 2.76 µm
You can compare that to the pixel size in your camera to see if you are achieving maximum resolution in your images.
Your pixel size is 3.8µm/pixel, which is the resolution you are getting.
For wider images, the Paracorr II expands the image scale by 15% so instead of 3.38' per millimeter on the focal plane, it will be 2.94'/mm
Your resolution will not be harmed by the 15% increase in scale, but the larger image size may be useful for planets.
For deep sky objects, the field will be narrowed some, from 1.91° on the diagonal of your chip to 1.66°
Hope some of that helps. If it's confusing, just ignore it.
Edited by Starman1, 31 August 2018 - 10:32 AM.
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#67
Posted 31 August 2018 - 10:54 AM
An 8" f/5 has an area in the center of the field that is completely without coma that is 2.22mm wide.
The image scale of an 8" f/5 on its focal plane is 3.38' per millimeter.
So the coma-free zone, where coma is smaller than the Airy Disc, is 7.5'
Inside that zone, a coma corrector will make no visible difference.
Jupiter is around 3/4' and Mars is < 1/2' and Saturn is about 3/4' and Venus only gets up to 1'.
So for any of these, a coma corrector isn't needed.
For any object larger than 7.5', or where you want good star images in the shot, you should use a coma corrector.
An 8" scope can resolve double stars down to about 0.56"
At the image scale of your scope, that is 0.00276mm, or 2.76 µm
You can compare that to the pixel size in your camera to see if you are achieving maximum resolution in your images.
Your pixel size is 3.8µm/pixel, which is the resolution you are getting.
For wider images, the Paracorr II expands the image scale by 15% so instead of 3.38' per millimeter on the focal plane, it will be 2.94'/mm
Your resolution will not be harmed by the 15% increase in scale, but the larger image size may be useful for planets.
For deep sky objects, the field will be narrowed some, from 1.91° on the diagonal of your chip to 1.66°
Hope some of that helps. If it's confusing, just ignore it.
It is confusing really, i don't want to ignore anything really, so better ask simple questions and just answer it straight forward without talking much about calculations.
1. For my 8" F5, what is a good or best corrector to match? so forget about arcseconds/pixel explanation, as long you know about calculations then you just recommend me best corrector and i just go for that without much thinking, and my main question is regarding DSO imaging, not planetary.
2. You mentioned the center of the image for planetary, now what will be the case if i will use a Barlow or focal extender to get those planets closer? and what about if i use ROI to cut mosy of the edges and the planet is all around the edges and drifting slowly until the video is completely captured? there is also ADC involved, filters also, so what are those will do for corrector involved?
For arcseconds/pixel, if i will use it for planetary then my cameras will have those pixel sizes, 3.75µm and 2.9µm, and maybe if i add more cameras i will be around those pixels maybe, 178mm putting 2.4µm, but 174mm giving 5.68µm, so i will have different pixel sizes, this will make things more complicated then, and i really don't know if all those results i saw of planets done with a dob or Newtonain done by different camera were using correctors or not.
#68
Posted 31 August 2018 - 01:25 PM
#69
Posted 31 August 2018 - 01:26 PM
#70
Posted 31 August 2018 - 01:33 PM
Just for my curiosity, is Paracorr really better than ASA 2"/3"/4" correctors?
Visually, probably, photographically probably not.
What is interesting is the ASA and OS show blur spots of their correctors while TV only shows blur diameter in a graph.
#71
Posted 31 August 2018 - 01:37 PM
Visually, probably, photographically probably not.
What is interesting is the ASA and OS show blur spots of their correctors while TV only shows blur diameter in a graph.
Ah i see, good to know, it saves me bucks then, i can think about TV Corr then and forget about ASA, it is only that i really want to have a reducer sometimes, so i don't know what is the best reducer to be used with Newtonian.
#72
Posted 29 July 2019 - 12:22 PM
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#73
Posted 29 July 2019 - 04:00 PM
I wish I found this thread a year ago. I bought the MPCC when I got into this hobby. It worked great for my f5 newt but when I bought an F4 astrograph, it just seemed like I could never find great focus. It turned out the fuzziness in the stars ended up being spherical aberration due to the two element design. Picked up the GSO 4-element CC and life is good.
And i am happy that i went with F4 coma corrector from beginning [Skywatcher Aplanatic] regardless i have F5 Newtonian, i knew one day i may go faster, and i saw a comparison enough that told me that MPCC was the least one, so i simply avoided, i just hope this F4 Aplanatic will serve me well for a while and i won't need to go with Paracorr or ASA ones yet.
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#74
Posted 29 July 2019 - 09:19 PM
Visual newt secondaries cut off part of the pattern so it looks like a smaller double spike. I can see it at f5 if I look for it near the edge, but it does not bother me. I won't mistake it for anything.
Jon, a P2 lover and advocate, says that he uses his P2 even at f5.5 simply because he has it right with him so why not. He likes the difference. But he likely would not buy one just for visual f5 coma. He bought his because his f4.06 newt needed it.
Coma is a vision test, linearly getting bigger away from center of view. If you have no astigmatism, you will have a lower tolerance for coma. Some people don't mind it down to f4.2, and that depends on what eyepiece afov you have, too.
#75
Posted 14 June 2020 - 09:17 AM
Thanks
Edited by Andre Moutinho, 14 June 2020 - 09:18 AM.