coma corrector advice
Posted 13 February 2013 - 11:05 PM
Posted 14 February 2013 - 07:05 AM
I got my collimation dialed in pretty good on my f4.6 12.5" dob. On some eyepieces I have coma at the edge of field. Will a coma corrector help with this? I know this may seem obvious but some of my eyepieces show almost no coma while others show a bit. I was making sure that it would work in this case.
A couple of questions:
What eyepieces are you using? Are you able to distinguish between off-axis astigmatism in the eyepiece and coma from the mirror. In my experience, unless you are using eyepieces on the order of the Naglers, Ethos's or the clones, astigmatism from the eyepiece will overwhelm the coma..
Posted 14 February 2013 - 08:17 AM
Posted 14 February 2013 - 10:50 AM
That said, if you have an F/5 or faster Newtonian or Dobsonian, a coma corrector will greatly improve the views, especially with the wide and ultrawide angle eyepieces. What you should do is try one out in person if possible with the sort of eyepieces you have now, or plan to buy in the near future to see if the improvement is worthwhile to you. You can get a used original Paracorr on the used market for 200 bucks or less, or you can buy one from other sources as well. If you want a coma corrector, there are alternatives to paying 500 bucks for one, even though the new Paracorr is admittedly better for very fast telescopes than the older models. Try one out, chances are you will see the improvement is worth the cost of purchasing one.
Posted 18 February 2013 - 05:56 PM
Posted 18 February 2013 - 06:38 PM
Posted 18 February 2013 - 06:59 PM
Posted 18 February 2013 - 07:02 PM
Posted 18 February 2013 - 08:30 PM
-- The coma corrector corrects coma where it exists, meaning the outer edges of the field of view. It seems to not have much of an impact on-axis (in the center of the field of view) so that makes planetary observing pretty much a wash with or without the corrector, since it is rare for anyone to position Jupiter or Saturn at the edge of the field of view.
-- If it works for your scope to improve images at the edges, just leave it in the focuser all the time. As far as I can tell there a CC delivers only positives, and no negatives, in fast scopes.
Posted 18 February 2013 - 08:37 PM
Is the high point coma corrector really that effective? It seems optimized for my f4.5.
Posted 18 February 2013 - 09:06 PM
Posted 18 February 2013 - 09:10 PM
Posted 18 February 2013 - 09:15 PM
Posted 18 February 2013 - 09:21 PM
Try focusing with the eyepiece all the way ito the CC. Still see coma at the edge? Pull the eyepiece 1/8" out of the focuser, refocus and look at the edge again? Coma better? Repeat until you find the position of the eyepiece where coma is least. It might be with the eyepiece pulled out of the CC to a set distance. You might need to add parfocalizing rings to your eyepieces to adjust them all for your CC.
Now, without refocusing the scope at all, put the next eyepiece in and pull it out of the CC until the stars are in focus. That will be the correct setting for that eyepiece.
In the same manner, you can find the optimum position for all your eyepieces.
Your eyepieces will all be parfocal from that point on, and the amount of coma correction will be equal on all your eyepieces.
You may find one of your eyepieces needs to move in farther than the CC body allows in order to get to the perfect position. Don't worry about that eyepiece and just use it all the way in.
So as long as you have parfocalizing rings, you don't need a tunable top. The tunable top just does the same thing by moving the top up and down instead of adding parfocalizing rings on the eyepieces. But, using the rings, one could do the same procedure with the Paracorr and only use one setting of the tunable top. Some creative types are doing just that.
In my scope, by adjusting the top, all my eyepieces focus in a 1-2mm range of focuser travel.
Posted 18 February 2013 - 09:40 PM
Posted 19 February 2013 - 12:33 AM
I take mine out when I am putting my scope away. I don't believe it negatively affects the planetary views but, that is just my opinion. I have heard other people say that they see a difference. Try it both ways and see what you think. That is all that matters anyway...
I also viewed Saturn and left the coma corrector in. Everything seemed be clear. The Cassini division stood out fine. Does a coma corrector affect planetary views. Is there a time when it should be really taken out?
Posted 19 February 2013 - 09:21 AM
Only one really cheap one by Meade still exhibited any.
Are you sure it was coma and not astigmatism in the eyepiece? Most eyepieces have problems in scopes fast enough to need a Paracorr.
Plantary viewing and the Paracorr. In a 14 inch f/4.5, the diffraction limited field is 2mm or about 4 arcminutes. That's an AFoV of 14 degrees at 200x, not a very big sweet spot.
Posted 19 February 2013 - 02:23 PM
Posted 19 February 2013 - 10:47 PM
Posted 20 February 2013 - 02:53 PM
It works well, but would be really good if more information on its optical performance could be made available.
Posted 20 February 2013 - 08:49 PM
Posted 20 February 2013 - 09:04 PM
Best coma correction will occur with the coma corrector at only one place in the optical cone coming from the primary, and with the focal planes of the eyepieces at a set distance from the lens.
This is accomplishable with parfocalizing rings, so it's not all that complicated. When you're done, all your eyepieces will be par-focal (if they can get to an optimum position), and coma correction will be at the best deliverable by the design of the corrector.
Posted 21 February 2013 - 12:34 PM
According to High Point you just put it in. It is adjusted for f4.5. Your link is dead.
Back focus is independent of focal ratio. If it is the same as the Astro-Tech, GSO, etc, then additional spacing will be needed. I have fixed the link above.
Posted 22 February 2013 - 11:39 AM
The coma correctors on the market are designed to have a set distance from the eyepiece's focal plane and the lens in the CC.
Reading through Chris' user guide, link fixed he says the distance is around 75mm, and added a 19mm spacer to his CC (and that was just to reach 66mm).
If that spacing is typical, and we wanted to get to the optimum (75mm), using your method instead of putting spacers into the CC, we would be putting a parafocalizing ring at (19 + 9) 28mm.
That seems like a looooong way out for a parafocalizing ring.
Do I misunderstand something?