Paracorr

[mill (Martin)]

No Colhut it is not a defect, it is just very hard to make an mirror system that is fast and without coma, just think $$$$$$$.
The better the mirror the better the picture but the better the price and even then nobody can get rid of coma unless you individually could polish that all out (don't even know if that is possible).

[mill (Martin)]

Ps.
Think about the light bouncing of the mirror.
In the middle it bounces up almost straight and at the edges it will be on an angle.
So the faster the mirror is the worse it gets.
Please anyone correct me if i'm wrong.

[casstony]

Here is some more detail about coma: http://www.opticalmechanics.com/about_coma.htm

[ColHut (Colin)]

Originally Posted by casstony

Here is some more detail about coma: http://www.opticalmechanics.com/about_coma.htm

Ahhh.
So either use very expensive eyepieces or longer focal ratios - which means smalle FOV so wider (than 2") eyepieces which are prohibitively expensive...

I see!

[OneOfOne (Trevor)]

Originally Posted by ColHut

So is it a defect in the shape of the mirror that is corrected for?

My understanding is that Coma in a Newtonian design is a result of the design. Just like chromatic aberation is a result of a refractor design, the light goes through a lens and you get dispersion of different wavelengths causing different focal lengths for each wavelength. You can correct for it (eg APO) but it is still there. The ParaCorr just helps to correct for the aberation caused by the Newt design, you can alter the design and maybe even get rid of it, but it wouldn't be a Newt, and would cost a lot more. On a photons per dollar basis, a Newt is probably the cheapest you can get!

[ausastronomer (John Bambury)]

Hi Col,

No its not a defect as such. No optical system regardless of its design or parameters is perfect. Some get very close and the more money you spend the closer they get. For instance even the best Ritchey Chretien Telescopes costing over $100K suffer from field field curvature. This is an aberation inherent in the design. Coma is an aberration inherent in any fast newtonian telescope, regardless of how perfect the mirror is. The worlds' best F4/1200mm focal length mirror will inherently have exactly the same amount of coma as the worlds worst F4/1200mm focal length mirror. What causes it is quite complex mathematically and optically and not worth going into here. Fact is it's there in every newtonian and the steeper the radius of curvature of the mirror the worse it is.

A paracorr is an optical device inserted into the focuser before the eyepiece and it corrects for coma.

CS-John B

[Don Pensack]

Use a label maker to label the settings of the Paracorr 1-5. Then, evaluate which setting of the Paracorr best corrects the edge of the field of each eyepiece. Start with the out-most setting, focus and look at the edge. Move the Paracorr in one setting, refocus, and compare the edge with the last image. There will be one setting that results in the best correction for that eyepiece.
Use your label maker to put a number on the side of the eyepiece for the appropriate Paracorr setting.
Then, when you're in the field, just match Paracorr number with eyepiece number--no memory or chart necessary.
I note that my eyepieces use all 5 of the Paracorr settings.

[MarkN]

Well I deliberated for an eternity but finally got a Paracorr for my Newt. I simply wasn't satisfied with what I was seeing.

I can only say it knocked my eyes out with the general improvement in viewing. In particular, open clusters such as NGC3532 take on new life. Clean, sharp stars right across the FOV and more surprisingly, tiny pinpoints of stars becoming apparent where they weren't before, making the view even more pleasing.

Clearly, this is one purchase I'll never regret.

Mark.

[wavelandscott (Scott)]

I wish everyone would stop posting "How happy they are with their Paracorr purchase" and related comments...

It makes me envious and will utlimately cause me to buy one too...

Just kidding...they sound like they are a helpful tool to extract the best performance from a reflector...

[OneOfOne (Trevor)]

Originally Posted by MarkN

Clean, sharp stars right across the FOV and more surprisingly, tiny pinpoints of stars becoming apparent where they weren't before, making the view even more pleasing.

Clearly, this is one purchase I'll never regret.

Mark.

My observations also, I seemed to be able to see stars that just weren't there if I took it out! Maybe I need to put in on my C11 to fix my collimation problem (another thread).

[JethroB76 (Jeff)]

Are there different models of paracorr?

I keep seeing references to paracorrs "with a tunable top", is this a newer model? Better?

[Don Pensack]

I feel the same way about the Paracorr. It's just "glued in" to the focuser. I never want to go back to the "going to lightspeed" view of space my scope had before.

There are two different versions of the Paracorr: a photographic version without a tunable top (which has a smaller diameter lens appropriate for imaging), and a visual/photographic version with a tunable top (needed to allow control of lens to eyepiece distance) and a larger lens (to prevent vignetting with eyepieces having large field stops).

[wavelandscott (Scott)]

[QUOTE=Don Pensack;239161]I feel the same way about the Paracorr. It's just "glued in" to the focuser. I never want to go back to the "going to lightspeed" view of space my scope had before.

QUOTE]

Maybe you mentioned it before and I just forgot but what scope(s) do you use it in?

[skies2clear]

Originally Posted by Don Pensack

I never want to go back to the "going to lightspeed" view of space my scope had before.

Nicely put Don!

CS

[Geoff45 (Geoff)]

It's an ugly fact of life that a single optical surface can't give you everything you want. There is no coma with a spherical mirror , but you get spherical aberration , which you can correct with a schmidt camera correcting plate. A parabolic mirror gives a perfect on axis image , but gets worse as you move away, so you need some correction to get rid of the coma. Same sort of thing with refractors--a single lens is hopeless, a doublet is better and a triplet (apo) is better still.

[Starkler (Geoff)]

Well after last nights viewing experience I'm the latest to jump onto the paracorr bandwagon.

I have one on loan along with 17 and 22mm naglers on a try before buying basis and in a nutshell, the paracorr cleaned up the EOF in every eyepiece I tried it with.

I have written before that I wasn't that impressed with naglers due to seeing astigmatic seagulls and flares in the outer field, well the paracorr cleans them right up

Heard about 14mm Pentax having field curvature? The paracorr nicely flattens the field and now shows sharp stars edge to edge even in my little 650mm focal length newt.

The 24 panoptic in the paracorr? Beautiful

Am I going to buy it? Definately!

[AstroJunk (Jonathan)]

Yeah, there is no turning back! And the better the optics the more important they are I find - how typical is that!

<off topic> Have you done a review of that fine scope of yours yet? i've been otherwise engaged for a while and not noticed.</off topic>

[Sentinel]

Just to remind myself how valuable the Paracorr is, I decided to observe a couple of hours without it, last night.

I just goes to show how you can get used to the Paracorr and not appreciate the job it does in sharpening the stars up. I have used the Paracorr exclusively for 6 years now, and tend to not appreciate it's worth until I do not use it.

[Don Pensack]

I periodically do the same thing. Many people do not use theirs at high power, so I occasionally remove it when I'm using 5-9mm.
Oops!
I immediately put it back in.
You might say I've gotten used to pin-point stars.
"Don't it always seem to go, that you don't know what you've got till it's gone"
Don

[erick (Eric)]

So, should I save my pennies to buy a Paracorr and use it with my el-cheapo eyepieces, rather than attempt to save several times that amount of money to buy a full suite of excellent (but expensive) eyepieces?

Could I indeed make a silk purse out of a sow's ear?