Can barlowing reduce coma??

[Bart]

Hello all

Just wondering if using a barlow on an F4.5 will reduce coma on an ultra wide EP? Just curious. Why doesn't he try it, I hear you say. Clouds, mainly. Thanks

Barry

[DirkSteele]

A Barlow itself will not reduce off axis coma. The visual appearance however will appear to have improved, but only because you have narrowed the field of view by using a Barlow and increased the magnification. Coma is worse the further from the centre of the field you move.

[alan potts]

Thats a good question Bart, I have no idea, will be watching and learning.

Alan

[John]

I think Matthew has it right. What you need is a coma corrector rather than a barlow - these can make a very significant difference.

It depends on the ultra-wide eyepiece being used as well though - most will show astigmatism at F/4.5 which will be distorting the edges of the field of view alongside the coma.

[KaStern_Former_Member]

Hello all

Just wondering if using a barlow on an F4.5 will reduce coma on an ultra wide EP? Just curious. Why doesn't he try it, I hear you say. Clouds, mainly. Thanks

Barry

Hi Barry,

coma is an off-axis-aberration. It is very obvious in fast newts.

Coma increases exponentally when the f/ratio gets bigger ( f/4 is bigger than f/8).

In a given mirror coma increases linear with axis distance.

If you use a normal 2x barlow in your newt you don`t get rid of coma.

This is because a normal barlow will not add coma.

Coma will not be any smaller with the barlow.

If you use a 200/1000mm Newt with a 10mm Plössl a star near to the field-stop of the eyepiece

will show the same amount of coma as it does in a 200/1000mm Newt with a normal 2x barlow

plus a 20mm plössl exepiece.

The reason is that in both the unbarlowed 10mm Plössl and the barlowed 20mm Plössl

the star has the same angular distance to the optical axis.

Even when you use a 10mm eyepiece alone and compare to the same 10mm eyepiece + 2x barlow

the visible amount of coma will be the same. This is because:

In the 10mm eypiece alone the star has a certain distance to the optical axis.

With an additional 2x barlow the distance is only half as big. So the coma is half as big.

But the star is magnified twice as big. And so the comatic blur will appear to have the same apparent size.

The barlowed Plössl will give better views though. That is because the Plössl suffers from astigmatism in fast scopes.

When barlowed the light cone is less steep and the eyepiece can handle it far better.

As a result there will be no more visible off-axis astigmatism.

To get rid of coma you have to do one of the following:

1) use a coma-free scope, for example like a good apochromat

2) use a coma-corrector with your newt

3) use a coma-correcting barlow (like the Klee-Barlow) for newtonians with your newt

4) use a coma-correcting eyepiece for newtonians (like the Pretoria-eyepiece) with your newt.

All these cure coma of the paraboloidal mirror because the introduce coma

of the same amount but with the opposite sign.

Here is what Dr Richard Buchroeder wrote upon the Klee Barlow:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=370444

"Fortunately, there is an alternative. Heinz Klee from South Africa designed a beautiful three-element, coma-correcting barlow lens which is now available from University Optics and is known as their 3x Barlow. It is superbly corrected for its own abberations and that of any Newtonian primary, which cannot be said for any cemented doublet barlow, which comprise the majority of Barlows on the market. We find that the residual field curvature of this 3x barlow comes close to compensating that from many of the Plossl types that have a 26mm focal length. The combination gives a coma-free, long eyerelief, superbly corrected flat-field eyepiece with about a 9mm focal length, and with more eyerelief than the same eyepiece used alone! Similar good results are obtained with shorter focal length eyepiece combined with the Klee barlow, but of course the eyerelief is less. The Klee barlow is curious and advantageous also in its extremely short length, whereas most barlows on the market are long and unattractive."

Cheers, Karsten

[great_bear]

I'm very sceptical that the Klee barlow reduces coma (especially since it was not designed to do so!)

I suspect that the reports of reduced coma when using that barlow are due to the common misdiagnosis of eyepiece astigmatism as coma. As Karsten says, Plossls (and similar simple eyepieces) suffer badly from this when used in fast newts.

[Bart]

Thanks very much folks, appreciate it. Out last night and found the 82° ES EPs had little perceptible coma but the 100° was quite noticeable. Barlowed the 20mm 100° and did think the Coma was reduced but so was the FOV, I thought. The FOV was very similar in the barlowed 20mm 100° to the 11mm 82°. Not much point in barlowing the 20mm so. Apart from the fact the combined 20mm and 2" barlow are a hefty combination that probably strains the focusser and ends up some distance from the actual telescope!

Thanks again.

Barry

[great_bear]

Out last night and found the 82° ES EPs had little perceptible coma but the 100° was quite noticeable. Barlowed the 20mm 100° and did think the Coma was reduced

In that case it is not coma but eyepiece astigmatism (which looks very similar)

- it's a common misconception as per my previous post.

[KaStern_Former_Member]

I'm very sceptical that the Klee barlow reduces coma (especially since it was not designed to do so!)

I suspect that the reports of reduced coma when using that barlow are due to the common misdiagnosis of eyepiece astigmatism as coma. As Karsten says, Plossls (and similar simple eyepieces) suffer badly from this when used in fast newts.

Hi Folks,

I can tell you that the Klee Barlow was specially designed by my fried Heinz Klee

to compensate for coma of any paraboloidal mirror down to f/4 and even faster.

It works quite well at f/3,5 too.

Mr Klee worked as a professional designer and designed advanced optics

for example for scientific use.

Mr Richard Buchroeder who commented on the Klee Barlow too worked as a

professional optics designer and in science. You can trust on his expertise.

Barlowed the 20mm 100° and did think the Coma was reduced

The eyepice astigmatism was reduced.

Cheers, Karsten