Diagnosis: Coma? Astigmatism? Other issue

Symptoms: excessive star flaring throughout much of the field of view.

Patient: Astroscan f/4.2 with an intake of a 14mm Epic.

Tests so far: Gave patient 22mm Epic, symptoms largely vanished. Tried a regimen of Televue Radians... 1 dose of 18mm, no symptoms to speak of. Switched dosage to 12mm... better than when on 14mm Epic, but still some problems. Tried 2nd round with different 14mm Epic (in case there was a problem with that batch of medicine), but symptoms returned.

Consulting physician Dr. Rank suggested combining his known patent med cure with Dr. Barlow's home remedy. Gave patient mixture of 1 part 28mm RKE and 1 part #126 Meade barlow. (Which is roughly equivalent to 14mm Epic). Symptoms largely vanished except in outer extremities. Main torso in better condition by a bit than with 12mm Radian. Similar findings with 22mn Epic and barlow.

Instrument calibration: switched eyeglasses to see if problems were being caused by new prescription. Similar findings.

Observations: What gives here? I always thought that coma (an obvious issue at f/4.2) is more of a problem with longer focal length, wider field eyepieces. And that because the fov would be larger. Here the reverse is happening. The 22mm is excellent, the 14mm is where the flaring appears. (Does this have something to do with star images being larger? The effects are most noticeable on brightest stars.)

Questions for residents: The barlow cure suggests that astigmatism is what is showing up in the 14mm Epic (yet it looks decidedly like comet flares pointing away from the center of the fov). If so, then what is showing up in the 12mm Radian? Some coma?

Can seeing issues bring out the coma more? (Mars looked like the Olympic rings).

Overall diagnosis: The 14mm seems to be a pretty good eyepiece otherwise, with good contrast and shows DSOs well even at f/4.2. The 14mm is also better at f/5.4, quite acceptable, but still shows some flaring away from central area.

Notes: Any insights from attending physicians would be appreciated!

Larry:

Ah, the fun and joy of an F4 Newtonian...

I am quite sure what you are looking it is the inability of your eyepieces to handle the F4.2 light cone of the Astroscan.

The only eyepieces that are certified to F4 are the Naglers, other eyepieces will show a variety of aberrations depending upon their particular design.

As you have discovered, a Barlow will help with an eyepiece deal with an F4 scope, this is because the light cone is no longer F4 but rather F8. Most eyepieces work rather well at F8.

One of my main scopes is a 12.5 inch F4.06 Discovery. My experience goes like this:

An F4 Newt can provide nice sharp stars across the field of view, without coma and astigmatism. However it takes very good (read expensive) eyepieces and a Paracorr to get the best. With an Astroscan you are pretty much stuck for the Paracorr because 1.25 inch Paracorrs are few and far between and those of us who have one are not likely to part with it.

Your best bet is to use a Barlow when possible. I have an Orion FunScope which is a 3 inch F3.7 clone of the Astroscan. With a 2X barlow, it provides nice views. My 20mm Erfle is horrible in the Discovery without a Barlow, quite nice with one.

Here's a few additional thoughts...

1. It is my opinion what is often thought to be coma in a fast Newtonian is actually astigmatism/coma/??? resulting from the inability of the eyepiece to handle the steep light cone. With the right eyepiece, the coma from the mirror is evident but not huge...

I read a post recently where it was stated that the reason Al Nagler decided to design and manufacture the Paracorr was because he looked through a fast Newt with one his eyepieces and realized that at last there were eyepieces capable of handling the fast light cones... Personally I believe this story.

2. Barlows do help drastically. There are some inexpensive eyepieces that have built in barlows, the Expanse/Synta/Scopestuff 9mm 66 degree widefield is one such. I have one of these and it gives quite sharp stars across the field of view. The 6mm also has a built in barlow but it has other problems. Scopestuff sells these for $39 including shipping.

3. I have never really understood why the scopes like the Astroscan and the Starblast are F4, sure that provides a nominally wider field of view and a more compact design but at a significant price, F5 is just so much nicer than F4. The Orion SpaceProbe 130ST is a 24 inch long 130mm F5 Newt and it provides passably sharp views without a Paracorr and with Plossl eyepieces. A 114mm version of this scope could be about 21 inches long, still very compact and capable of a 2.7 degree FOV.

So...

Welcome to the exciting and frustrating world of fast Newtonians. My experience tells me there is really only one reason to own an F4 Newt, that is to make it more portable. In my case, my 12.5 inch F4.06 has a 52 inch long OTA which means it will fit quite readily into a small car, something a longer F5 scope cannot do.

But otherwise, the pain with collimation, the need for fancy eyepieces and a Paracorr, well it just aint worth it...

jon isaacs

While coma is certainly at work here, astigmatism is likely overwhelming the patient's 14mm Epic cohort. The appearance of peripheral minimal-blur ("best focus") astigmatic star images is also conic or "comet shaped," albeit a more "spiked" or tuft-like flare (rather than the conic "smear" of coma). Here's an interactivity test which should demonstrate the presence of astigmatism in the 14mm cohort...

By alternately positioning the two playmates closer together and further apart (either side of their optimal play separtation), you should be able to more clearly observe the Epic's astigmatism. Resulting star images will alternately "squint" along perpendicular axes, on either side of that optimal play separation. If coma were the only condition (i.e. Epic not infected with astigmatism), this same test would show no evidence of the 90-deg. alternating squint distortion.

Your suspicion regarding comatic &/or astigmatic star images being more visible in the shorter cohorts (e.g. 14mm versus 22mm) is correct: you were likely seeing the affects of higher magnification of the distorted Airy disc.

The Radians' physical makeup prevents their being similarly affected by astigmatism, particularly within their torsos (relative to comparable Epics). This is simply a function of genetics. Astigmatism remains evident and visible, however, in the Radians' extremities.

The stronger appearance of the 14mm Epic's astigmatism (i.e. more advanced or widely spread than in its 28mm sibling) is a direct result of the patient Astroscan's inherent f/4.2 condition. Introducing the Barlow significantly modifies that inherent condition (effectively changing it to f/8.4). As such, the sibling 28mm Epic becomes more compatible, and the appearance of astigmatism is reduced. In other words, you changed the Astroscan -- not the 28mm Epic (hence the much-smaller degree of observed astigmatism seen in the 28mm sibling).

As for poor seeing conditions (much stronger dynamic distortions and defocus), they will not exacerbate the appearance of coma or astigmatism. Rather, said distortions will likely overwhelm and obliterate the appearance of comatic &/or astigmatic conditions.

Although the 14mm Epic's condition in incurable, it is stable and will not worsen. So long as he and the Astroscan get along (at least to your own eye), there's no reason they shouldn't continue to play together for the rest of their lives.

Cheers & best wishes.
-Dan