Paracorr Type 2

[oldfruit]

I have to agree John, whilst it may be present it is not too bothersome and probably not so noticeable to my inexperienced eye. At F/4 it was glaringly obvious, hence the initial enquiry.

[Piero]

In my f7.4 Tak refractor, I can see it at the edge with my Lunt 20mm 100 Deg.. not disturbing, but it's there.

In a f5 or f5.3, coma is certainly visible. Of course it's up to the telescope owner to decide whether it is bothersome and / or to do something about it.

 

Consider a dim star on axis. On fast optics without coma corrector, that star can become invisible at the edge due to the fact that it's light is spread on a larger area. This doesn't mean that you don't have coma. Same for seeing colour on a star off axis. Rather than chromatic aberration, that is more likely due to coma.

I am not saying that a coma is a must. I'm just saying that coma is there. It affects the off axis star brightness, as light is spread, and therefore it can reduce the chance of seeing dim targets. That's not my opinion, just how a fast newtonian works.

Edited September 27, 2019 by Piero

[John]

6 minutes ago, Piero said:

In my f7.4 Tak refractor, I can see it at the edge with my Lunt 20mm 100 Deg.. not disturbing, but it's there.

In a f5 or f5.3, coma is certainly visible. Of course it's up to the telescope owner to decide whether it is bothersome and / or to do something about it.

Coma in a refractor ? - I thought it was a reflector thing ?

On coma af F/5.3, I tend to take the view that if I don't see it, and I do look from time to time, it's not going to bother me

I've often followed a binary right across the field of my 6mm Ethos (265x) and it's remained sharply defined and split until it slips behind the field stop. I can't ask more than that 

 

[Piero]

Fast optics show coma, whether refractor or reflector. The Televue 100 refractor doesn't because it has lenses in place to correct coma.

Edited September 27, 2019 by Piero

[John]

7 minutes ago, Piero said:

Fast optics show coma, whether refractor or reflector. The Televue 100 refractor doesn't because it has lenses in place to correct coma.

I'm surprised to hear that. I've seen other issues such as field curvature with refractors but never coma.

https://starizona.com/tutorial/coma/

 

[Piero]

4 hours ago, John said:

I'm surprised to hear that. I've seen other issues such as field curvature with refractors but never coma.

https://starizona.com/tutorial/coma/

I apologise with the OP as this post is a bit off-topic.

Citing from the link above "Most refractors have little or no coma, contributing to their being well-suited to wide-field viewing and imaging."

This doesn't exclude coma in refractors. 

In addition, coma can be related to the telescope optical design (e.g. Newtonian telescope), but also induced by misalignments (miscollimated axes). If the focuser of a refractor is not collimated, the telescope can show coma, even on axis.

 

My second comment above came from notes I took about 2 years ago when I bought the Takahashi. I just returned from a session outside and checked the 20mm Lunt 100 deg with the Tak-100. I could not detect field curvature. On the other hand I could not exclude that the aberration was a minor astigmatism at the very edge of the eyepiece. Spotting it took some effort, as I personally struggle seeing the field stop in a such a wide AFOV eyepiece. That's also the reason why I seem not to get along with 100 deg AFOV eyepieces, and so this eyepiece doesn't get much use.. Shame as it is a very good performer, light for its class, and has an excellent eyecup.

I have seen coma with this eyepiece on my two F5.9 dobsons (8" and 12", no coma corrector) every time. It is tolerable to my eye, but it's obviously visible from about 60 deg AFOV. 60 deg is rather consistent with my other eyepieces. On-axis stars are aberration free. This is now (thankfully!) also with the 12" dob after fixing the issues I had with the mirror supports.

In addition, coma is more visible in the 12" than the 8". This should not be a surprise considering that stars are brighter in the larger aperture, and therefore the comatic smear becomes more apparent. 

 

It makes me wonder whether misalignment-unrelated coma is less visible 1) under light polluted skies, 2) with dusty optics, or 3) if the telescope is not collimated within tolerances.

Points 1 and 2 can affect the darkness of the background sky, therefore reducing the visibility of the comatic tail. Point 3 adds coma on axis, which can make one accustomed to the problem.

Edited September 28, 2019 by Piero

[John]

You should do some more scope and eyepiece reviews Piero - your eye is clearly very well tuned for spotting these aberrations

 

 

[Piero]

John, I don't think there are things like perfect views, optics, or mechanics, but just things that either work bad or within an acceptable tolerance. 

I am critical with my equipment because certain things can be improved. Others will just need to be accepted as they are. It is just a matter of recognising them. I see this as a continuous learning process. 

Your comment pushed me to test that combination again. So, thank you for your feedback. This didn't change my opinion on that eyepiece, but the additional test let me understand the eyepiece and the refractor a bit more, which is a positive thing, I feel.

[jetstream]

The Paracorr II should be delivered the end of next week and as Piero says I could really use the aperture stop for the Glatter laser.  An interesting thing to try is de collimating a newt in small steps to see what happens through the eyepiece.

[Louis D]

While using my TSFLAT2 field flattener visually in my AT72ED, I do not see any evidence of coma in the refractor's image at the edge.  However, with it removed, the defocused stars at the edge due to the severe field curvature of such a short focal length refractor masquerade as coma-aberrated.  Stars at the edge need to be refocused to see if they come to a better focus before declaring coma in a refractor.  With true coma, refocusing does not improve the tightness of the image.

[Don Pensack]

Alas, coma correctors do not correct astigmatism in an eyepiece, so even if coma is gone, astigmatism may still make the star images non-round.

In a refractor, this is likely to be the issue you see.  And if the focal length is short, field curvature in the instrument can defocus the astigmatism, making it appear even worse.

Coma isn't an issue in refractors, however.