How wide can you go in a C8?

[parallaxerr]

Question - How wide an AFOV EP can you go in a C8, before significant abberations become visible. And, what abberations would they be? Coma, field curvature?

A little bit of subjective question I know, but I'd be interested to hear peoples experiences on various AFOV EPs in the C8. I am currently using 50° AFOV EPs which are sharp and flat to the edge and would like to know at what point, the extra AFOV of a new EP may become undesirable/unusable.

[Richard Hather]

I use a 13mm 100 degree ethos in my C6 and I see no field curvature at the edges or coma.

Maybe if I had more experience I would notice the subtle differences but I'll be honest I've looked and scrutinised and I can't see any?

Just a lovely wide view and bright and clear objects to observe.

I know the limit for my scope because of the small baffle tube is around 24mm 68D or a 32mm plossl 50D for the C8 I presume you can push it further as the baffle tube in a C8 is bigger.

Im gunna push the limits of my scope though and try a 20/21mm 100D EP which would be slightly above the limit of my scope but the eye is forgiving so I want to see for myself.

So to summarise I reckon the widest you could go could easily be a 28/31 mm 82 degree EP as the baffle tubes bigger but I'm sure other members know for certain.

Richard

[cloudsweeper]

Jon - assuming the C8 is the same tube as the 8SE, I don't think the 'scope gives much coma or field curvature, so those effects would most likely be from the EP.

I've been using a Revelation 42/65 and a 30/68, and they are good except at the edges where there is some astigmatism. 

The 30/68 has now been replaced with an ES 30/82, and I am very pleased with it.  There is just a little FC at the edges, but I haven't detected astigmatism.  And additional FOV is nice, although you have to move your head/eye more to appreciate it of course!

Doug.

[YKSE]

To my understanding, the dominant aberration in C8 is usulally FC, the FC is about the same magnitude as about a 700mm FL refractor, younger eyes can have the ability to acommodate it, but not me, I can see the FC, with my 70° AFOV eyepieces, even though nothing serious.

Another potentail aberrations in C8 is SA, that'll be caused by much too long back focus when extra stuffs are added there, such as binoviewer without OCS.

If the aberration seen significant or not, it's much dependant on personal references, IMHO.

My impressions is that most SCT owners use EPs around 70° AFOV as their wider field EPs.

Vignetting seen in C8 is more likely caused the EPs or diagnals than the scope, using EPs with 46mm field stop (such 40mm  Skywatcher AERO) will have just perceivable vignetting when I look for it, the 38mm rear port size of C8 doesn't cause any serious viegetting as one might fear, simply because our eyes are not very sensitive to gradual vignetting.

[BGazing]

I use Paragon 40 (SW Aero) and perceive no vignetting. Not that I am looking for its specifically.

[michael.h.f.wilkinson]

I use a Nagler 31T5 (a.k.a. "Panzerfaust") and an LVW 42mm in my C8 to get 1.25 and 1.34 deg true FOV respectively. My other Naglers work fine with the scope. I do not notice any serious aberrations. I did see some FC in the TMB Paragon I used to have. I am not sure it is due to the scope or to the EP.

[BGazing]

3 hours ago, michael.h.f.wilkinson said:

I did see some FC in the TMB Paragon I used to have. I am not sure it is due to the scope or to the EP.

Well, there is some FC around the edges, but that is the function of the scope. But Paragon is a remarkably good EP for that money and weight, I have to say. 

[Stu]

8 minutes ago, BGazing said:

Well, there is some FC around the edges, but that is the function of the scope. But Paragon is a remarkably good EP for that money and weight, I have to say. 

Agreed, the Paragon is very good indeed  

[michael.h.f.wilkinson]

8 minutes ago, BGazing said:

Well, there is some FC around the edges, but that is the function of the scope. But Paragon is a remarkably good EP for that money and weight, I have to say. 

I regret selling it. It is excellent. I have heard that the LVW42 was designed with use in SCTs in mind, so it might have field curvature to match the SCTs. Not sure this is true (different SCTs require different curvature)

[allen g]

in my 8 inch meade f/10 can go 31 nagler and that takes me to approx 1.29