Televue plossl pushed beyond its limits

[GavStar]

I’ve been experimenting with my new Takahashi Epsilon together with my night vision monoculars and 40mm plossl.

My aim has been to use as fast a setup as possible to enable as much light to get to the night vision momoculars as possible. However, with the Epsilon working at f3.3, I think I’m hitting the limits of the Televue plossl and getting aberrations (astigmatism) towards the edge of the field.

Ive added some phone photos of the eyepiece views - conditions were awful, full moon and observing from a LP (17.8 sqm site). 

Is it astigmatism from the eyepiece I’m seeing towards the edge of the fov or is it something else?

[Ruud]

It looks like you get arcs from field rotation. Is that at all possible?

[GavStar]

Just now, Ruud said:

It looks like you get arcs from field rotation. Is that at all possible?

I don’t think so since I see these aberrations live at the eyepiece (and also if I do sub 1 second exposures)

[vlaiv]

Not sure it is down to the eyepiece alone.

Even center field stars don't look good. But then again this is not long exposure AP - this is taken at the eyepiece.

How is the collimation of the scope? At that speed it is going to be critical to get it spot on.

Btw, which model are we talking about? New Tak E130D, if I'm not mistaken, has hyperbolic primary and ED lens element to create flat field - in this case it is very important to position NVD at exact distance / spacing prescribed by scope specs.

[GavStar]

8 hours ago, vlaiv said:

Not sure it is down to the eyepiece alone.

Even center field stars don't look good. But then again this is not long exposure AP - this is taken at the eyepiece.

How is the collimation of the scope? At that speed it is going to be critical to get it spot on.

Btw, which model are we talking about? New Tak E130D, if I'm not mistaken, has hyperbolic primary and ED lens element to create flat field - in this case it is very important to position NVD at exact distance / spacing prescribed by scope specs.

I think the tracking wasn’t perfectly set up so some of the centre stars are a bit elongated.

Looking at in and out of focus stars at the centre, the collimation looks fine to me. I know collimation with this scopes isn’t easy so have left it alone so far.

This is the new 130d. I’ve had an extended play with the spacing between the corrector and the eyepiece and think I’ve got it broadly right? If it was out materially wouldn’t I see coma on the stars at the edge? I don’t think I can see coma, just astigmatism??

[fwm891]

To me there's a few problems - mainly field rotation from the polar axle alignment even with short exposures your magnifying the alignment errors. Good experiment though.

[GavStar]

29 minutes ago, fwm891 said:

To me there's a few problems - mainly field rotation from the polar axle alignment even with short exposures your magnifying the alignment errors. Good experiment though.

But I get the same elongation of edge stars at the eyepiece live views and at very short sub 1 second exposures? 

[fwm891]

18 minutes ago, GavStar said:

But I get the same elongation of edge stars at the eyepiece live views and at very short sub 1 second exposures? 

OK. I suggest then you go back to the start taking images at each stage as each piece of kit is added into the chain until you find which is causing the problem

[vlaiv]

3 hours ago, GavStar said:

I think the tracking wasn’t perfectly set up so some of the centre stars are a bit elongated.

Looking at in and out of focus stars at the centre, the collimation looks fine to me. I know collimation with this scopes isn’t easy so have left it alone so far.

This is the new 130d. I’ve had an extended play with the spacing between the corrector and the eyepiece and think I’ve got it broadly right? If it was out materially wouldn’t I see coma on the stars at the edge? I don’t think I can see coma, just astigmatism??

Corrector spacing will create astigmatism. I've recently seen a good diagram depicting sagittal vs tangential astigmatism depending on spacing (to close or to far away), let me see if I can find it...

I've searched, and searched and no luck, anyway, this is the image:

and one type is associated with too close spacing, and other with too far away, but I can't recall which is which - you should probably try it out and see if you get one type and the other - and record spacing, so you can "aim" at the middle.

(your image shows sagittal astigmatism)

[Louis D]

Have you tried putting a quality 2x Barlow in the chain to see if slowing down the light cone makes any difference?  How about using other premium eyepieces like Naglers and Pans?  Do they show the same effect?

[GavStar]

I found this thread

which suggests I’ve got too much spacing between the corrector and the eyepiece so next time I observe I’m going to go super short on the spacing to see if I can get the other star pattern in the diagram in the thread.

[GavStar]

Just got a 41mm panoptic to directly compare against the 55mm plossl. The panoptic has much better edge stars (see phone image below). It’s now clear to me that it is the 55mm plossl that is doing the aberrations. The panoptic copes with the very fast f ratio of 3.3 much better...

[alan potts]

Gav, feel you must bear in mind, as I am sure you have F 3.3 is very very fast for any eyepiece, TV I believe quote F 4 as about the limit for edge sharpness. As for the photo above all considered I feel the 41mm, which I also use a lot, has done a fine job.

Alan