Mirror vs prism diagonal for fast refractor

[haitch]

If you think about it... What is a correct upright image in astgronomy???

I used to have a WO prism diagonal but it wasn't as contrasty and anything bright gave a strong diffraction spike through it.

[vlaiv]

1 hour ago, haitch said:

If you think about it... What is a correct upright image in astgronomy??? 

I used to have a WO prism diagonal but it wasn't as contrasty and anything bright gave a strong diffraction spike through it.

Although I know what you mean by image orientation, and the quicker one learns that orientation is not important - the better (what is important is relative positions of objects), there is actually a good reference frame to define correct upright image.

It's after all relative to observer - if image in telescope matches that of particular observer without using any instrument - "plain eye sight", then we can say that it is correct upright image for that observer.

That kind of reminds me of the famous fallacy - "Why does mirror invert only left to right and not upside/down?" (it does not do only left/right - it is only due to "standard" position that we think of mirror being placed that it inverts only left/right - hold the mirror in your lap for example and look at it from above - image of objects in front of you will be flipped vertically).

[Gemineyes]

I recently got a Sky-Watcher 120mm f/5 for rich field observing also. I paired it with a TS Optics 2" Crayford, and an APM erecting prism diagonal. So far, in two brief checks, the APM prism is doing a bang up job. We have had very heavy smoke/haze lately plus moonglow recently - I used a 24mm TV Panoptic and star images at near the lower end of the useful magnification range are very nice. very small diffraction spikes, and field is in focus, with a touch of curvature, nearer the outer 8-10% or so of the field, which is compensated by observing slightly off axis and then they are crisp to maybe the last 1% or so. Totally so far very happy.  

I will do more testing but the combination is a winner and chroma on this doublet achromat seems so far to be very well controlled using this prism. I currently plan to use it mostly on my Porta II head on a Manfrotto 028 tripod - Excellent field-ready setup! 

 

Darren