Is a wide-field scope suitable for EAA?

[AKB]

Over the last few years, I've used a 9.25" Hyperstar and a small (or small ROI) camera to do EEVA.  The thinking is: aperture is king.  Of course, image quality also matters and, despite all the tweaking, it has to be said, that a Hyperstar is not the highest optical quality... although perhaps good enough?  I've certainly been happy with my results.

But it's time to move on, and I've gone (hopefully) a bit bigger and a bit better... a RASA 11.  Of course, before it can be used seriously there are a few adjustments to be made.  FLO was brilliant in helping me sort out a problem with the focuser motor, so now all I need to do is sort collimation, tilt, and back focus.  The collimation itself seems good, although not perfect, I'm leaving it alone at the moment.  One thing to remember is that the optical axis need not be aligned with the physical axis, so it's not worth trying to sort that out.  I've made a start with tilt, but last night was cold, and short-lived since the freezing fog came in quite quickly.   Nevertheless, I made a first pass on tilt.  I think I have more to do (I know I have) and back-focus is certainly still off, but I'm starting off with evaluating this against some rather well-known, bright, and generally large objects, so that I can make easy comparisons as adjusments progress.

One thing is already clear: this is certainly a fast system.  Here's some examples from last night (using only a clear glass filter – other options yet to be evaluated, as has the choice between mono and colour.)

But will it be any good on more distant, smaller, and fainter objects?  The specs say it has a spot size of around 4.4 microns over the whole imaging circle, and I won't be using anything like that.  So I have high hopes.

Tony

 

 

 

 

 

 

Edited December 3, 2023 by AKB

[PeterC65]

I agree that aperture is important, even for EAA (I've just upgraded my mount so that I can upgrade my Newtonian to 8"). But I absolutely love the views I get with my tiny FMA135. It only has a 30mm aperture, but maybe because the magnification is low, the images are crystal clear (that may also be down to the field flattener that is built in). It struggles with fainter objects but still often manages to pick them up. I tend to have it riding piggyback on a larger scope so just leave it stacking away for longer when the object is faint.

 

[saac]

Those are a cracking set of images so if they continue like that I'd say you have a really great setup there.  I love your M45 -  5 minutes , impressive. 

Jim 

[Martin Meredith]

Fantastique! Should be good for some of the larger galaxy clusters too. I often only capture the central region but there are sometimes stacks of galaxies in the surrounding region. Looking forward to seeing more from this setup.

[ollypenrice]

If we really want mental clarity, what matters is aperture per pixel. There is just no standard, commonly used, term for this but I think there should be.

I'd love a RASA 11. Our RASA 8 (actually Paul Kummer's RASA 8)  has proved to be very stable once set up. The biggest surprise is the way it holds focus so well but another treat is the absolute freedom from dewing caused by the camera heat and fan.

Olly

[AKB]

2 hours ago, ollypenrice said:

If we really want mental clarity, what matters is aperture per pixel. There is just no standard, commonly used, term for this but I think there should be.

Up to (or, down to) a limit, surely.  Arbitrarily reducing the pixel size much below, say, the spot size (or Airy disc?) isn't going to help.

How about “p ratio” ?

Edited December 5, 2023 by AKB

[ollypenrice]

1 hour ago, AKB said:

Up to (or, down to) a limit, surely.  Arbitrarily reducing the pixel size much below, say, the spot size (or Airy disc?) isn't going to help.

How about “p ratio” ?

Sorry, I was thinking only of the speed of the system, not its resolution. Flux Per Pixel, maybe?

Olly

[vlaiv]

Aperture per pixel - is just F/ratio.

In order to truly capture the speed of system we also need relation of pixel to angles in the sky - arc seconds per pixel.

We combine the two and we get the speed formula - aperture at resolution.

Here is mental helper:

4" at 1"/px is baseline

8" at 1"/px will be x4 as fast

4" at 2"/px will be x4 as fast

8" at 2"/px will be x16 as fast

Another way to look at it is by sensor size. This is because pixel size is "variable" quantity - we can bin it to get larger pixels, but if we do that - we reduce total number of pixels for a given sensor size.

On the other hand - larger sensor will let us use larger telescope (more light gathering) to get the same field of view (and with binning - we can keep sampling rate - "/px at wanted value).

1 hour ago, AKB said:

Up to (or, down to) a limit, surely.  Arbitrarily reducing the pixel size much below, say, the spot size (or Airy disc?) isn't going to help.

This limit depends on FWHM of stars in the image, and those partly depend on spot size.

Spot size in microns is not telling much. Spot size in arc seconds it much better in telling the story of resolution.

Fast scope - say 8" F/2 with 4um spot size - is x3 less sharp than say 8" F/6 with 4um spot size, because 4um at three times shorter focal length translates into x3 bigger spot size in arc seconds (or compared to objects in the sky).