Help with choice of a second scope for DSOs and why do so many come up as under-sampling?

[SiD the Turtle]

Looking for my next scope to start some proper deep sky imaging and after some advice. Current setup is:

• Celestron 127 MAK
• HEQ5 unmodded
• Skywatcher 50ED guidescope + ZWO ASI120MC-S guide cam.
• Canon 600D but looking to invest in a dedicated camera next year.
• Controlled via Astroberry/Raspberry Pi.

As you can guess, the very high focal ratio was good as my beginner's scope but a) it takes some long, hard to track subs to get a decent picture and b) the reduced field of view means I can't image anything really beyond galaxies.

So what's next? Budget is probably around £500-£700 for the scope and any associated bits like field flatteners.

I like the look of something like the William Optics Zenithstar 73 and William Optics in general. I know there's the argument that they're pricey but a few pros are:

• Built in top rail to mount my guidescope (don't like dealing with the flexure issues with the way it's currently mounted).
• Built in bahtinov mask.
• Decent focuser mechanism I can hang a camera and maybe later a focuser and filter wheel off of without issues.
• Refractor which is lower maintenance than a reflector.

Downside is needing to purchase a field flattener, but that seems to be the case with pretty much all scopes. The exception is the RedCat but they're like gold dust thanks to COVID and I'd like to have a scope I can attach a motorised focuser to in future.

So questions:

1. Is that a suitable scope for a second purchase, considering my existing kit?
2. What's the deal with these scopes and CCD suitability? According to the calculator such a scope with my Canon 600D or something like the ASI1600-Pro comes in as under-sampled, and even more under-sampled with the flattener/reducers. But I see so many of the big YouTubers, people on here etc with similar combos.

Thanks in advance!

[SiD the Turtle]

Anybody able to comment?

[fifeskies]

Your Canon 600D (I have the same) and an ASI1600 both have fairly large sensors as CCD go.

Matched to a 4inch ED scope (I have a Vixen 4 inch and an 80mm (3in+) Skywatcher they fit well as they can then make best use of their resolution as regards sampling.

The smaller 72 scopes (3in -) will undersample , but not by a lot , and you should not be put off by this.

The online guides are just that , a guide not an absolute rule.

Having the larger sensor means that you can get good use out of the camera if you go up in aperature.        ( Ha Ha Ha ..... IF !!!! ).

 

 

Another way to look at it is a 73mm will give a wider view of the sky , so more ends up crammed onto a single pixel.

This means you lose a small amount of detail you could have captured, but it is not far away from the maximum you can get under typical sky conditions anyway.

The 80mm is narrower and the 4 inch about 3 times smaller area so gives finer detail , if conditions allow.

It needs a very clear night for the 4 inch to make full use of its extra resolution at this scale.

But it rather depends on what you want to look at , some of the best targets are better in a wider field anyway (eg M31) , which allows single frame capture rather than having to start doing mosaics.

 

as an illustration here is an image I got of M31 with my 600D on my 80ED with a 0.8 reducer/flattener

 

Edited November 6, 2020 by fifeskies

[geordie85]

https://www.astrobuysell.com/uk/propview.php?view=167719

This could be a good buy for you with some budget left over for a flattener.