My friend Rodney Watters who lives close to me in the Central West of NSW, Australia told me that he was imaging the wonderful distorted galaxy NGC 1532. He encouraged me to image it as well, with a view to comparing the result for his shorter focal length APO refractor with that for my longer focal length SCT.

Rodney's image is at left and mine to the right.

Rodney’s set up was as follows:

Tak TSA 120, 5” APO refractor with 0.75X reducer

Focal length: 669mm

QSI 683 camera, with 5.4 micron pixels

Image scale 1.66 arc secs per pixel

FOV: 1.6 degrees x 1.2 degrees

60 hours data collection, with drizzle integration

https://www.astrobin.com/n7sx2f/C/?nc=user

My set up:

14” SCT (Edge HD 1400)

SBIG 16803, with 9 micron pixels

Focal length: 3910mm

Image scale 0.47 arc secs per pixel

FOV: 0.54 degrees x 0.54 degrees

16 hours of data collection

https://www.astrobin.com/4vl7rj/?nc=user

Rodney's imaging rig produces beautiful wide field images, with all the benefits that go with that, but with a moderate image scale. My set up gives a smaller FOV, but a smaller image scale.

In general and simplisticaly, the resolution in an image is a function of either the seeing or the sampling. One will be controlling. It is widely accepted that to have "good" sampling the image scale needs to be ~ 1/3rd the seeing. If the image scale is smaller than this, it is said to be oversampled, with minimal gain in resolution. Note there will always be some gain with better sampling, but the returns are diminishing. If the image scale is larger than 1/3rd the seeing, then the resolution achieved in the image is not as good as the seeing allows and it is said to be under sampled.

Accepting the 3X "Nyquist" multiplier means that for Rodney's set up, when the seeing is less than 5", he is under sampled. For mine, I become under sampled when the seeing is less than 1.5". Given that in our region the seeing typically runs in the 2" - 3.5" range, this means that the resolution of fine detail in my images should be superior to that of Rodney's images.

Although this is not entirely obvious when looking at either image in its entirety, it becomes clearer that there is quite a substantial difference in resolution, when zoomed in on the galaxy.

This is not to say one set up is better than the other. It is very clear that many wonderful images are produced with a level of under sampling, with the benefit that a wider FOV brings, however, as this comparison demonstrates, there is a clear resolution benefit in being adequately sampled.

As a final caveat, it is difficult to control all the variables in such a comparison, especially things like the seeing and the processing that both Rodney and I applied to the datasets. Nevertheless I think this is clear enough to support the conclusions I have drawn. I hope you agree.