Skywatcher 150PDS vs Equinox 80 for astrophotography

[jeffwjz]

Hi, I'm currently using a skywatcher 150PDS on a HEQ5 Pro mount for mono ccd astrophotography. Recently I noticed someone selling a Equinox 80 Apo refractor at a reasonable price and I'm tempted to have one. But before that I'm trying to work out the difference the apo can bring and decide if I really should go for that.

The main reason I'm looking at this Equinox 80 is that it will give me a better field of view for certain astro targets. F ratio wise, the Equinox 80 is F6.2, and with a field flattener it will be of a very similar F ratio as my current 150PDS. But because of the size of aperture, 150PDS has almost twice the diameter of Equinox 80, does it mean that I need 4x the guiding time if I'm using Equinox 80 compared to my 150PDS, to have the same luminance?

Any advise on this topic?

Thanks.

[vlaiv]

It's worth getting if you need wider FOV for certain targets.

Exposure time for same SNR compared to 150PDS is not as straight forward to calculate - it depends on host of factors.

In general, you won't need x4 as much exposure if you use the same camera. Although aperture is almost x4 larger (well, quite a bit less than that, because of two reflective surfaces and central obstruction and slightly less than twice diameter), focal length is shorter too - meaning sampling resolution is also lower - larger piece of sky falls onto single pixel and that means that more light of target is gathered by single pixel.

It may turn out that Equinox with field flattener / focal reducer is even "faster" than 150PDS when all things are accounted for.