Minimum Apo aperture/ Focal ratio for planetery imaging?

[Drakester]

Hi All,

Hopefully not a dumb question...

I understand Apo's are mainly for widefield & DSO, but if I wanted to additionally squeeze some planetery imaging from an Apo refractor what's realistically the smallest aperture I could use? and what f ratio?

Thanks.

[Deneb]

Maybe a 100ED with a focal length of 900m & focal ratio of f/9.

Nadeem.

[Drakester]

Thanks Nadeem, but will f/9 be a bit slow and compromise the DSO/Widefield stuff?

I'm researching my first scope purchase and am still hoping to find the Apo "holy grail" which will perform for planets & DSO.

Cheers

[Earl]

You can add a reducer to lower the ratio for dso work

[Rossco72]

I'm researching my first scope purchase and am still hoping to find the Apo "holy grail" which will perform for planets & DSO.

Cheers

I don't think it exists but if it does then you better let us all know...

[Whippy]

I've tried using my ED120 with a x3 barlow for lunar and planetary and while it works, it's now wholly suitable IMO. I think an SCT might be a better bet. Use a 6.3 reducer for DSO work while a barlow would make the focal length long enough for lunar and planetary.

Tony..

[Interested]

If you consider that magnification is roughly 2 x aperture in mm then an 80mm Apo will have a useful mag of about 160. That said, I have taken my scope to just over 300x with no problem.

My point above is that I don't think you will achieve what you want with a small Apo (stands by to be corrected by someone out there!)

I bought my scope for widefield/DSO and not too interested in planetary. If I had been, I would have gone for an SCT and a focal reducer (I might still do so at some point as there appears to be a lot more add-ons for SCT's to get the best out of them!)