F15 or F7.5 refractor views

[moriniboy]

Seen quite a few 80mm F15 scopes for sale recently and am wondering why, they seem to be the ideal scopes for lunar / planetary.

If l used a different ep to get the same magnification as the F15 in my F7.5 refractor would the views be that different?

[John]

If the F/7.5 was an achromat you would notice some chromatic aberration around brighter stars, planets and the moon. This would be largely absent from the F/15. If your F/7.5 is an ED (eg: Skywatcher ED80 etc) then there would not be much to choose between them in my opinion. The F/7.5 would be easier to mount steadily though - these long tubed refractors place a lot of demand on mounts too ensure that they are vibration free - it's not the weight of the tube that the issue, it's the length.

[AndyH]

Not 80mm, but similar idea..........

60mm F16.7 achro vs 66mm F6 ED

My Carton 60mm F16.7 completely and utterly blows away, beyond belief, the views(at the same mag), through the Astro-Tech 66mmF6 ED frac I had. It's not even close. The 66mm wins on FOV of course and was a great, great scope. Not in the same league as it's smaller and longer brother.

Clour correction on the 66mm was excellent. I needed it to be as I mainly used it for birding. Nothing more annoying than getting a purple fringe round a distant bird in a tree, backlit by a bright sky. Basically it was this scope rebranded - http://www.longperng.com.tw/goods.php?act=view&no=2

Andy.

[hunterknox]

It's because a lot of optical aberrations are easier to control at longer f ratios. Making an excellent f15 is a lot easier than making an excellent f7.5 or f6.

[John]

Not 80mm, but similar idea..........

60mm F16.7 achro vs 66mm F6 ED

My Carton 60mm F16.7 completely and utterly blows away, beyond belief, the views(at the same mag), through the Astro-Tech 66mmF6 ED frac I had. It's not even close. The 66mm wins on FOV of course and was a great, great scope. Not in the same league as it's smaller and longer brother.

Clour correction on the 66mm was excellent. I needed it to be as I mainly used it for birding. Nothing more annoying than getting a purple fringe round a distant bird in a tree, backlit by a bright sky. Basically it was this scope rebranded - http://www.longperng...p?act=view&no=2

Andy.

Thats great Andy

FWIW my Vixen ED102SS (f/6.5) matched the views put up by a Skylight 4" F/15 a couple of years back. The Vixen is a darn sight easier to mount and cost me around half the price of the Skylight

[nightfisher]

I have the 105 f15,and the views are free of false colour, superb, but by eck its a pain to mount

[Kevin Ellis]

Assuming your interest is the Moon, planets and double stars.....

The F15 will be kinder on your eyepieces than the F7.5. Simple and cheaper eyepieces like the plossl or Edmund RKS will work well in the F15. With certain exceptions (ie Radians), your eyepiece will also have greater eye relief for a given magnification. For x150 you will need an 8mm eyepiece in the F15 but a 4mm eyepiece in the F7.5, unless you use a barlow.

[Mr Fiery Jack]

The longer fl in an achromatic scope will always produce the most pleasing views...tricky to mount though.