Focal Ratio help

WaTcHuLoOkNaTm9 (Harry) · #1 29-12-2015, 12:17 PM

Hey there,

I am new to this forum and astronomy but have a quick question. I am stuck between these two scopes: bintels bt200 8inch one is 80 cm f/4 and 100 cm f/5. The f/4 is 5 bucks more expensive but I was hoping someone could tell me the advantages of each scope while considering if the larger scope is to big for the mount. Also, if I got the f/4 could I get eyepieces to get to the equivalent magnification?

Thank

(Sorry ignore repost)

Wavytone · #2 29-12-2015, 12:38 PM

I would buy the f/5 not the f/4. The f/4 is more for those intending to do imaging, not optimal for visual use for two reasons - the secondary mirror is comparatively larger (bad) and while some eyepieces are OK at f/4, many are not.

The f/5 is better for visual use. f/6 or f/7 even better, IMHO, if you can accommodate the longer length of the optical tube assembly.

WaTcHuLoOkNaTm9 (Harry) · #3 29-12-2015, 12:50 PM

Thanks for that,
so you would say the f/4 is more for astrophotography and the f/5 for visual use? I will most likely go for the f/5 then thanks. Will the heq5 goto mount hold the f/5 comfortably and is astrophotography out of the question with the f/5. However, with astrophotgraphy will the images created have more detail due to the exposure times? Also, with the f/5 regarding viewing, will i be able to see the rings of Jupiter clearly? and nebula with colour or is colour only astrophotgraphy.

Cheers

Kunama · #4 29-12-2015, 01:25 PM

The HEQ5 should be fine for either the F4 or F5. There is no reason you couldn't image with an F5 scope, it will also be a bit more forgiving with regard to focusing and collimation.

I think you meant the rings of Saturn, and yes that will show them,
Colour will only really be visible in the various star colours, you would need much more aperture to see colour in nebulae.
I have an 18" F5.6 which does show colour but not everyone's eyes are up to the task. I recently showed several people some of the common targets (M42, 47Tuc etc)
One young lady exclaimed " Wow look at all that blue/green and red " My own eye could only make out the blue green.

raymo · #5 29-12-2015, 01:27 PM

Many Newtonians will not reach focus for astrophotography, as many new
owners have found out to their cost. All Skywatcher Newtonians are suitable for imaging. If you decide on one from Bintel ask them whether
the one you select will reach focus. If both their f/4 and f/5 models will
reach focus, definitely go for the f/5. Easier to focus, and more tolerant of miscollimation of the mirrors, and more tolerant of different types of eyepieces.
The HEQ5 will handle the f/5 comfortably, that's the rig I use for imaging.
raymo

WaTcHuLoOkNaTm9 (Harry) · #6 29-12-2015, 05:30 PM

Thanks for the replies I guess il grab the f/5. They both are advertised as being for astrophotography but il ask just in case. I was wondering, if you used a 5 min exposure to take the photo would it show colour because it gathers more light. Idk. Thanks for the help

barx1963 (Malcolm) · #7 29-12-2015, 07:46 PM

Just a point, if going for visual use, an EQ mount is the less optimal answer. There are some that disagree, but the vast majority of visual observers these`days use dobs. Much easier to setup and use.

Cheers

Malcolm

raymo · #8 29-12-2015, 09:45 PM

An exposure of just a few seconds will show some colour in the
brighter nebulae.
raymo

WaTcHuLoOkNaTm9 (Harry) · #9 30-12-2015, 09:12 AM

Thanks for the replies guys. Will probly go for the f/5 but I was thinking. I am on the Mornington peninsula mount Eliza. If anyone on here is from here is it possible to go as big as a 12 inch dob?

Nab (Darren) · #10 30-12-2015, 11:10 AM

If you are not intending to do serious astrophotography - and spend a lot more money, you would (as Malcolm suggested earlier) be much better off with a dobsonian mount. You can go large (like the 12) or more modest like an 8 and still have an outstanding visual experience. You can also pocket the savings for accessories and/or eyepieces when you have some more time behind the scope you can decide what you really want/like.

A 12" dob is large and heavy. If you haven't seen one in real life it you pay to visit bintel and get a feel for what you will have to be handling. I have an 8 and at just over 20kg total I can just comfortably move it around assembled.

Cheers
Darren.

barx1963 (Malcolm) · #11 30-12-2015, 06:11 PM

Quote:

Originally Posted by WaTcHuLoOkNaTm9

Thanks for the replies guys. Will probly go for the f/5 but I was thinking. I am on the Mornington peninsula mount Eliza. If anyone on here is from here is it possible to go as big as a 12 inch dob?

The Mornington Peninsular Astro Society is very active, worth checking out here http://www.mpas.asn.au/
Dobs come in a huge range of sizes, I have used 8", 12" and 20" as my personal scopes (currently have an 8" and a 20") and I loved using the 12" that I had for 4 years.
While they are biggish, with a bit of care they are quite easy to handle if you are of average fitness. A good idea is to get along to an observing night to see some scopes in action. MPAS is an idea or if a camp interests you we have one coming up at Snake Valley near Ballarat in March and the ASV has one the week before near Heathcote.

Cheers

Malcolm