Virgo Cluster

[Pinwheel (Doug)]

I saw the Virgo cluster of galaxies last night but can't photograph it as my 12" dob is an F5. As you astro photographers are so good at this, how about posting some photos of it. I'd love to see the Virgo cluster in more detail.

[cometcatcher (Kevin)]

What do you have against F5?

How about Jo's excellent M86 and surrounding area? http://www.iceinspace.com.au/forum/s...d.php?t=119778

Or http://www.iceinspace.com.au/forum/s...d.php?t=119997

[Pinwheel (Doug)]

Originally Posted by cometcatcher

What do you have against F5?

Nothing. My F5 has no back focus & to get it, I need to cut 50mm off the tube. I'm not ready to cut up my new 12" just yet also my scope is a Dob without any tracking.

[raymo]

The f/5 statement could be misleading to newbies Doug. The f number is irrelevant; it's just the construction. My f/5 has enough back focus.
I understand your frustration from earlier posts in other threads, and sympathise with you. I don't recall how much more back focus you need,
would a low profile focuser not be enough? Whoops, sorry, gone off subject.
raymo

[Pinwheel (Doug)]

Originally Posted by raymo

The f/5 statement could be misleading to newbies Doug. The f number is irrelevant; it's just the construction. My f/5 has enough back focus.
I understand your frustration from earlier posts in other threads, and sympathise with you. I don't recall how much more back focus you need,
would a low profile focuser not be enough? Whoops, sorry, gone off subject.
raymo

Good point, I'm guesstimating 25-35mm so. Just for interest, can you measure how long your F5's tube length is.

[cometcatcher (Kevin)]

The Baader MKIII coma corrector will also buy you a few mm of inside focus and you will need one anyway for prime focus.

[raymo]

My 8" f/5 OTA is 910mm overall. Using the same construction
dimensions scaled up the 12" would be approx. 1355mm. I
imagine a low profile focuser would gain you at least 30mm,
maybe a little more; combined with the aforementioned coma
corrector you might make it. There are often ways of moving the mirror
up the tube without chopping the end off, depending upon the
type of mirror cell, and how it is installed.
raymo

[Pinwheel (Doug)]

Originally Posted by raymo

My 8" f/5 OTA is 910mm overall. Using the same construction dimensions scaled up the 12" would be approx. 1355mm.

Well that works out perfectly, my tube is 1405mm so take off 50mm would make it 1355mm too! Now you've got me thinking should I cut it back...

[cometcatcher (Kevin)]

Originally Posted by Pinwheel

Well that works out perfectly, my tube is 1405mm so take off 50mm would make it 1355mm too! Now you've got me thinking should I cut it back...

Before you start cutting... Do you have a tracking mount for the big dob? Without it you could do planetary but not galaxies.

[raymo]

As I said, why not look into moving the mirror up inside the tube, rather than chopping it off. Maybe make some 50mm spacers to go between the end of the tube and the cell. I don't know how the GSO is constructed.
raymo

[Pinwheel (Doug)]

Originally Posted by raymo

As I said, why not look into moving the mirror up inside the tube, rather than chopping it off. Maybe make some 50mm spacers to go between the end of the tube and the cell. I don't know how the GSO is constructed.
raymo

Actually I'm investigating this option first. I have an idea for a 50mm x 300mm circular plastic or aluminum honeycomb spacer to fit under the mirror. Then it's just a matter of getting 6 bolts 50mm longer to secure the mirror to the assembly.