WO Refractor - WOt am I missing?

tombps (Tom) · #1 28-12-2010, 04:53 PM

I think this will be a "love me, I'm thick" type of question.

Having treated myself to a William Optics 66 Petzval for my wife to give me at Christmas (from the WO Ebay website; flame!), I've just brought it out into daylight for the first time. I have an 8" SCT, a 5" Mak, and a 10" Dob that resides at a holiday spot, so this is for the relaxed and wider views.

Trouble is, I can't focus. With the focuser racked out, I still need to back the diagonal out and lift the eyepiece to find focus. The scope didn't come with diagonal, so I'm using the one from my Meade SCT and the standard Meade 26mm.

Nothing I've seen in my research suggests there should be any more extension, but I have no experience of short focal-length refractors, so I may be missing something obvious. Hopefully it isn't one of the lens elements.

Cheers,
Tom

multiweb (Marc) · #2 28-12-2010, 04:58 PM

You'll find most refractors need extension tubes to reach focus at prime.

rogerg (Roger) · #3 28-12-2010, 06:27 PM

Quote:

Originally Posted by multiweb

You'll find most refractors need extension tubes to reach focus at prime.

Really, even with a diagonal? My 80mm and 90mm meters scopes have the focusser out about 15mm when using a standard 26mm ep and WO 2" diagonals. But, I have no experience with anything shorter than my 80mm

multiweb (Marc) · #4 28-12-2010, 06:32 PM

Quote:

Originally Posted by rogerg

Really, even with a diagonal? My 80mm and 90mm meters scopes have the focusser out about 15mm when using a standard 26mm ep and WO 2" diagonals. But, I have no experience with anything shorter than my 80mm

I have two (an 80mm and a 90mm) and both need a good 180mm of extension tube before I can reach focus. Depends on the F/ratio I guess. Mine are F/6 and F/7 I think.

rogerg (Roger) · #5 28-12-2010, 06:36 PM

ok, I don't understand why the difference between yours and mine, mine are F/6 and F/7 (roughly) too. Megrez (typo in previous message of mine "meters" should be "megrez").

multiweb (Marc) · #6 28-12-2010, 06:46 PM

Quote:

Originally Posted by rogerg

ok, I don't understand why the difference between yours and mine, mine are F/6 and F/7 (roughly) too. Megrez (typo in previous message of mine "meters" should be "megrez").

Just measured my ED80. Focuser about 3/4 out + extension tube I have 165mm to focal plane from lip of tube. Will have to measure the other one but I think it's even more from memory.

dannat (Daniel) · #7 28-12-2010, 08:07 PM

I had a WO petzval with a SCT thread at the end of the focuser & i was able to reach focus with a 2" dielectric diagonal no worries..ot was for photography that i needed an ext tube....i know this is a silly question but is the focuser too tight..are you using its full travel?

tombps (Tom) · #8 28-12-2010, 08:12 PM

Well, a bit more research reveals that it is common for short refractors especially to need an extension. It seems that a WO 2-inch diagonal itself will do the job also, so in my ignorance I didn't realise the diagonal could have that much effect on the light path, if I'm understanding that.

This makes it interesting as I was going to put (eventually) a brick of a DSLR on the end. Have to be careful about that.

Thank you for the discussion. This scope is a transitional scope towards a larger refractor in the future, so I need to know about these things.

Cheers,
Tom

tombps (Tom) · #9 28-12-2010, 08:22 PM

Quote:

Originally Posted by dannat

....i know this is a silly question but is the focuser too tight..are you using its full travel?

Nothing's too silly for me at this stage. Yep, I was getting the full length. Apparently, the long, curved light path in an SCT means that the ratio of movement to focus", if that makes sense, is a lot more forgiving than in a short tube. Even changing to a different diagonal can hugely affect where the focal plane will be.

The SCT is what I'm used to, so it's always been the case that everything focuses easily, even if I have to rack the focuser a bit after attaching the camera. From what I've now read, considerable adjustment needs to be made to a short refractor when the camera goes on.

Glad I found all this out on a cloudy day!

rogerg (Roger) · #10 28-12-2010, 08:58 PM

Quote:

Originally Posted by tombps

This makes it interesting as I was going to put (eventually) a brick of a DSLR on the end. Have to be careful about that.

My megrez 80 came with an extension to use in place of the diagonal when using a camera. The extension is about 100mm in length. With it on the focuser is about 15mm out.

Dennis · #11 28-12-2010, 09:57 PM

The WO 66 Petzval does require the longer light path of a 2” diagonal in order for it to come to focus. This was explained to me by Ron at Sirius Optics when I purchased mine some 5 or 6 years ago.

However, I have found that it does come to focus with my Orion Deep Space Star Shoot CCD camera (1st generation) using a Takahashi 1 ¼ inch prism diagonal (i.e. not a mirror) – the longer light path through the prism makes this possible. I haven’t tried it with an eyepiece though.

Cheers

Dennis

Miaplacidus (Brian) · #12 28-12-2010, 11:15 PM

Or you could buy some binoviewers. Problem solved! (albeit, expensively.)

tombps (Tom) · #13 29-12-2010, 09:50 AM

Quote:

Originally Posted by rogerg

My megrez 80 came with an extension to use in place of the diagonal when using a camera. The extension is about 100mm in length. With it on the focuser is about 15mm out.

Actually, it's the weight I'm worried about. The camera on the end of an extension must make a heavy lever.

mithrandir (Andrew) · #14 29-12-2010, 10:27 PM

Quote:

Originally Posted by tombps

Actually, it's the weight I'm worried about. The camera on the end of an extension must make a heavy lever.

Can't you slide the tube in the rings or dovetail to a position where the balance is better? If mount capacity is no issue, adding a sliding balance weight on a dovetail works too. That beats trying to slide a heavy scope to balance the declination.