Jump to content


Photo

Used 18" Obsession, what to look for?

  • Please log in to reply
13 replies to this topic

#1 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 08 July 2013 - 02:17 AM

I am looking at a used obsession this week. It's a 1997 model, but the mirrors were recently re-coated.

Since I've mostly been an sct guy, what should look for?


I've read lots and lots but it's always good to check for tips.

Thanks!

#2 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4447
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 08 July 2013 - 07:01 AM

A couple of generic suggestions not specific to Obsession:

--Look to see that the scope is at least roughly collimated, i.e., viewing through the focuser shows proper alignment of the reflections. This will tell you that everything fits together properly. If a laser is available you can check for movement of the spot as the scope is raised and lowered in altitude. Spot movement will indicate that there is a source of flexure, such as wear on the truss connectors.

--How are the motions in azimuth and altitude? They should be smooth but not loose.

--Make sure the mirror surface has no clamshells or other defects. Small issues can be tolerated at an appropriate price.

--How is the finish of the scope? Maybe you can renew the finish if necessary; what you don't want is a lot of parts that have become warped or severely corroded by exposure to the elements.

JimC

#3 okieav8r

okieav8r

    I'd rather be flying!

  • *****
  • Administrators
  • Posts: 4350
  • Joined: 01 Mar 2009
  • Loc: Oklahoma!

Posted 08 July 2013 - 08:10 AM

Jim offers some good advice. Observe through the scope with the owner if you can. When you check for movement of the spot, also make sure nuts, bolts, and screws are tightened properly. What kind of shape is it in cosmetically? Not that it's all that important, but it can give you a clue as to how well it was cared for.

#4 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 12711
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 08 July 2013 - 02:48 PM

Mainly to personally check it out; set it up for some viewing. Any major defects will become known. Make sure that it can be collimated.

#5 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 08 July 2013 - 09:35 PM

Thanks for the hints about checking for flex and for loose bolts. I wouldn't have thought of those.


Will definitely check Collimation and check the movement too.

#6 omahaastro

omahaastro

    Viking 1

  • -----
  • Posts: 500
  • Joined: 30 Aug 2006
  • Loc: Omaha, NE

Posted 10 July 2013 - 12:18 AM

Primary is probably a Nova or a Galaxy. Take eyepieces... check focus, you may need to adjust length of poles, or work collimation bolts to adjust height. If you're not able to make use of wheel barrel handles... carefully consider whether you want to lift 80 lbs of mirror box (or perhaps you'll have it permanently setup somewhere).

#7 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 10 July 2013 - 10:41 AM

Thanks. Current plan is to have the Obsession in the garage and just roll it out when I want to use it. How long does it take the mirror to cool if it's only about 20 deg above ambient if the fan is running? I live in Phoenix, so it's warm most of the the year.

I'll probably keep my C11 for taking along when we are camping as I don't think I can load the Obsession into the camper and keep it safe in there when not using it. I live in a yellow zone so it's not too bad at home.

#8 Sebastian Quirk

Sebastian Quirk

    Lift Off

  • -----
  • Posts: 13
  • Joined: 04 Jan 2012

Posted 10 July 2013 - 01:26 PM

I live in Phoenix, and had an 18" Obsession for awhile, and now have an 18" Teeter, basically the same scope in terms of cooling properties.

I have a "exhausting" fan set in a sealed rear baffle coupled with the Teeter-style boundary layer fans. Had a similar set-up on the Ob. The boundary layer fans improve the view for well over 2 hours, so I'd say it takes 2 hours in general for things to settle down to a reasonable degree. Truly, it never reaches ambient. But first hour I would typically use binocs or do something else while the scope cooled.

Endorse the advice to check all bolts and tighten as necessary. Both my dobs would/do loosen over time....but I trailer the scope and things vibrate more. That's the number one reason for collimation drift for these scopes in my experience: loose bolt somewhere. Split block (which holds the truss tube) loose on an Ob. Teeter alt bearing bolts loosen also. Spider vane bolts.

Use blue loctite judiciously to reduce this tendency on the split block screws, but it'll happen either way...

Also carefully check the tailgate, mirror cell and collimation bolts. Make sure the mirror is resting on the pads, and check the back of the mirror for evidence of trauma. More than a few times I've seen damage from mis-handling in older mirrors that have been around the block...If there is a crack or splintering anywhere on the mirror, walk away.....

#9 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 10 July 2013 - 03:37 PM

Thanks, great advice!

#10 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 24 July 2013 - 10:10 AM

Got my list ready... hopefully going to see it tonight if the guy is available! I've been sitting on the fence so long it's starting to hurt... time to get off and go see it and hopefully buy it tonight!

#11 davidpitre

davidpitre

    Soyuz

  • *****
  • Posts: 3574
  • Joined: 10 May 2005
  • Loc: Central Texas

Posted 24 July 2013 - 08:50 PM

Check the condition of the secondary mirror as well. Look at the plywood all over. Make sure it is in good condition (a worn finish is not a big deal if the price is right , and can be easily touched up) with no swelling or delamination.
An 18" Obsession is a great sized scope.

#12 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 27 July 2013 - 11:15 AM

Thanks for all the help guys. The 18" is now sitting in the back of my Subaru Outback.

The man was very very helpful and showed me everything, so much so that he answered most of the questions you guys gave me. I also had the collimation laser on and when from zenith to about 20deg and no movement of the laser! :D

Will give a first light report when I get a chance.

Thanks again!

#13 Erik Bakker

Erik Bakker

    Gemini

  • *****
  • Posts: 3156
  • Joined: 10 Aug 2006
  • Loc: The Netherlands, Europe

Posted 27 July 2013 - 01:29 PM

Kev,
Congrats on your new super scope. You will likely enter a new era of happiness under the stars :jump:

#14 Kevdog

Kevdog

    Apollo

  • *****
  • Posts: 1277
  • Joined: 11 Jul 2012
  • Loc: Desert Hills, AZ

Posted 30 July 2013 - 02:20 PM

The finish did have a bit of wear on it from the AZ heat, but the wood itself is in great shape as was the rest of the scope. He really took care of it.

Here's my 1st and 2nd light reports. Good stuff!

http://www.cloudynig...5996537/page...






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics