Results 1 to 4 of 4
Like Tree5Likes
  • 1 Post By Smokehouse
  • 1 Post By SteveNH
  • 2 Post By Smokehouse
  • 1 Post By SteveNH

Thread: RC Focusing Issue/Question

  1. #1
    Smokehouse's Avatar
    Smokehouse is offline White Dwarf
    Points: 1,423, Level: 22
    Level completed: 23%, Points required for next Level: 77
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!365 Days+ Registered Achievement!First 1000 Experience Points
    Join Date
    Jun 2015
    Location
    Texas
    Posts
    26
    Points
    1,423
    Level
    22
    Thanks
    0
    Thanked 0x 0 Posts

    Default RC Focusing Issue/Question



    Just received a GSO (Mallincam) 10" Truss Tube RC. I will be mainly using it for "near time" viewing on a monitor via a Mallincam Universe. First night out I decided to do an alignment using a diagonal and lens vs a camera. Now i knew I may have to add extension rings to get within the focal plane but the issue I ran into is driving me crazy. I could not get a star in focus. What I am seeing is a beautiful donut star (collimation looks good) but that is as good as I can get the star into focus. If I go out with the focuser the pattern gets even more defuse, if I go in with the focuser I get a donut but cant go any further in with the focuser. Thats all the travel I have. I initially thought of putting the extension rings between the base and the focuser of which I did (one 2" and two 1" rings in different combos) but still cant get a tight star pattern, just a tight donut.

    What am I missing?

    Thanks in advance, as this is driving me crazy!!

    Jeff
    frdpwr427 likes this.

  2. #2
    SteveNH's Avatar
    SteveNH is offline Main Sequence
    Points: 3,576, Level: 39
    Level completed: 51%, Points required for next Level: 74
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!200+ Posts Achievement!365 Days+ Registered Achievement!
    Join Date
    Apr 2015
    Location
    Millbrae
    Posts
    137
    Points
    3,576
    Level
    39
    Thanks
    7
    Thanked 66x 53 Posts

    Default Re: RC Focusing Issue/Question

    Hi Smokehouse,

    What you are experiencing is a common problem that many people face when trying to connect their telescopes with a camera for prime focus photography. The problem is not enough back focus. In other words, the focal plane is too far in, so that your sensor cannot reach it. Extension rings are the opposite of what you need - that is, not more distance, but less distance from your telescope optics. I am not familiar enough with commercial Ritchey-Chretien OTA construction to know how to remedy the OTA, if that's even possible, since I understand that the design specifies a strict primary to secondary distance as well as from the secondary to the focal plane for the optics to behave properly.

    For Newtonians and refractors, one way to solve this problem, aside from shortening the tube, is to use the type of Barlow lens that throws the image plane back from the original position, so you have more room to work with. I've done this with some of my refractors. For reflectors, sometimes the mirror cell is moved forward in the tube a few inches, as I did for my 8" Newt to accommodate an SLR at prime focus. Hopefully the experts here will have a working solution for your RC.
    frdpwr427 likes this.

  3. #3
    Smokehouse's Avatar
    Smokehouse is offline White Dwarf
    Points: 1,423, Level: 22
    Level completed: 23%, Points required for next Level: 77
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!365 Days+ Registered Achievement!First 1000 Experience Points
    Join Date
    Jun 2015
    Location
    Texas
    Posts
    26
    Points
    1,423
    Level
    22
    Thanks
    0
    Thanked 0x 0 Posts

    Default Re: RC Focusing Issue/Question

    Well I had success tonight after Jack Huerkamp (Mallincam) sent me a troubleshooting guide. I removed the diagonal and just used a lens and started installing extension rings. After 3" of extensions I got the focal plane in the middle of the focuser travel. I had tried this the other night but was using a diagonal with no luck, with the diagonal removed it worked. I had thought the same thing as you as you are saying, "less distance" but with the diagonal removed the 3" extension was the key. Weird!!!

    Thanks

    Smoke'
    SteveNH and frdpwr427 like this.

  4. #4
    SteveNH's Avatar
    SteveNH is offline Main Sequence
    Points: 3,576, Level: 39
    Level completed: 51%, Points required for next Level: 74
    Overall activity: 0%
    Achievements:
    2 Posts Achievement20 Posts Achievement!50 Posts Achievement!200+ Posts Achievement!365 Days+ Registered Achievement!
    Join Date
    Apr 2015
    Location
    Millbrae
    Posts
    137
    Points
    3,576
    Level
    39
    Thanks
    7
    Thanked 66x 53 Posts

    Default Re: RC Focusing Issue/Question

    Congratulations on getting it solved so simply! I completely missed it when you said you had other equipment in the optical train. Diagonals always seem to take up so much more optical path than you'd think! It makes sense that the OTA would be designed with ample back focus, considering the imaging equipment that they'd expect to be attached to it. I'm glad you got it worked out.

    Steve
    frdpwr427 likes this.

 

 

Similar Threads

  1. 8EdgeHD focusing issue
    By gomezdb in forum Astronomy Beginners Forum
    Replies: 2
    Last Post: 06-29-2015, 05:06 AM
  2. Focusing Issue
    By chris_annee in forum Astronomy Beginners Forum
    Replies: 4
    Last Post: 08-12-2013, 12:24 PM
  3. Collimated but having focusing issue
    By snoopoy33 in forum Astronomy Beginners Forum
    Replies: 4
    Last Post: 04-27-2012, 04:47 PM
  4. LX200 Focusing Issue
    By Prince William in forum Meade LX200 Telescope Forum
    Replies: 5
    Last Post: 10-13-2009, 02:47 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin® Version 4.2.0
Powered by vBulletin®
All times are GMT. The time now is 11:10 PM.