Jump to content


Photo

Focuser Question

  • Please log in to reply
8 replies to this topic

#1 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 24 January 2013 - 01:15 PM

I have a 12.5" second hand telescope. The scope originally had a damaged secondary holder and the secondary was pretty funky with scratches...it was also too big...3.1". I replaced the spider, secondary holder, and secondary mirror. The new mirror is 2.7". All of the hardware is 1800Destiny. This has ironed out many small issues with the scope, but for one...and this issue existed from start. When fully collimated, while looking through the collimating eyepiece if you rack the focuser all of the way out the very bare right edge of the mirror is cut off in the view. It just takes off enough of the view to remove sight of the edge of the mirror clips on that side. When racked in the whole mirror is visible. I just don't like loosing that small sliver of mirror when racked out. I have heard reference to the focuser needing to be square. Is this the issue I am experiencing? Do I need to shim the focuser on one side?

#2 nicknacknock

nicknacknock

    Viking 1

  • *****
  • Posts: 741
  • Joined: 20 Feb 2012
  • Loc: In a galaxy far far away...

Posted 24 January 2013 - 01:55 PM

I think your secondary is not aligned with the focuser. There are many guides around so I am just pasting the link to one by Starizona:

http://starizona.com...ating_newt.aspx

#3 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 24 January 2013 - 05:22 PM

I dont think that is an issue. When using a sight tube the center of the mirror is directly under the crosshairs.

#4 Vic Menard

Vic Menard

    Aurora

  • *****
  • Posts: 4792
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 24 January 2013 - 05:49 PM

...When fully collimated, while looking through the collimating eyepiece if you rack the focuser all of the way out the very bare right edge of the mirror is cut off in the view. It just takes off enough of the view to remove sight of the edge of the mirror clips on that side. When racked in the whole mirror is visible.

Usually, when you rack the focuser out, increasing the distance from the secondary mirror, the secondary mirror looks smaller, so it's more likely to be fully visible. The primary mirror reflection (with mirror clips) inside the secondary mirror will often fill, and then spill over, the secondary mirror.

From your description, I'm unsure which is happening. When you note that the scope is "fully collimated", do you mean the axial alignment is correct?

It sounds to me like your secondary mirror placement is still not fully corrected. If the secondary mirror looks "round" and the clipping (whichever type) is occurring on the side closer to or farther from the primary mirror, the error is due to incorrect offset, and the correction is (usually) done at the secondary mirror. But if the clipping is occurring perpendicular to the OTA axis, then the correction will probably require shimming the focuser. All of this assumes that the secondary mirror edge looks round (relative to the bottom of the focuser draw tube and the primary mirror reflection).

I just don't like losing that small sliver of mirror when racked out. I have heard reference to the focuser needing to be square. Is this the issue I am experiencing? Do I need to shim the focuser on one side?

The focuser doesn't need to be square as much as it needs to be aligned to the secondary mirror. It doesn't hurt to have both--but getting precise squaring is unnecessary for most applications. How tall is the focuser fully racked in and fully racked out?

#5 Vic Menard

Vic Menard

    Aurora

  • *****
  • Posts: 4792
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 24 January 2013 - 05:50 PM

I dont think that is an issue. When using a sight tube the center of the mirror is directly under the crosshairs.

The center of which mirror?

#6 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 24 January 2013 - 06:45 PM

It is a high profile focuser. Racked in it is 3.5". Racked out it is about 4.75. When racked in all appears in the sight tube aligned and visible. When racked all of the way out the edge of the secondary...the upper right edge to right middle is slightly out of view by a hair, but the sight tube stays centered over the secondary and the primary center mark.

#7 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 24 January 2013 - 06:48 PM

Secondary and primary lined up in the secondary.

#8 Vic Menard

Vic Menard

    Aurora

  • *****
  • Posts: 4792
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 24 January 2013 - 07:17 PM

...When racked in all appears in the sight tube aligned and visible.

Axial alignment:
1.) Intersection of sight tube cross hairs aligned with primary mirror center spot (focuser axis)
2.) Reflection of sight tube crosshairs aligned with primary mirror center spot (primary mirror/optical axis)
Secondary mirror placement - all three circles concentric
1.) Bottom edge of sight tube
2.) Real edge of secondary mirror
3.) Reflected edge of primary mirror


...When racked all of the way out the edge of the secondary...the upper right edge to right middle is slightly out of view by a hair, but the sight tube stays centered over the secondary and the primary center mark.

The fact that the cross hairs of the sight tube are still aligned to the primary mirror center spot, we know that the focuser axis is aligned and the focuser draw tube travel is linear.

If one edge of the secondary mirror slips out of view when the focuser is racked out, then the secondary mirror is slightly offset incorrectly. If it's only slight, as you've described, I wouldn't worry about it too much, as it only affects balanced fov illumination, which will almost certainly be undetectable visually.

Keep the axial alignments in tolerance and your image performance will be unimpaired (by collimation).

#9 Atl

Atl

    Viking 1

  • *****
  • Posts: 501
  • Joined: 13 Apr 2012

Posted 24 January 2013 - 11:15 PM

You are correct. It was the secondary positioning. just changed the secondary and made the same stupid alignment mistake I made with the last mirror...trying to get it done in 2 steps instead of multiple passes. I undid all of the adjustments and started over adjusting everything in passes. First the secondary then primary rinse and repeat....slowly. now when racked out it all fits...just...in the eyepiece. Mistakes seem to come from the second guessing based on lack of experience. I feel mastering this right now is fundamental to progressing.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics