Optical Telescope Focus Backlash June 2000 Here's some instructions from Dave about the focus backlash. The current settings are BACKLASH0 = 2000 remove focus backlash BACKLASH1 = 100 relax the focus What this does is as follows. When you ask opd to move optical focus to a focus position

(where

is current position plus the amount by which you've asked the focus to change), it first moves to

+BACKLASH0, then drives to

, and finally drives to

-BACKLASH1. The focus position reported on the "now at" line is

-BACKLASH1. The sign's of the values put in for BACKLASH0 and BACKLASH1 ought to be the same. Dave doesn't know whether gravity tends to push the focus to smaller

or larger

. A week ago, to focus a star at the zenith, the

had to be increased from what it was at lower elevation. That would say that gravity tends to change the focus by reducing the effective

and to compensate, one must increase

. If we're seeing a change in the focus at high elevation, then it's likely that changing the signs of the BACKLASH terms will change the size of this effect. There's three possiblities: -2000 makes it better, worse, or doesn't affect it. We won't know until we try, but the experience last week would suggest that negative values ought to reduce this effect. The notes I made the last time I changed these values (June 1999) warn that the new values of BACKLASH don't take effect until after the SECOND move of the focus. It ought to be apparent, however, when this happens so long as BACKLASH1 is non-zero. After changing the signs of them in the opdrc file, move the focus by 0 steps until it gives a different final position. It ought to change by twice the value of BACKLASH1 (if one changed its sign). To put the focus to the same position as before changing the signs, you'll then want to move the focus in the opposite direction by this amount to give the same position as before changing the signs. The BACKLASH1 parameter was added after I thought that I saw the focus drifting for a few minutes after changing the focus. That is, I would move the focus until I found a sharp image, then take another exposure without moving the focus and it was out of focus. That night I had to move the focus to a place short of where I thought it should be and let it drift into focus from there. But after BACKLASH1 was implemented and I tried various combinations, it wasn't obvious that the focus was drifting that night. So the current values are not well determined. If anyone suspects that the focus is drifting, he might try changing BACKLASH1 to see if it can reduce the drift. A spare copy of the current opdrc is in /home/smtobs/op/opdrc.oct20. If the /etc/opdrc file is edited, make a copy of the final result with an appropriate name in the same directory. -Bill From: IN%"dashby" "Dave Ashby" 1-JUN-1999 17:52:03.46 To: IN%"bpeters" "Bill Peters" CC: IN%"dashby" "Dave Ashby" Subj: opd mods Return-path: Received: from steffens.as.arizona.edu by GAFFEL.AS.ARIZONA.EDU (PMDF V4.2-13 #2381) id <01JBW7Y5QQ680034KG@GAFFEL.AS.ARIZONA.EDU>; Tue, 1 Jun 1999 17:51:56 MST Received: from astro.as.arizona.edu (astro.as.arizona.edu [128.196.208.2]) by steffens.as.arizona.edu (8.9.1/8.9.1) with ESMTP id RAA06415; Tue, 1 Jun 1999 17:51:51 -0700 (MST) Received: from GAFFEL.AS.ARIZONA.EDU (gaffel.as.arizona.edu [128.196.210.186]) by astro.as.arizona.edu (8.9.1/8.9.1) with ESMTP id RAA24498; Tue, 1 Jun 1999 17:51:50 -0700 (MST) Received: from the_tick (gilden.as.arizona.edu) by GAFFEL.AS.ARIZONA.EDU (PMDF V4.2-13 #2381) id <01JBW7XY5ZI80033FE@GAFFEL.AS.ARIZONA.EDU>; Tue, 1 Jun 1999 17:51:47 MST Date: Tue, 01 Jun 1999 17:51:43 -0700 (MST) From: Dave Ashby Subject: opd mods To: Bill Peters Cc: Dave Ashby Message-id: MIME-version: 1.0 Content-type: TEXT/PLAIN; charset=US-ASCII Content-transfer-encoding: 7BIT X-Sender: dashby Hi Bill, I thought I would send you some notes about the changes that I have made to the focus command in the optical pointing daemon. I have added two parameters to the /etc/opdrc file. BACKLASH0 - This is the same as the term used to date. The value we have been using so far is -2000. I've changed the sign to see how it works. BACKLASH1 - Relaxation term you requested. This should have the same sign as BACKLASH0. If you log onto kueppers as smtobs, you will have write access to opdrc. If you edit opdrc, the opd will reread the file so there is no need to restart opd. There are now only two options to the focus command. The following is the response help command. help focus focus [-a] [-m steps] opt a - autofocus mode opt m - manual focus mode steps - number of steps to move The manual focus uses the new backlash terms, but the autofocus doesn't. I am not done with the modifications to the autofocus mode. I have attached a copy of the current opdrc file. -Dave. #-----------------------------------------------------------------------------# # opdrc / optical pointing daemon resource file # #-----------------------------------------------------------------------------# # Comment lines start with '#' in the first column and continue to the end # # of the line. # # Any text after the data value is a commend. # # Space, tab, ',' and '=' are all data # #-----------------------------------------------------------------------------# # camera info CCDTEMP = -25.0 C target temperature if the CCD # Communication MAILBOX = OPOINT local mailbox name TEST_MAILBOX = OP_TEST # Files and Directories Directories VARFILE = /var/lib/op/opvar contains current scan number RAWDATA = /var/spool/op/raw raw data directory REDDATA = /var/spool/op/red reduced data directory ERRLOG = /var/log/opd.log opd log file APFILE = /etc/apogeerc apogee rc(ini) file #OPRRC = /etc/oprrc opr resource file # Focus stuff BACKLASH0 = 2000 remove focus backlash BACKLASH1 = 0 relax the focus