On Mar 17, 2020, at 5:19 PM, uncarollo2 <chris1011@...> via Groups.Io <chris1011@...> wrote:



I have a 1600 mount with a 1200mm fl 175EDF refractor in Hawaii. I just spent 2 weeks testing keypad software using this setup. The 1600 has encoders and is a sweet mount that is dead nuts accurate. On my first night the trade winds were blowing strong and my star excursions were up to +- 3 arc seconds from one exposure to the next. Overall tracking results were on the order of 1.2 to 1.5 arc sec rms. Even so, each 10 minute exposure had perfectly round stars of 7 arc sec FWHM.

When the trades stopped blowing a couple of nights later, the rms tracking values dropped to 0.25 and less. One night I had .09 rms tracking in a 20 minute exposure. Stars never moved more than 1/4 arc sec from the zero line.

Now, to make it even more interesting, I never used a guider for the entire 2 weeks that I was imaging. Rather I was testing the pure tracking ability of the encoder mount with a tracking model. I took hundreds of exposures with almost no rejects. I had only a couple of exposures with slight oval stars which were caused by 30mph wind gusts.

The encoders eliminate periodic error and make it possible to drive both axes at very precise custom rates that are created in a tracking model. The model I was testing happens to be the one we are adding to the new keypad software, but it applies equally to the APCC-APPM tracking model. Once you have modeled the sky, and you don't move the mount or scope, you can indeed eliminate guiding. I spent the early part of one night creating the tracking model and used it for 2 weeks. Worked perfectly every night.

If you don't have a permanent setup with good polar alignment, then guiding works fine too - as long as you have good parameters dialed in.

Rolando

-----Original Message-----
From: Cheng-Yang Tan via Groups.Io <cytan299@...>
To: main <main@ap-gto.groups.io>
Sent: Tue, Mar 17, 2020 8:43 am
Subject: Re: [ap-gto] Mach2 and APCC last night

Hi Terry,

FYI, although I don't have a Mach2, I have an AP1100 with absolute encoders. For me, the PHD2 rms guide errors are *strongly* dependent on seeing conditions. Last week on 06 March, when the seeing was terrible around here, even with encoders, the guiding was ~0.9 arcsec rms! But on Sunday night, 15 Mar, guiding was very good ~0.3 arc sec rms because the seeing was good. So although encoders help with getting rid of PE, don't expect miracles if seeing is bad.

YMMV

cytan

On Tuesday, March 17, 2020, 08:05:34 AM CDT, Terri Zittritsch <theresamarie11@...> wrote:

On Mon, Mar 16, 2020 at 10:22 PM, Allen Ruckle wrote:

Terry, 
    I am surprised to see the tracking you report as being .5 arc Sec. 35° above the Southern Horizon and .8 seconds of arc at 23° above the Horizon.
It would seem to me that if the Mach II mount with the encoders that almost completely eliminates all periodic error would have that much tracking error.

   Does the atmosphere produce that much error?  I share an observatory with a friend who has a 14.25" Newtonian f5.6, fork mounted with a 11" Byers gear on the RA and a SiTech II drive corrector. We  have gotten guiding with PHD-2 of .21 RA and .27 Dec  rms. using the Tik management feature.  The Observatory is located at the SDAA dark sky site in Tierra Del Sol about 60 miles east of San Diego at about 3600' elevation.

   I am waiting to be notified that the AP Mach2 I ordered last year.   I have been thinking it would have more accurate tracking than what I already have access to.   am I going to be disappointed?

Allen, The atmosphere creates significant star movement in my less exotic locale (at 155M in Vermont under the jet stream).      And what I'm reporting is guiding (i'm actively guiding), not tracking.  So your .21 and .27 results in around .34 arc seconds total (root of sum of squares).   What I report you should compare to the .34.    I think almost all of what i see is the atmosphere and some due to my lack of precision in setting the right guiding parameters and some over-correction, that which then needs to be corrected.   Plus, I am a portable mount, and about the best polar alignment I get is around an arc minute, maybe a bit less, but this will result in some drift that needs correction.  

Since I've never owned another A-P mount, it's hard for me to compare.   Comparing to my old mount, it's significantly more controlled.. to where I feel I can start honing my guiding and A-P skills rather than continually account for idiosyncrasies of the mount.


Terri