Re: Focus repeatability issue

Roland Christen

I don't believe seeing has any effect on focus position. Temperature has the most effect. The aluminum tube shrinks with falling temps and optical lenses also change focal position with dropping temps. During that time the focus position can vary considerably, so you may have to refocus after every frame until the air temperature stabilizes or drops at a slower rate.


-----Original Message-----
From: Jim Fakatselis <pashasdad@...>
Sent: Tue, Jun 23, 2020 10:00 am
Subject: Re: [ap-gto] Focus repeatability issue

I would generally expect seeing effects to be more random in nature not as monotonous but I may be incorrect.  Not much experience.  

On Jun 23, 2020, at 10:40 AM, Jim Fakatselis via <pashasdad@...> wrote:

I live on Long Island in NY.  

On Jun 23, 2020, at 10:14 AM, Michael Hambrick via <mike.hambrick@...> wrote:

It could be due to variation in the seeing. I have been wanting to ask the experts on the forum what they are actually looking at when they talk about seeing in terms of arc seconds, and how they measure it.

Where are you imaging from ? I have also been watching on the weather reports where they have been saying that there is a very prominent Saharan dust cloud moving west and is already affecting the southern parts of the US (Florida & Gulf Coast states). I expect that this dust will degrade the quality of the seeing. I guess the good news is that it reduces the chances of tropical cyclone formation.

Best Regards

Michael Hambrick
TSR Global Manufacturing Support
PO Box 2000
Orange, TX 77631-2000
Phone: +1 (409) 882-2799
email: mike.hambrick@...

From:        "Jim Fakatselis" <jfakatse@...>
Date:        2020-06-23 09:06 AM
Subject:        Re: [ap-gto] Focus repeatability issue
Sent by:

Here’s the tabular data.


On Jun 23, 2020, at 9:53 AM, Jim Fakatselis <pashasdad@...> wrote:

I am imaging with a QSI683WS8 and a Starlight Instruments focuser with the Starlight stepper motor on an AP130mm EDF refractor Using MaximDL.
I was attempting to measure the filter offsets with my AstroDon series E gen 2 filter set. Taking four sets of measurements repeatedly with 5 of the 7 filters I use most, L, R,G,B,Ha.  If I try to immediately refocus, my focus setting continues to drift monotonically. Very erratically. Can’t get repeatability at all.
This past evening four V curve runs on just the Luminance filter in rapid succession, (less than 2-3 min per V curve), over and over produced readings like this:
15020, 15101, 15140, 15213 as an example.
The other filters behaved similarly.  I can provide Excel plot of data with linear ramp.

Reported temp from HSM stepper is well within 1 deg, measurements were made about 2-3 min apart in rapid succession. Little to no temp effect should be present.  

Any ideas on what I should look for to determine source of this erratic behavior?

The last few weeks there has not been an issue like this, this seemed to develop suddenly. No change to imaging train, it’s been untouched.   Seeing?

Thanks in advance,

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