Focus repeatability issue


Jim Fakatselis
 

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,
Jim


Jim Fakatselis <jfakatse@...>
 

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,
Jim


Michael Hambrick <mike.hambrick@...>
 

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




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




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,
Jim




Jim Fakatselis
 

Interesting...
I live on Long Island in NY.  
Jim


On Jun 23, 2020, at 10:14 AM, Michael Hambrick via groups.io <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
ARLANXEO
TSR Global Manufacturing Support
PO Box 2000
Orange, TX 77631-2000
Phone: +1 (409) 882-2799
email: mike.hambrick@...




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




Here’s the tabular data.

<mime-attachment.jpg>

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,
Jim




Jim Fakatselis
 

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 groups.io <pashasdad@...> wrote:


Interesting...
I live on Long Island in NY.  
Jim


On Jun 23, 2020, at 10:14 AM, Michael Hambrick via groups.io <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
ARLANXEO
TSR Global Manufacturing Support
PO Box 2000
Orange, TX 77631-2000
Phone: +1 (409) 882-2799
email: mike.hambrick@...




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




Here’s the tabular data.

<mime-attachment.jpg>

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,
Jim




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.

Rolando

-----Original Message-----
From: Jim Fakatselis <pashasdad@...>
To: main@ap-gto.groups.io
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 groups.io <pashasdad@...> wrote:


Interesting...
I live on Long Island in NY.  
Jim


On Jun 23, 2020, at 10:14 AM, Michael Hambrick via groups.io <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
ARLANXEO
TSR Global Manufacturing Support
PO Box 2000
Orange, TX 77631-2000
Phone: +1 (409) 882-2799
email: mike.hambrick@...




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




Here’s the tabular data.

<mime-attachment.jpg>

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,
Jim



Stuart <stuart.j.heggie@...>
 

Jim, I am wondering if what you're seeing is that the scope is still cooling and the optics and tube are still changing shape. Just because the outside temps are stable does NOT mean that the scope is thermally stable too.

If I were to do this experiment, I would opt to do it in the pre-dawn hours on a night that has a very stable temperature. Otherwise what you may be seeing is the ambient is stable the scope is catching up.


On Tue, 23 Jun 2020 at 10:40, Jim Fakatselis <pashasdad@...> wrote:
Interesting...
I live on Long Island in NY.  
Jim


On Jun 23, 2020, at 10:14 AM, Michael Hambrick via groups.io <mike.hambrick=arlanxeo.com@groups.io> 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
ARLANXEO
TSR Global Manufacturing Support
PO Box 2000
Orange, TX 77631-2000
Phone: +1 (409) 882-2799
email: mike.hambrick@...




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




Here’s the tabular data.

<mime-attachment.jpg>

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,
Jim




--

Stuart
http://www.astrofoto.ca/stuartheggie/


Dale Ghent
 

Are you sure it was truly clear sky where you had your telescope pointed? Faint high-level clouds can affect the measured HFD/HFR of the star(s) that MaximDL is using. Clouds move, and that could explain the quickly varying values you are getting.

Also consider the resolution of your setup - a single step in your case might be quite small so the CFZ in your case might /seem/ wide because the high resolution of your setup puts a lot of steps in that zone and landing anywhere in that zone will be ok.

It is tough to find exacting repeatability when creating focus offsets in any system simply due to mechanical and atmospheric effects. The CFZ is always going to be more than 1 single step, so my recommendation is do do a run and then use those derived offsets and look for the repeatability of the HFD/HFR values those produce. If they seem consistent and your stars look good, then you have good offsets to work with.

/dale

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



Barry Megdal
 

The comments other have made about CFZ (Critical Focus Zone) are very relevant.  You need to find out what the step size of your focuser means in actual linear distance (e.g. microns), to see if the variation you are observing is even an issue.

 

Don Goldman and I wrote an article for Sky and Telescope (https://www.dropbox.com/s/j0iqk7026yfgklg/get%20focusedfinal.pdf?dl=0 ) a number of years ago which calculated a new, more restrictive definition of the CFZ, but once you know the step size and the optical parameters of your scope you can see if this variation is even an issue – Focusmax or other focusing software will always show some variation – the question is just whether it is significant.  The difference between what we published and the traditional definition of CFZ was that we showed that the traditional definition was roughly equivalent to a ¼ wave error, but 1/10 wave is more appropriate for critical imaging, so that is what I used in doing the math.

 

-        Barry

 

Dr. Barry Megdal

 

President

Shb Instruments, Inc.

19215 Parthenia St.  Suite A

Northridge, CA 91324

www.shbinstruments.com

(818) 773-2000  (818)773-2005 fax

bmegdal@...

 

Faculty (retired)

Dept. of Electrical Engineering

Caltech

 


Jim Fakatselis <jfakatse@...>
 

That makes excellent sense Barry, I appreciate your feedback.  I remember reading that article back then bit never put 2 and 2 together. Thank you!  
Let me run some numbers and I may email you back if that’s ok. 
I did this around 1:30 am local time when temp conditions should have been stable.  Perhaps upper layers of the atmosphere were turbulent, don’t know, or I may never know.  My issue was a monotonic increase, not only noise around that value.  
I have all the numbers to calculate CFZ. Thanks again,

Best regards,
Jim

On Jun 23, 2020, at 2:35 PM, Barry Megdal <bmegdal@...> wrote:



The comments other have made about CFZ (Critical Focus Zone) are very relevant.  You need to find out what the step size of your focuser means in actual linear distance (e.g. microns), to see if the variation you are observing is even an issue.

 

Don Goldman and I wrote an article for Sky and Telescope (https://www.dropbox.com/s/j0iqk7026yfgklg/get%20focusedfinal.pdf?dl=0 ) a number of years ago which calculated a new, more restrictive definition of the CFZ, but once you know the step size and the optical parameters of your scope you can see if this variation is even an issue – Focusmax or other focusing software will always show some variation – the question is just whether it is significant.  The difference between what we published and the traditional definition of CFZ was that we showed that the traditional definition was roughly equivalent to a ¼ wave error, but 1/10 wave is more appropriate for critical imaging, so that is what I used in doing the math.

 

-        Barry

 

Dr. Barry Megdal

 

President

Shb Instruments, Inc.

19215 Parthenia St.  Suite A

Northridge, CA 91324

www.shbinstruments.com

(818) 773-2000  (818)773-2005 fax

bmegdal@...

 

Faculty (retired)

Dept. of Electrical Engineering

Caltech