Back Focus Troubles

Andrew J

I could use some help and guidance to better understand Back Focus.

The first question relates to the impact Filters have on Back Focus. For several years I have been trying to get a definitive answer on the how adding filters to an imaging chain affects the Back Focus calculation and in turn how this impacts the length of the spacers required to achieve the recommended Back Focus for a particular scope configuration. For example, I have TEC140ED with Field Flattener that requires 85mm of Back Focus. I also have Chroma filters, that requires an additional 1mm of Back Focus per the manufacture. To keep the math easy, let’s assume my Camera and Filter Wheel (without filters) consume 35mm of the Back Focus. When I add the Chroma Filters to the equation, do I need a 49mm (85 – 35 – 1) or 51mm (85 – 35 + 1) spacer to achieve optimal Back Focus? This would seem like an easy thing to answer, but I just got off the phone with telescope and eyepiece manufacturer, and depending on who I talked to I got completely opposite answers.

My understanding is that when they say the filter adds 1mm, it means just that, it ADDS 1mm to the Total Back Focus between the OTA and the camera sensor. Sticking with the example above and the 85mm of native Back Focus, adding Chroma filters to the optical path would changes the Total Back Focus from 85mm to 86mm. If correct, then the answer to the question above would mean I need a 51mm spacer (85mm native BF + 1mm for filter = 86mm Total BF; 86mm Total BF – 35mm Cam/FW = 51mm spacer). Hoping the knowledgeable experts here can confirm if my understanding is correct.

My second question is, why does Back Focus spacing matter? When using a refractor with a Crayford style focuser, it is not clear why getting precise Back Focus spacing is so important when the camera sensor will be moving in or out to achieve focus anyway. Referring back to the example above, assume instead of the 51mm spacer I went completely nuts and added a 41mm spacer instead so that my Back Focus spacing was 10mm short. Assuming I had 10mm of travel available on my focuser, what would be the difference between having the 51mm spacer and the focuser racked to say 49mm to achieve focus vs. having a 41mm spacer and the focuser racked to 59mm to achieve focus? It is the same 100mm distance from the back of the OTA to the camera sensor either way. So why does it matter which spacer is use? I have spent a fair amount of money over the years with Precise Parts in an attempt to precisely achieve the Back Focus recommendations for various scope configurations. I never really stopped and asked why this is necessary. Hopefully someone here who understands optics can reassure me that the money spent on getting the “optimal” back focus was a worthwhile cause.

Thanks in advance for the education.

Andrew J

Re: Added album Photos of Forum Members in their A-P Logo Hats

Karen Christen

Hi Kenneth,

Karen

AP

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Kenneth Tan
Sent: Tuesday, June 1, 2021 1:26 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Added album Photos of Forum Members in their A-P Logo Hats #photo-notice

I have a strange problem. I am using a c925 on a AP 1600 . I can run it without guiding for about 30 s with fairly round stars.  The moment i try to guide with the phd on the asiair pro, it is all over the place. Unpredictable spikes in both axes.  Has anyone encountered this problem? I will try with phd proper the next time.

Kenneth

--
Karen Christen
Astro-Physics

Re: Added album Photos of Forum Members in their A-P Logo Hats

Kenneth Tan

I have a strange problem. I am using a c925 on a AP 1600 . I can run it without guiding for about 30 s with fairly round stars.  The moment i try to guide with the phd on the asiair pro, it is all over the place. Unpredictable spikes in both axes.  Has anyone encountered this problem? I will try with phd proper the next time.

Kenneth

Re: Added album Photos of Forum Members in their A-P Logo Hats

That’s a fun album, Mike! There are quite a few hats shipping recently, so I hope to see some in the album over the summer.

Clear Skies,

Marj Christen

Astro-Physics

Machesney Park, IL 61115

Phone: 815-282-1513

www.astro-physics.com

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of M Hambrick
Sent: Tuesday, June 1, 2021 8:49 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Added album Photos of Forum Members in their A-P Logo Hats #photo-notice

It would help to have the link to the album>

https://ap-gto.groups.io/g/main/album?id=264707

Re: CP5 update?

Yes, that email is from me! Thanks for checking with us though. You can never be too careful.

Liam

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Marj Christen
Sent: Tuesday, June 1, 2021 9:49 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] CP5 update?

Yes, it is legitimate but only affects a few people. Please follow the instructions in the letter thank you.

On May 31, 2021, at 9:49 PM, weihaowang <whwang@...> wrote:

Hi,

A few days ago, I got an email from AP regarding a special firmware update for CP5, saying that it
will require a special firmware load.

I don't see any discussion about this here, so I wonder if this is legitimate and if I should proceed.

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/

Re: Mach 1 ra motor failure to move the the west.

Roland Christen

What could have been wrong?
Bad connection to either of the two motors will cause this. Most likely a loose cable connection to the motor box. Second thing could be a poor connection at one of the connectors inside the cable. Third might be a broken wire inside the motor box that is just touching the pin on the connector.

Rolando

-----Original Message-----
From: crib409acme425 <jacobsonc@...>
To: main@ap-gto.groups.io
Sent: Mon, May 31, 2021 6:47 pm
Subject: [ap-gto] Mach 1 ra motor failure to move the the west.

the other night I set my Mach 1 up with a RASA 8 as a trial before New Moon. this was a new scope to me.  it was balanced , no cabling issues and in fact with the clutches loosened the scope was free to spin in ra and dec with a light touch.  Now to the trouble.  I noted it was not tracking in RA plus when ever I tried to use the Keypad to force it to move to the west the RA motor noise would last about a second stop with a second short burst and that was it.  I could go to the East and use the keypad to move N and S.  The battery was well charged.  Two mornings before I used this same Mach1 for the Lunar eclipse with out issue.
That Night I took the whole thing down and reassembled in my garage.  In the am I tried the K pad on my 1200 mount.  No problems.
I hooked the electronics up again, on the Mach 1, using the same battery as the night before and everything worked including tracking a phantom star .
It has worked ever since as I have tried to see if it would fail.  What could have been wrong? I did not look at the stall light I guess because it would move N, S and East and freely move with clutches released.
The other issue I have is that the ra gear train noise in ra slewing is not always of  the same frequency.  It seems to vary a bit.  thanks cjacobson

--
Roland Christen
Astro-Physics

Re: My AP1100 CP4 died (I think ..., need confirmation)

Roland Christen

Don't remove the flux. It does no harm. Removing it is NOT recommended!

Rolando

-----Original Message-----
From: Cheng-Yang Tan via groups.io <cytan299@...>
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Mon, May 31, 2021 9:41 am
Subject: Re: [ap-gto] My AP1100 CP4 died (I think ..., need confirmation)

Hi Marj,
I got a piece of non-conductive foam that had the right thickness to compress the board down when the cover is screwed back on. See photo.

Interestingly, I see flux residue on the pins of the processor but nowhere else. This could be due to age or something else. See attached photo. I didn't bother cleaning it with flux remover because the CP4 works. I have photos that I took under a microscope if you're interested. Anyway, this is for your info.

My CP4 serial number is CP4-0476.

cytan

On Sunday, May 30, 2021, 10:00:27 PM CDT, Marj Christen <marj@...> wrote:

Cytan,

Some of the very first CP4s did not have the foam and yours may be one of those.

Clear Skies,
Marj Christen
Astro-Physics
Machesney Park, IL 61115
Phone: 815-282-1513
www.astro-physics.com

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Roland Christen via groups.io
Sent: Sunday, May 30, 2021 8:50 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] My AP1100 CP4 died (I think ..., need confirmation)

I'm glad you solved it. Actually Marj came up with the solution, so I am naming her "Employee of the Day". Maybe I'll make a Pizza for her tomorrow, with all her favorite toppings.

By the way, there should have been a small square of foam on top of the board to keep it in place. On later versions of the CP4 we used a different standoff that has screws to keep the board in place.

Rolando

-----Original Message-----
From: Cheng-Yang Tan via groups.io <cytan299@...>
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Sun, May 30, 2021 7:37 pm
Subject: Re: [ap-gto] My AP1100 CP4 died (I think ..., need confirmation)
Hi Roland,
That was the problem! I pushed on the main board and could feel the board going back into its socket. It moved quite a bit. I have no idea why it would have moved out of its socket because I've never dropped it. But then I'm the second owner, so maybe it was jarred a little bit loose during this second shipping and I do transport it in and out to setup every time. And over time, it popped out.

I've tested it fully cabled up and no more problems: it talks via USB and slews find in DEC.

Thanks again!

cytan

On Sunday, May 30, 2021, 05:11:06 PM CDT, Roland Christen via groups.io <chris1011@...> wrote:

Push on the main board. It may be loose. It can happen if you dropped the CP4 at some point. If it becomes loose, the LED will not light up and there will be no communication. It is held in place by standoffs. Push on all 4 corners. There is a long connector underneath, and you can see if it is fully seated.

Roland

-----Original Message-----
From: Cheng-Yang Tan via groups.io <cytan299@...>
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Sent: Sun, May 30, 2021 4:16 pm
Subject: Re: [ap-gto] My AP1100 CP4 died (I think ..., need confirmation)
I pushed on it. But it didn't feel loose. But after doing that, the LED lit.

I'll work on it later this evening ...

Thanks!

cytan

On Sunday, May 30, 2021, 04:09:40 PM CDT, Cheng-Yang Tan via groups.io <cytan299@...> wrote:

You mean this board that I put a rectangle around it?

On Sunday, May 30, 2021, 03:37:01 PM CDT, Marj Christen <marj@...> wrote:

Hi Steve,

One other thing to try. Open the cover of your CP4 and push down on the small circuit board to reseat the connections. It is possible that it came loose from the main board. It is highly unusual for a CP4 to simply die. Let us know if that works for you.

Clear Skies,
Marj Christen
Astro-Physics
Machesney Park, IL 61115
Phone: 815-282-1513

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of Steve Salvo
Sent: Sunday, May 30, 2021 1:15 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] My AP1100 CP4 died (I think ..., need confirmation)

Yes, initially it lite red.  As soon as I directed east, the yellow light immediately went on and the mount would not move in the east direction. After restarting, the mount would move west, but not east.  Directing east caused the light to turn yellow and motor stall.

Steve

On May 30, 2021, at 14:09, Cheng-Yang Tan via groups.io <cytan299@...> wrote:
﻿
Do you remember whether your CP4 LED lit up?

Anyway, it was a very weird night. After powering up, with the CP4 LED lit up, I wasn't able to talk to the CP4 with USB. I changed USB cables and used my backup laptop and both wasn't able to talk to the CP4. But I was able to talk to it via TCPIP. In APCC, I can move the mount in RA but not DEC. That's when the stall error popped up.

After taking it back indoors, the CP4 LED no longer lights up :(

cytan

On Sunday, May 30, 2021, 01:04:04 PM CDT, Steve Salvo via groups.io <sws105@...> wrote:

My CP4 was showing similar issues.  Turned out to be mount limit warnings.

see below from Rolando.
Roland Christen
07/31/20

'm afraid to repeat these tests as there could be potential problems with the hardware,
The "Motor Stall" warning will occur when unsafe limits are exceeded. This is not a warning that something is wrong with the motor or electronics, it is simply telling you that the software has turned off the power to the motors to prevent your scope from being damaged. At this time we use the same error code for all situations where the motor power has been turned off. It probably could be changed to indicate not "motor stall" but "motor power deactivated".  However, we have many other software challenges right now and this one will have to wait a bit. Be assured that nothing bad has happened to your electronics or the motors.

Rolando

--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics

--
Roland Christen
Astro-Physics

Re: CP5 update?

Yes, it is legitimate but only affects a few people. Please follow the instructions in the letter thank you.

On May 31, 2021, at 9:49 PM, weihaowang <whwang@...> wrote:

Hi,

A few days ago, I got an email from AP regarding a special firmware update for CP5, saying that it
will require a special firmware load.

I don't see any discussion about this here, so I wonder if this is legitimate and if I should proceed.

Cheers,
Wei-Hao

--

Homepage:

Astrobin gallery:
http://www.astrobin.com/users/whwang/

Re: AP130GT + older rings without center top hole

John Jennings

Nice setup. You probably don't need much guiding with the encoders anyway....

Re: Added album Photos of Forum Members in their A-P Logo Hats

M Hambrick

It would help to have the link to the album>

https://ap-gto.groups.io/g/main/album?id=264707

Re: Added album Photos of Forum Members in their A-P Logo Hats

M Hambrick

I also took the liberty of adding the photo that David Johnson's posted on May 9 of his wife wearing her A-P hat.

Mike

Added album Photos of Forum Members in their A-P Logo Hats

M Hambrick <mhambrick563@...> added the album Photos of Forum Members in their A-P Logo Hats: Here is a photo album where forum members can upload photos of themselves wearing Astro-Physics logo hats in whatever exotic location (astro-related or not) they choose

Re: APPC emergency Stop Window keeps disappearing

Ray Gralak

Hi Tom,

This has happened three times now. It works one day and then the next is gone.
Anyone know how to restore it?
Can you be more specific? In the latest APCC, the Emergency Slew window closes when a slew completes. So, do you mean the window dowsn't open at all? If that is the case, then maybe another window is hiding it?

Or, if you "Save Window Positions" enabled and have multiple monitors, maybe it is being saved off screen? Or maybe it was saved in the minimizing state? Usually, turning off "Save Window Positions" in APCC's Settings menu will handle these cases.

-Ray

-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Tom Blahovici
Sent: Monday, May 31, 2021 7:18 PM
To: main@ap-gto.groups.io
Subject: [ap-gto] APPC emergency Stop Window keeps disappearing

HI
This has happened three times now. It works one day and then the next is gone.
Anyone know how to restore it?
Thanks, Tom

CP5 update?

weihaowang

Hi,

A few days ago, I got an email from AP regarding a special firmware update for CP5, saying that it
will require a special firmware load.

I don't see any discussion about this here, so I wonder if this is legitimate and if I should proceed.

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/

APPC emergency Stop Window keeps disappearing

Tom Blahovici

HI
This has happened three times now. It works one day and then the next is gone.
Anyone know how to restore it?
Thanks, Tom

Re: AP130GT + older rings without center top hole

Cheng-Yang Tan

My camera + OAG is the old workhorse: STF8300 and OAG8300 + ST-i guide camera.

Not sure that you can get the above setup from Diffraction Limited anymore.

cytan

On Monday, May 31, 2021, 07:00:03 PM CDT, John Jennings <johnrogerjennings@...> wrote:

Perfect. That's the exact setup I want to use.

How about your off axis guider, pretty good results? What guide scope and camera?

Re: AP130GT + older rings without center top hole

John Jennings

Perfect. That's the exact setup I want to use.

How about your off axis guider, pretty good results? What guide scope and camera?

Mach 1 ra motor failure to move the the west.

crib409acme425

the other night I set my Mach 1 up with a RASA 8 as a trial before New Moon. this was a new scope to me.  it was balanced , no cabling issues and in fact with the clutches loosened the scope was free to spin in ra and dec with a light touch.  Now to the trouble.  I noted it was not tracking in RA plus when ever I tried to use the Keypad to force it to move to the west the RA motor noise would last about a second stop with a second short burst and that was it.  I could go to the East and use the keypad to move N and S.  The battery was well charged.  Two mornings before I used this same Mach1 for the Lunar eclipse with out issue.
That Night I took the whole thing down and reassembled in my garage.  In the am I tried the K pad on my 1200 mount.  No problems.
I hooked the electronics up again, on the Mach 1, using the same battery as the night before and everything worked including tracking a phantom star .
It has worked ever since as I have tried to see if it would fail.  What could have been wrong? I did not look at the stall light I guess because it would move N, S and East and freely move with clutches released.
The other issue I have is that the ra gear train noise in ra slewing is not always of  the same frequency.  It seems to vary a bit.  thanks cjacobson

Re: "Limit Reached" at meridian, not meridian limit.

Eric Claeys

I think I figured it out.  When I tested it, DEC was 89 54' 14.3 sec, which is close to 90 degrees, and when you're close to 90 degrees the meridian limits don't work.  When I backed off DEC a degree, the "Time before limit reached" went to 1.5 hours, which is what I'd expect given the position of the scope.

"Limit Reached" at meridian, not meridian limit.

Eric Claeys

I just created Meridian Limits but am getting unexpected behavior.  When tracking, I get "Limit Reached" when the mount gets to the Meridian, not when it gets to the Meridian Limit.  I've successfully used this many times in the past and don't know what's changed.  The picture below shows my limits, and shows the "Limit Reached" message even though the mount is nowhere near the Meridian Limit.
Am I doing something wrong?

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