Re: Calculate flattener spacing for new camera from old camera

Bill Long

You really are though. The tolerance on the AP130 FF is +/-2mm on a 35mm chip. You are using a 4/3 chip. It will have so little issues getting a perfectly flat field that you are sweating over nothing.

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Nick Iversen <inoddy@...>
Sent: Tuesday, September 1, 2020 9:07 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Calculate flattener spacing for new camera from old camera

I'm not really making this too hard - I'm doing it the easy way by using calculation instead of trial and error. Even if I got the spacing from QHY it would not be accurate enough to make both cameras parfocal. My way gets both cameras properly back-spaced AND parfocal.

Re: Calculate flattener spacing for new camera from old camera

Eric Claeys

Michael, can you post your diagram as an attachment?  The resolution of in-message pictures is pretty low, versus attachments which are high resolution and easier to read.
Thanks!

Re: Calculate flattener spacing for new camera from old camera

Nick Iversen

I'm not really making this too hard - I'm doing it the easy way by using calculation instead of trial and error. Even if I got the spacing from QHY it would not be accurate enough to make both cameras parfocal. My way gets both cameras properly back-spaced AND parfocal.

Re: Calculate flattener spacing for new camera from old camera

Bill Long

You are making this too hard. Contact QHY and they will tell you the spec for the camera you have. Then use the data on the a-P site to determine the right spacing for your flattener. I use the same one, so 80.8 is correct, but you need to account for your filters (which we dont know what those are either) which would increase the space you need. 3/<thickness of filter in mm>. So for Astrodon or Chroma filters, which are 3mm thick, you add 1mm. So you want 81.8mm of spacing in that scenario.

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Nick Iversen <inoddy@...>
Sent: Tuesday, September 1, 2020 8:13 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Calculate flattener spacing for new camera from old camera

The camera is a QHY9 (since you asked) but there have been a number of different models of this and they have different spacing. The QHYCCD we site says 35mm but I know this is wrong.

Since the 13035FF changes the 130GTX from a focal length of 819 mm to 873 mm I'm going to use the formula:

change in focus in front of
13035FF  = 873 / 819 times the change in focus behind the 13035FF

So if I have the move the focuser in by 1mm to get focus with the new camera then I need to move the camera 873 / 819 = 1.066mm closer to the 13035FF  to get back to the required 80.8 mm spacing. If that doesn't work then I'll try 819 / 873. If that doesn't work I'll use trial and error with shims.

Re: Calculate flattener spacing for new camera from old camera

Peter Nagy

lqy at qhyccd dot com

Peter

Re: Calculate flattener spacing for new camera from old camera

Peter Nagy

If you send an email to Yang of QHY, he will quickly reply back to you. Tell him exactly the version of QHY9 you have. Or attach a picture if your QHY9 camera. His email address is:

He'll be happy to respond.

I'm a little skeptical of your formula, I don't think it works that way.

Peter

Re: Calculate flattener spacing for new camera from old camera

Nick Iversen

This will help you see what I'm trying to do. My process will be:

1) replace the old camera with the new camera.
2) refocus and calculate the distance d required
3) restore the focus position and then move the camera by distance d in the same direction.
4) go back to (1) and repeat the loop until convergence.

I would like to speed up convergence by using a formula for the distance d to move the camera in step 3. E.g. move the camera by 1.066d

Re: Calculate flattener spacing for new camera from old camera

Nick Iversen

The camera is a QHY9 (since you asked) but there have been a number of different models of this and they have different spacing. The QHYCCD we site says 35mm but I know this is wrong.

Since the 13035FF changes the 130GTX from a focal length of 819 mm to 873 mm I'm going to use the formula:

change in focus in front of
13035FF  = 873 / 819 times the change in focus behind the 13035FF

So if I have the move the focuser in by 1mm to get focus with the new camera then I need to move the camera 873 / 819 = 1.066mm closer to the 13035FF  to get back to the required 80.8 mm spacing. If that doesn't work then I'll try 819 / 873. If that doesn't work I'll use trial and error with shims.

Re: Calculate flattener spacing for new camera from old camera

Bill Long

The total distance from the 13035FF to the chip is only 102.9mm IF you remove the provided spacer from AP. That is really important to mention.

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Michael Hambrick via groups.io <mike.hambrick@...>
Sent: Tuesday, September 1, 2020 7:32 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Calculate flattener spacing for new camera from old camera

Hi Nick

There are two distances that you must be concerned with: the distance from the back of the flattener to the CCD chip which is fixed, and the overall focus distance for which you use your focuser to achieve the optimum focus. Does the sketch below help make sense of it ? The fixed back focus from the flattener to the chip is dimension X. The total focus length is Y and is adjusted with the focuser. In the example below I had to get a spacer with length D from Precise Parts in order to achieve the proper back focus for the flattener.

In your case, the total distance from the back of the 13035FF to the surface of the CCD chip is 102.9 mm. This is equivalent to the dimensions A through E in the sketch below, and it must be maintained in order to get a flat field. You may need to order a custom length spacer to meet this distance requirement. The dimension Y is a function of the optics, and I think it should be the same as for your existing camera.

I hope this visual helps.

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:        "Mike Dodd" <mike@...>
To:        main@ap-gto.groups.io
Date:        2020-09-01 09:04 PM
Subject:        Re: [ap-gto] Calculate flattener spacing for new camera from old camera
Sent by:        main@ap-gto.groups.io

On 9/1/2020 9:48 PM, Nick Iversen wrote:
> Mike - Pointing the scope to a bright star and focusing on it doesn't
> solve the problem. I would have to spend a lot of trial and error with
> shims to focus the camera by changing the back focus distance.

I'm confused. The purpose of a focuser is to move the camera to the
telescope's focal point.  If you have a focuser, why do you need shims?

I thought "back focus" is the distance between the back of a lens and
the image of an object at infinity. Why won't the focuser bring the new
camera into focus?

Maybe I don't understand what you're trying to accomplish.

--- Mike

Re: Calculate flattener spacing for new camera from old camera

Michael Hambrick <mike.hambrick@...>

Hi Nick

There are two distances that you must be concerned with: the distance from the back of the flattener to the CCD chip which is fixed, and the overall focus distance for which you use your focuser to achieve the optimum focus. Does the sketch below help make sense of it ? The fixed back focus from the flattener to the chip is dimension X. The total focus length is Y and is adjusted with the focuser. In the example below I had to get a spacer with length D from Precise Parts in order to achieve the proper back focus for the flattener.

In your case, the total distance from the back of the 13035FF to the surface of the CCD chip is 102.9 mm. This is equivalent to the dimensions A through E in the sketch below, and it must be maintained in order to get a flat field. You may need to order a custom length spacer to meet this distance requirement. The dimension Y is a function of the optics, and I think it should be the same as for your existing camera.

I hope this visual helps.

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:        "Mike Dodd" <mike@...>
To:        main@ap-gto.groups.io
Date:        2020-09-01 09:04 PM
Subject:        Re: [ap-gto] Calculate flattener spacing for new camera from old camera
Sent by:        main@ap-gto.groups.io

On 9/1/2020 9:48 PM, Nick Iversen wrote:
> Mike - Pointing the scope to a bright star and focusing on it doesn't
> solve the problem. I would have to spend a lot of trial and error with
> shims to focus the camera by changing the back focus distance.

I'm confused. The purpose of a focuser is to move the camera to the
telescope's focal point.  If you have a focuser, why do you need shims?

I thought "back focus" is the distance between the back of a lens and
the image of an object at infinity. Why won't the focuser bring the new
camera into focus?

Maybe I don't understand what you're trying to accomplish.

--- Mike

Re: Calculate flattener spacing for new camera from old camera

Bill Long

What make and model is this camera? It should say on it...

From: main@ap-gto.groups.io <main@ap-gto.groups.io> on behalf of Nick Iversen <inoddy@...>
Sent: Tuesday, September 1, 2020 6:48 PM
To: main@ap-gto.groups.io <main@ap-gto.groups.io>
Subject: Re: [ap-gto] Calculate flattener spacing for new camera from old camera

Peter - I don't know the back focus for the new camera. Hence the need to do it empirically.

Mike - Pointing the scope to a bright star and focusing on it doesn't solve the problem. I would have to spend a lot of trial and error with shims to focus the camera by changing the back focus distance. I'd rather do it by calculation. But if I don't get a formula that's the way I'll do it.

Re: Calculate flattener spacing for new camera from old camera

Mike Dodd

On 9/1/2020 9:48 PM, Nick Iversen wrote:
Mike - Pointing the scope to a bright star and focusing on it doesn't
solve the problem. I would have to spend a lot of trial and error with
shims to focus the camera by changing the back focus distance.
I'm confused. The purpose of a focuser is to move the camera to the telescope's focal point. If you have a focuser, why do you need shims?

I thought "back focus" is the distance between the back of a lens and the image of an object at infinity. Why won't the focuser bring the new camera into focus?

Maybe I don't understand what you're trying to accomplish.

--- Mike

Re: Calculate flattener spacing for new camera from old camera

Peter Nagy

That's why I asked for brand and model of your new camera.

Peter

Re: Calculate flattener spacing for new camera from old camera

Roland Christen

There is no formula. The distance from the back of the flattener to the chip is a fixed number which is listed on the website under each field flattener.
All cameras list the distance from the chip to the front surface, which is normally less than the distance required by the flattener. The rest of the distance is made up with a spacer.

Rolando

-----Original Message-----
From: Nick Iversen <inoddy@...>
To: main@ap-gto.groups.io
Sent: Tue, Sep 1, 2020 8:48 pm
Subject: Re: [ap-gto] Calculate flattener spacing for new camera from old camera

Peter - I don't know the back focus for the new camera. Hence the need to do it empirically.

Mike - Pointing the scope to a bright star and focusing on it doesn't solve the problem. I would have to spend a lot of trial and error with shims to focus the camera by changing the back focus distance. I'd rather do it by calculation. But if I don't get a formula that's the way I'll do it.

Re: Calculate flattener spacing for new camera from old camera

Nick Iversen

Peter - I don't know the back focus for the new camera. Hence the need to do it empirically.

Mike - Pointing the scope to a bright star and focusing on it doesn't solve the problem. I would have to spend a lot of trial and error with shims to focus the camera by changing the back focus distance. I'd rather do it by calculation. But if I don't get a formula that's the way I'll do it.

Re: Calculate flattener spacing for new camera from old camera

Peter Nagy

What brand and model is your new camera?

If your new camera has different back focus, then simply change the spacing to meet the 13035FF flattener back focus requirement. Once you get the proper spacing, it should behave exactly like your old camera. The focus should not change or be different.

Peter

Re: Calculate flattener spacing for new camera from old camera

Mike Dodd

On 9/1/2020 8:37 PM, inoddy@hotmail.com wrote:
I have a 13035FF flattener and a camera with correct spacing and correct
focus. I want to find the correct spacing for a new camera with unknown
sensor plane depth.
Truly unknown? Have you no specs for both cameras? If not, perhaps you can look into the camera and estimate the difference in spacing to the sensors.

My idea is to replace the old camera with the new camera and find how
far the focus changes in mm. The change in focus distance must be
mathematically related to the change in spacing that I need. I doubt
that it is 1 to 1 because the flattener changes the focal length of the
scope.
Are you over-analyzing this? Why not install the new camera and point the scope to a bright star or the Moon, and then focus it?

When I installed a new camera, I simply adjusted the focuser until the target was in focus, and that was it. I never calculated anything in advance.

--- Mike

Calculate flattener spacing for new camera from old camera

Nick Iversen

I have a 13035FF flattener and a camera with correct spacing and correct focus. I want to find the correct spacing for a new camera with unknown sensor plane depth.

My idea is to replace the old camera with the new camera and find how far the focus changes in mm. The change in focus distance must be mathematically related to the change in spacing that I need. I doubt that it is 1 to 1 because the flattener changes the focal length of the scope.

Does anyone know the calculation?

Re: Drift Alignment To Help With Unguided Images

Worsel

Curtis

One of the advantages to a corrected Dall-Kirkham (either PW of AGO) is the spherical secondary, which means collimation is easier to dial in than an SCT.  I check collimation on the 14.5" AGO every 6 months.  It rarely needs adjustment, but if it does, it is easy.

I use the A-P 27TVPH occasionally as well.  Brings the f ratio down to to about f/5.  (0.75 x 6.7)   Vignetting occurs (full-frame DSLR), even given the big image circle of the 14.5, but that can be managed.

Bryan

Re: Drift Alignment To Help With Unguided Images

CurtisC

The guy who makes AG Optical's optics is a neighbor and personal friend of mine, so -- just speaking for myself -- I'd look at AGO first.  As for PlaneWave -- they moved from Calif. to Michigan, which converts delivery from a simple drive to the factory into a \$1000 crating and shipping adventure.  Of course, I'd have to pay for shipping an AGO scope, too, since his plant is in Alabama.  But, of the two telescope manufacturers, the other consideration puts AGO at the top of my personal list.

HOWEVER -- when I was throwing around retirement money a few years ago, to build my observatory and buy my equipment, I specifically chose a refractor (TEC140ED), because I didn't want to worry about flopping mirrors or sagging secondaries and endless attempts to get the collimation just right.  I don't have enough remaining years to waste my time on that.  My only regret is that I like galaxies.  The TEC140ED is fine for extended nebulae and galaxy clusters, but not so much for capturing details of individual galaxies.  I have occasionally looked at the EdgeHD 11 as an economical alternative to get some additional aperture, without exceeding the capabilities of my Mach1GTO mount, but the preceding discussion pretty much nixes that idea.

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