Author Topic: Using 2 different focal lengths  (Read 1480 times)

Offline rdryfoos

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Using 2 different focal lengths
« on: 2016 September 11 11:11:39 »
I have always wanted to take an image with my Televue np101is at 540mm and then add data of selected areas using my C11 Edge at 1,960mm.  From what I have seen and read, I understand this might be an automatic thing that is easily accomplished by Star Alignment and then integration.  But I have never done it.  Does the longer focal length get scaled to the smaller focal length FOV?  Will there be seams, or areas around the long focal length image that are similar to what happens when you align any image--there is a border that may require cropping--obviously this would not be good.  I probably will try it using just a luminance (or Ha if NB) for details (color with 540 mm fl will no doubt be better than the 1960mm).   Would it be better to just add all the subs--say Ha with 540 and 1960 into staralignment then integrate them as a single stack--or should I add the 1960fl Ha image to the non-linear 540mm fl image as a liminance layer like you would add a luminance to any non linear image?

Any recommendations?

Thanks----Rodd

Offline Zocky

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Re: Using 2 different focal lengths
« Reply #1 on: 2016 September 11 11:26:15 »
... Does the longer focal length get scaled to the smaller focal length FOV? 
You should do exactly the opposite, scale smaller FOV to the FOV of longer focal length. 
Skywatcher ED 80/600 with FF/FR x0.85; HEQ5-pro mount
SBIG ST-8300M, FW5 with Baader LRGB Ha7nm filters
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Offline rdryfoos

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Re: Using 2 different focal lengths
« Reply #2 on: 2016 September 11 11:51:12 »
... Does the longer focal length get scaled to the smaller focal length FOV? 
You should do exactly the opposite, scale smaller FOV to the FOV of longer focal length.
It is an automatic thing right? PI does it--so I shouldn't have to do anything but put into star alignment and integration.  The final image will have a FOV similar to teh small scope--not teh big one--so I don't understand why I shoudl scale to the big one?

Offline Zocky

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Re: Using 2 different focal lengths
« Reply #3 on: 2016 September 11 12:02:45 »
First you should do Star alignment. Select all frames for alignment and for reference image select the frame taken with longer focal length. PI should do the rest. After alignment proceed to Image Integration. 
Skywatcher ED 80/600 with FF/FR x0.85; HEQ5-pro mount
SBIG ST-8300M, FW5 with Baader LRGB Ha7nm filters
https://www.flickr.com/photos/zoran-novak/

Offline Zocky

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Re: Using 2 different focal lengths
« Reply #4 on: 2016 September 11 12:10:25 »
...so I don't understand why I shoudl scale to the big one?

Because if you scale larger FOV to a smaller one, you won't get seamless image.
Skywatcher ED 80/600 with FF/FR x0.85; HEQ5-pro mount
SBIG ST-8300M, FW5 with Baader LRGB Ha7nm filters
https://www.flickr.com/photos/zoran-novak/

Offline rdryfoos

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Re: Using 2 different focal lengths
« Reply #5 on: 2016 September 11 12:17:01 »
...so I don't understand why I shoudl scale to the big one?

Because if you scale larger FOV to a smaller one, you won't get seamless image.
By "scale" you mean use as a reference image (the big one) correct?  Will I see a difference in the image side by side if I add a good amount of data with the longer FL?  Is this something that is generally considered worth it? (assume good data)

Offline Zocky

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Re: Using 2 different focal lengths
« Reply #6 on: 2016 September 11 12:25:11 »
Is this something that is generally considered worth it? (assume good data)

Yes, I believe so. As long as you use longer focal length for Luminence and shorter for RGB, you should get a decent result.
Skywatcher ED 80/600 with FF/FR x0.85; HEQ5-pro mount
SBIG ST-8300M, FW5 with Baader LRGB Ha7nm filters
https://www.flickr.com/photos/zoran-novak/

Offline rdryfoos

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Re: Using 2 different focal lengths
« Reply #7 on: 2016 September 11 12:35:00 »
Is this something that is generally considered worth it? (assume good data)

Yes, I believe so. As long as you use longer focal length for Luminence and shorter for RGB, you should get a decent result.
Thanks--I will give it a go.  Maybe a Lum (or Ha) mosaic with FL 1960 with a single image at 4.3 (540 with ,8x reducer).  That will give me a fairly wide field at 2.58 arcec/pix and a luminence channel of .57 (or 1.14) arcsec/pix.