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Determining barlow magnification

Astrophotography DSLR DSO Equipment Imaging
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#1 Rayje1997

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Posted 28 March 2023 - 09:24 AM

Hello!

 

I'm not exactly a beginner, but I felt that this question belonged in the beginner forum.

 

It is galaxy season currently and I my main imaging scope and camera aren't generally well suited for galaxies. With the frankly HUGE pixels and the relatively short focal length (650mm) of the scope, it is generally not long enough to capture much detail. Ideally I would either get a longer scope, or a camera with smaller pixels, but budget is a little tight at this present moment so that's not an option for this season. Maybe next year. I do have a 3x barlow, but imaging at f/15 is a bit much even for me and I don't need THAT much more focal length.

 

So the question is this, the barlow lens element comes off of the barlow I have and I could easily screw that onto a 1.25" nosepiece for my camera to reduce the effective amount of magnification. The good thing is that the barlow will make it easier to reach focus with my DSLR on my newt, but the bad part is, I'm not sure exactly how much moving it closer to the sensor will reduce the magnification factor. Is there some formula I can use to determine barlow distance from the sensor vs effective magnification? And before anybody says that you shouldn't barlow for imaging, I KNOW this is mostly an experiment and I just want to know what effective focal length and f-ratio I will be working with.

 

Thanks in advance!



#2 John Rogers

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Posted 28 March 2023 - 09:45 AM

The most precise method to determine effective focal length would be to take an image of a star field and plate solve to determine the scale.  The scale ("/pixel) can then be used to calculate the focal length.

 

The nova.astrometry.net website can be used to plate solve the image in a few minutes.


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#3 Rayje1997

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Posted 28 March 2023 - 09:59 AM

The most precise method to determine effective focal length would be to take an image of a star field and plate solve to determine the scale.  The scale ("/pixel) can then be used to calculate the focal length.

 

The nova.astrometry.net website can be used to plate solve the image in a few minutes.

I use ASTAP with NINA for plate solving normally but I have never looked for that particular info. I hadn't considered using plate solving to tell me that but I will give it a try now that you say it. Thanks for the tip!



#4 Der_Pit

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Posted 28 March 2023 - 03:18 PM

If you want to dive into it, this image from Wikipedia contains all the relations:

 

Barlow_02.png

 

What you're after, the magnification, is called β here.  It's however not trivial to get all the focal length info, and actually figure out what the distances in your setup are.  So the shoot-and-platesolve suggestion is a quite reasonable one grin.gif


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#5 Rayje1997

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Posted 28 March 2023 - 04:58 PM

If you want to dive into it, this image from Wikipedia contains all the relations:

 

Barlow_02.png

 

What you're after, the magnification, is called β here.  It's however not trivial to get all the focal length info, and actually figure out what the distances in your setup are.  So the shoot-and-platesolve suggestion is a quite reasonable one grin.gif

I actually will probably do some of the math just for fun, but I agree that practically the "try it and see" method really does sound like what I'll be going with.
 



#6 Rayje1997

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Posted 30 March 2023 - 07:25 AM

Update: I tried this last night and it turns out that I'm looking at about 2.25x. I had to do a combination of plate solving and math to come up with that, and in NINA the plate solving would not work properly until I changed the scope's focal length to 1,500. The actual effective focal length was 1,462.5mm at f11.5. That's pretty slow, but I went ahead and did a 5 hour session on M101 and we will see what comes out of it. I will post the processed results here once I have that done, but what I can tell you before I even do that is that if someone else wants to do this as a serious solution to not having enough FL, get a better barlow, and stick with something like a 2x so that you end up somewhere around 1.5x. What I mean by get a better barlow, is that because of my cheap $20ish barlow, there is a lot of field curvature in the image as you can see in this single 60 shot. 

 

M101 Test Image


#7 rj144

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Posted 30 March 2023 - 07:40 AM

You generally don't want to use a barlow for DSO.


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#8 Rayje1997

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Posted 30 March 2023 - 07:57 AM

You generally don't want to use a barlow for DSO.

I know, I said that in my original post. This is really just an experiment while the moon is bright. I can actually deal with the slowness, the only thing I can't deal with is the field curvature. It was also a test to see if I can guide effectively enough to image at around 1,500mm because I have been considering purchasing a small sct or maksutov for targets that I want to get closer on. Bright moon nights make great testing nights!


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#9 Rayje1997

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Posted 30 March 2023 - 08:38 PM

Here is the final result of this test. After stacking it looks like there might have been some chromatic aberration introduced by the barlow as well as the aforementioned field curvature. Overall, of course I don't recommend doing this for the wealth of reasons that others mention all the time, BUT if you just want to goof around on a bright moon night like I did, it was pretty fun and it's cool to see a galaxy at a larger scale than you would normally capture. I will be purchasing an SCT or a mak for this purpose in the future, probably something around 1,200mm or so that will also be used for planetary imaging with a barlow. If you read this thread, I hope you got something from it!

 

M101 barlow test



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