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Use of barlow lenses when capturing Neptune

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#1 jsnatale


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Posted 15 September 2021 - 11:21 AM

Neptune is now close to opposition, and I would like to try to find it, and take pictures of it one night soon. I have an ASI 224 MC camera, and a CPC 800 HD Edge telescope with go-to. 


I have read that at least 150X magnification is needed to see the planet adequately. What does that mean in terms of which barlow lens to use?  A 2X or 3X? I can't calculate magnification since there is no eyepiece in the image train when using my camera. For those of you that have imaged Neptune with an 8" telescope, what barlow are you using, if any? From what I have read it is nothing more than a tiny blue dot, and can be extremely difficult to find. I want to get the most out of my telescope and camera, and get a few pictures of it while it is close to opposition. 


For those of you that have responded to my last post on Neptune - thanks. This post deals more with barlow lenses, so please feel free to comment on this as well. 


Also if any of you have captured Neptune before, please post pictures, and let me know what you are using for your telescope and image train. I am curious what is possible with different telescope/barlow/camera options. 

#2 matt_astro_tx



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Posted 15 September 2021 - 12:14 PM

So in astrophotography no one uses "magnification" as a factor.  It's all about image scale and sampling.


For optimal sampling you want to operate at a focal ratio that is 5-7 times the size of your camera's pixels.  Here's a breakdown.

  1. Your camera, the ASI224MC has 3.75um pixels
  2. Multiply 3.75 x 5 and you get 18.75
  3. Multiply 3.75 x 7 and you get 26.25
  4. What this means is that your optimal planetary focal ratio is in the range of F/18 to F/26
  5. Your CPC 800 Edge HD telescope is a F/10 instrument, so you'll want either a 2x barlow (which gets you to F/20) or a 2.5x barlow (which gets you F/25).

Make sense?


This combination of your camera, scope, and a 2x or 2.5x barlow will produce the best images possible for you.


Ultimately though you need to know that above all else the seeing is what will determine how good your images are.

Edited by matt_astro_tx, 15 September 2021 - 12:17 PM.

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#3 james7ca


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Posted 15 September 2021 - 12:18 PM

There is really no meaningful way to calculate an equivalent visual "magnification" when imaging.


In any case, below is an animation that I did comparing the relative sizes and color of Uranus and Neptune when imaged with a 9.25" EdgeHD (at its native f/10) and with an ASI183MM camera. I won't try to claim accurate color, but both RGB images got exactly the same processing and it is well known that Neptune appears much more blue than Uranus.

Attached Thumbnails

  • Uranus and Neptune.gif

Edited by james7ca, 15 September 2021 - 01:03 PM.

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#4 sfugardi



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Posted 15 September 2021 - 07:04 PM

For Neptune, don't use a barlow since it is too dim. Your best option is prime + RG610 filter with your 224MC to capture any storms. Don't bother imaging Neptune unless you have 4/5 seeing. The best focus is when you can see Triton on the same screen, go for long runs at least 10' 20-25fps. The trick is to find it with the scope, then with the camera. Good luck!




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#5 Tulloch



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Posted 15 September 2021 - 07:15 PM

Here's a recent image of Neptune and Triton I took with my C9.25" SCT, 2.5x PowerMate and ASI224MC. I set my camera up to image at 20 fps at a gain setting of 500 and imaged for 5 minutes. Stacked in AS!3, sharpened in Registax, Triton brought up using the "levels" tool in Photoshop. The hardest thing about imaging Neptune is finding it in the first place, here are some tips to put it on the camera sensor.



Neptune is plenty bright enough to capture at f/21, you can read more about this image here.



2021-08-26-1436_5-L-Neptune_AS_F3000_lapl6_ap1_Driz30 ps2sm200.png


Hope this helps, Andrew

Edited by Tulloch, 15 September 2021 - 07:16 PM.

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#6 jsnatale


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Posted Yesterday, 06:09 AM

Thanks everyone. This is really great information, and the pictures are fantastic. Tulloch and James7ca- I have the smaller version of your telescope (8 vs 9.25). Hopefully I can eventually have the same success you both had with the slightly smaller aperature. Not sure how much difference the additional 1.25" in aperature makes, or how it fits into the entire viewing equation. 


Between this and my other recent post on the subject, I now have a lot of really great information. Again, thanks! 

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