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# Help figuring out magnification/focal length

4 replies to this topic

### #1 gfunkernaught

gfunkernaught

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Posted 29 September 2020 - 08:38 AM

I suck at math, so I need help figuring something out. I did some research and found that one Barlow Length of a typical 2x Barlow is 100mm from the Barlow lens to the eyepiece or camera sensor. So that 100mm is possible because of the the tube. Moving the Barlow lens farther away from the EP/sensor will increase magnification, moving closer will decrease magnification. So based on the 5xpixel size=recommended focal length formula I've seen for planetary imaging, my canon eos ra which has a pixel size of 5.36 microns, I had to set my magnification to 2.5x, which I had to place the lens 150mm from the sensor. I found this chart on Starizona's website for focal length and pixel scale here https://starizona.co...netary-imaging/
I want to start using my ZWO ASI290mm mini for planetary imaging, which has a pixel size of 2.9 microns. The chart suggests two different focal lengths based on seeing conditions, one being 2900mm for good seeing, and 7200mm for exceptional seeing conditions. So based on the 100mm=2x magnification formula, how would I find the distance required to achieve a focal length of either 2900mm or 7200mm? Like how can I find the magnification power multiplier from a given focal length? Again, I suck at math. Any help would be appreciated and if my question isn't elaborate enough please let me know.

Edited by gfunkernaught, 29 September 2020 - 08:40 AM.

### #2 gfunkernaught

gfunkernaught

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Posted 29 September 2020 - 08:47 AM

I'm dumb, I think I just answered my own question but could totally use critique. I just divided the recommended focal length by my telescope's focal length, and got the magnification multiplier number. So 7200/2032=3.5, which means the barlow lens would have to be 250mm from the sensor. 2900/2032=1.4, which means the the barlow would have to be about 50mm from the sensor. I'm not 100% confident in my math skills but I think this sounds right.

### #3 Couder

Couder

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Posted 29 September 2020 - 08:49 AM

Don't know if I can help or not. I've taken many pictures, never used any math. I set up, select the eyepiece(s), and start taking pictures. I am obviously  an expert astro photographer - I have no bad pictures. I will share my secret - throw away/delete all your bad ones. You too can become an expert.

Just Be Cuz

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Posted 29 September 2020 - 12:47 PM

Moving to Beginning and Intermediate Imaging

### #5 Stelios

Stelios

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Posted 29 September 2020 - 01:45 PM

With a 2.9um camera, you want an F *ratio* of from around 5*2.9 ~= 15 (normal seeing) to 7*2.9 ~= 20 or maybe a little higher (excellent seeing). So you need either 1.5x or 2x Barlow as you almost certainly have an 8" SCT at F/10.

If you think in terms of focal ratios, the math is MUCH easier

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