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Astrophotography barlow lens help please

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#1 Dean9tech

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Posted 09 August 2020 - 03:52 AM

Heloo, I bought skywatcher 200/1000 newtonian telescope on heq5 Pro mount, I am imaging with Canon t3i and I am looking to buy barlow lens for planetary imaging. Now what magnification do I need to make the most details on planets? and to make a planet as big as possible in my field of view? I have money only for one barlow for now although I understand I will need more of them in the future. Could you give me some advice.

#2 Tapio

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Posted 09 August 2020 - 04:15 AM

Usually f/20 - f/25 is considered good for planetary imaging.

So a 4x (or 5x) would be good.

 

Mind you DSLR is not the best tool for planets (a cheap - $200sh cmos camera would be better).

But you can do it and best is to take a video.

And a good thing is that t3i has a special crop video mode:

https://www.filmkit....d-is-invaluable



#3 Tulloch

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Posted 09 August 2020 - 05:20 AM

The Canon DSLR system is great for planetary, but only the 5x crop mode in LiveView gives you the quality you need. Before I purchased my dedicated planetary camera (the ASI224MC), I used my Canon 700D with BackyardEOS and was able to get some acceptable images of the planets with my 6" Evolution which compared favourably to the 224MC.

https://www.cloudyni...6-sct-test-two/

 

The best focal ratio to use is 5x the pixel size, so for your T3i/600D (with pixel size 4.29 microns) the best ratio to use is around f/21, so you are chasing a multiple of 4x. However, 4x Barlows are not common (and the ones that are around are expensive), so I would actually go for a cheaper 3x Barlow myself, which will actually end up giving you closer to 4x due to the distance between the DSLR sensor and the end of the Barlow. The page below shows how most Barlows change their magnification with distance.

http://www.televue.c...d=52&Tab=_photo

 

Read this article for how to use the LiveView 5x zoom, Jerry is not a fan of the 3x crop mode in the T3i. I would strongly urge you to follow the directions on his website, they worked for me.

https://www.astropix...resolution.html

 

And before you do anything, watch these tutorial videos - they will teach you a lot (at least, they helped me a lot smile.gif)

http://planetaryimagingtutorials.com/

 

Hope this helps,

 

Andrew


Edited by Tulloch, 09 August 2020 - 05:21 AM.

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#4 Dean9tech

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Posted 10 August 2020 - 08:27 AM

Thank you guys that helps a lot.

#5 t-ara-fan

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Posted 10 August 2020 - 11:46 PM

>> The best focal ratio to use is 5x the pixel size,

Could you explain why that is?

#6 Tulloch

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Posted 11 August 2020 - 01:58 AM

>> The best focal ratio to use is 5x the pixel size,

Could you explain why that is?

Here's the maths :) Backed up by real world experience also.

 

https://www.cloudyni...w/#entry5317455

 

Andrew



#7 luxo II

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Posted 11 August 2020 - 06:50 AM

Agree with Tapio; my ZWO-ASI533 camera has 3.76 micron pixels so a focal ratio around f/18. Two of my scopes are f/12 so f/18 is easily achieved with a 1.5X Barlow; alternatively I can use a 2X Barlow at about 50mm from the sensor to achieve similar.


Edited by luxo II, 11 August 2020 - 06:51 AM.


#8 t-ara-fan

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Posted 11 August 2020 - 01:58 PM

Here's the maths smile.gif Backed up by real world experience also.

 

 

Thank you.  It makes total sense.

I have a related question.

 

Let's say you were going to buy a camera. And the choices were 4.78um or 2.4um pixels.  Assuming noise, QE, and framerate were similar, would it be better to go with the smaller pixels and need less barlow magnification?  My telescope optics are great, not sure if a barlow or PowerMate is as good.




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