Astrophotography barlow lens help please
Posted 09 August 2020 - 03:52 AM
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:
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.
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.
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.
And before you do anything, watch these tutorial videos - they will teach you a lot (at least, they helped me a lot )
Hope this helps,
Edited by Tulloch, 09 August 2020 - 05:21 AM.
- Dean9tech likes this
Posted 10 August 2020 - 11:46 PM
Could you explain why that is?
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.
Posted 11 August 2020 - 01:58 PM
Here's the maths 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.