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First photos with Full-spectrum DSLR mod and refractor show much room for improvement

beginner astrophotography filters refractor dslr
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#1 skyace

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Posted 24 January 2021 - 04:32 PM

I just recently received my Canon EOS T3i back from having had the full-spectrum mod performed (i.e. stock filters removed and clear filter installed), and put this on my William Optics Zenithstar

61II for some test frames under Orion last Wednesday.  What I'm seeing in both the 5 second and 60 second exposures, however, seems to indicate I may have some filter issues resulting in star bloat.

 

I'm using in Astronomik L-2 UV-IR APS-C clip-in filter, and also have a 2 in Optolong L-Pro installed in a field flattener for light pollution (Bortle 5 where I'm at).

 

I understand the refractor is an APO Doublet, and the conditions weren't completely ideal (moon was 46%), and I also have a sturdier tripod on order to better support the Skywatcher Star Adventurer 2i and optics... but it still seems like things aren't quite right.  I used the Bahtinov mask for focus before imaging and I got an X pattern, and am reasonably certain that it wasn't knocked out of focus when I went to Orion. 

 

Do I need a more aggressive UV-IR filter like the Astronomik L-3? Is this more likely a focus issue?

 

I will also mention that the night before I shot the same target with a full-frame mirrorless, non-modified camera (so I didn't use the Astronomik L-2, but otherwise had the same setup) and did not see the bloated stars. 

 

I'm a beginner, and an aspiring astrophotographer, so I could be missing something due to lack of experience.  Any tips or suggestions are welcome smile.gif

 

The attached thumbnails are unedited (I shot in RAW, but these were exported thumbnails):

Exposure: 5 s on one, 60 s on the other

ISO: 1600

Here's a link to the full RAW files for these two: https://adobe.ly/3c1Xc26



#2 NatureKnyt

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Posted 24 January 2021 - 04:36 PM

Seems clearly (ba-dum) that focus is the primary issue. I don't know enough to be sure what impact have two filters in there has. The L-Pro would have the UV/IR block so the L-2 isn't needed I think. 

 

disclaimer: I'm a moron. 



#3 imtl

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Posted 24 January 2021 - 05:14 PM

Your focus is off to start with. And you do not need to double filter.

 

NatureKnyt, you're not a moron



#4 sg6

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Posted 24 January 2021 - 05:16 PM

Would agree with the above the L-2 seems to be doing little or nothing. However why the pairing would cause bloat is therefore questionable.

 

Bloat could be that you have more wavelengths getting through and so the image is bigger from more different focal lengths. Likely to be at the Red end as the original DSLR filters would have heavily removed anything above 650nm and your filters (both) will be passing 650nm to 700nm and a lot more.

 

I suspect it is the additional red end that is the cause of the bloat.

 

Your T3 will have cut the Ha down to 22% and now you are passing 95% of the Ha, and similar up to 700nm. The original filters will have started to reduce transmission at around the 600nm mark.



#5 skyace

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Posted 25 January 2021 - 11:34 PM

Thanks for feedback! I didn't even think about the fact that the L-Pro would perform the UV-IR block function and would make the L-2 unnecessary. Looking at the transmission curves, it looks like the L-Pro would let less of the spectrum at the top end than even the L-3. I think my approach now will be to just have the L-pro filter when in light-polluted skies, and use the L-2 when in darker skies.

 

And thanks for the confirmation to check focus. Next mission I'll grab a focus using the Bahtinov setup, and then I'll find the target and take a test frame to confirm.



#6 skyace

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Posted 07 February 2021 - 05:40 PM

The next time out went much better...I'm getting used to the setup more, and including some confirmation of focus on images as part of my routine.  One other thing I hadn't thought of that could have been a factor. Dew. The first time out was tremendously cold and though the Colorado air is usually pretty dry, the optics could have cooled to the dew point without my noticing (I did not have a dew heater at the time).

 

Here's a link to my latest results: https://www.astrobin.com/zt03on/

 

Thanks again for the help, all those who responded!




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