Question about Focus with DIY-Astromodded Canon T3i - Forums

Topic starter

Dec 18, 2023

Hello everyone. Thanks in advance for answering my question.

I'm on the verge of astromodding my Canon T3i. Mainly by removing the LPF2 filter, and keeping the LPF1 in place. The only question I have left before astromodding it is regarding backfocus/autofocus without the LPF2 filter, as I am NOT planning to replace the LPF2 filter with any replacement glass elements.

With the LPF2 filter removed, can I still achieve proper backfocus/autofocus functionality during normal daytime photography use (assuming I am successful at putting the sensor back exactly where it was prior to modding) if I simply attach a 2" UV/IR cut filter to the front of my camera lens with some adapter rings? Or would I need the UV/IR cut filter to be between the lens and the sensor, rather than in front of it in the form of a 2" filter?

I would buy a clip-in UV/IR cut by Astronomik, as I know those will work in helping me achieve proper focus/autofocus, but that would run me $130 CAD, and that's a hefty cost to me as a SVBony UV/IR cut is only $20. Plus, the main reason I have for DIY-Astromodding is the fact that I save a LOT money, and if I have to buy a $130 filter anyway, then I may as well go the extra bit and just get someone to properly astromod it for me.

andrea tasselli

Frequent contributor

Dec 18, 2023

As far as I know you need to reposition the sensor itself to have the same capabilities as of before. If you plan on retaining the LPF1 filter than you don't really need the IR-cut filter so you might as well spare the effort. This with the 2 Canon I have had modified (1300D and 7Dii).

Carl Elgario

Topic starter

Dec 19, 2023

andrea tasselli:
As far as I know you need to reposition the sensor itself to have the same capabilities as of before. If you plan on retaining the LPF1 filter than you don't really need the IR-cut filter so you might as well spare the effort. This with the 2 Canon I have had modified (1300D and 7Dii).

So basically, no need for an IR-cut, but I will need to move the position of the sensor itself (I think forward, from what I've read) a certain amount? How would I go about doing this precisely?

andrea tasselli

Frequent contributor

Dec 19, 2023

Carl Elgario:
So basically, no need for an IR-cut, but I will need to move the position of the sensor itself (I think forward, from what I've read) a certain amount? How would I go about doing this precisely?

That's correct, the sensor needs to move forward by 1/(n'+1)*(filter thickness), where n' is the refractive index of the filter. How you do it depends on the camera but I assume you add spacers between the sensor board and the camera case.

Well Written Helpful Insightful Concise

danieldh206

Dec 19, 2023

You can purchase a new sensor filter with the same thickness that allows Ha. I know the videos on line make Astro modding a Canon look straight forward but it is not, especially if you want to use the camera with lenses after the mod. If you only use with telescopes the mod is easier because you can easily adjust for a 55mm back focus. But trying to get the 44mm back focus for Canon lenses is a pain.

Helpful Concise

Carl Elgario

Topic starter

Dec 19, 2023 · Edited: Dec 19, 2023

You can purchase a new sensor filter with the same thickness that allows Ha. I know the videos on line make Astro modding a Canon look straight forward but it is not, especially if you want to use the camera with lenses after the mod. If you only use with telescopes the mod is easier because you can easily adjust for a 55mm back focus. But trying to get the 44mm back focus for Canon lenses is a pain.

Learned that the very hard way just now. Not only did I screw up my sensor position/tilt to the point where I will need to dedicate a LOT of time to recalibrate it, but I stripped a bunch of screws. If the videos and tutorials made the fact that you may deal with stuck screws and will need high quality screwdrivers more clear, I would've saved my time and money and just got it professionally modded. What a disappointment. Ugh. At the end of the day it was my decision and my fault, no one elses, but it's still quite a disappointment regardless.

Well Written

Jens

Dec 19, 2023

I moddified my Canon 600D. I literally just took the filter out and put the sensor back in. Now it's important that the tightening screws for the sensor go exactly where they were before. put a line on the screw and the screwhole and make them lineup again after you're done.
I unfortunately didn't do that and had to eyeball it. Honestly it turned out to be ok.
I didn't calculate the missing filter or anything like that.
If you go onto my account, the North america, veil, soul Nebula and the andromeda Galaxy were all done with that camera.

Just make sure to get yourself a UV/IR cut filter or a Dual Narrowband filter. otherwise the photos will look pretty bad and blurry. I was pretty dissapointed
when I first tested the camera without those filters.

If you need more information about it, feel free to ask

Helpful

Jens

Dec 19, 2023

Aw man, I saw that you tried it, yeah stuff like that happens. I had one stripped screw on the outside of the camera, I just drilled it out haha.

If you were in switzerland, I could send you my camera. I don't need it anymore because I upgraded to a cooled astrocamera.

I was really happy with the camera though, great way to do astrophotography.

Supportive

Carl Elgario

Topic starter

Dec 19, 2023

Jens:
Aw man, I saw that you tried it, yeah stuff like that happens. I had one stripped screw on the outside of the camera, I just drilled it out haha.

If you were in switzerland, I could send you my camera. I don't need it anymore because I upgraded to a cooled astrocamera.

I was really happy with the camera though, great way to do astrophotography.

In an incredible turn of events, I powered through, and actually managed to remove the LPF2 filter, get everything back in AND in working order, as well as achieve proper back focus that allowed me to focus at infinity! I managed to get my tilt in a good enough order by simply measuring the lengths of the black “knobs” next to the torx screws that adjust sensor position, and that was also somehow successful as I don’t have ridiculous tilt.

Now, my back focus is not precise for autofocus Canon lenses, and I absolutely won’t be able to use autofocus lenses anymore, but I’m honestly not complaining, as this is gonna be an astro-dedicated dslr anyway. I saved myself over $250 CAD and still have a working dslr without any crazy tilt, so I am very happy.

Helpful Engaging

Jens

Dec 19, 2023

Awesome to hear! I'm glad it worked out for you! I think the best desicion I've made was to go for a L-enhance dual narrowband filter. With that I'm able to image from my balcony from the little town I live in without issue.

Supportive

Armin Lukas

Dec 24, 2023 · Edited: Dec 24, 2023

To gain the autofocus with the 600D you need to adjust the screws for focus-adjusting. With the 600D you need to turn exactly 150 degrees clockwise to position
the sensor.
I also own a 600D without the LPF2. And the sensor was adjusted with this method.

Behnam Darvish

Dec 24, 2023

My suggestion is that you better not touch or reposition the sensor as you may easily end up causing tilt in the sensor. I have even seen professionally repositioned sensors in modded cameras that are suffering from tilt. Put on a UV/IR cut clip-on filter between the lens and the sensor. They are generally thick enough to bring you back to focus again and they are cheap. The only drawback is that you only lose a small fraction of the incoming light. Also if the filter is very cheap, it may cause halos around bright stars and objects just like any other cheap filters.

Helpful Concise

Monty Chandler

Dec 25, 2023

Consider not screwing up your perfectly good dslr and consider an astrophotography camera? Auto-focus is something you do to your telescope, not your camera. Cheers