7 replies to this topic

[The0s]

Hi everyone,

 

As you can probably tell from the title, I'm having a bit of trouble of getting into focus with a Redcat 51 II-U and an older astromodded DSLR (Canon XSi). I just recently got a T-ring to play around with using my DSLR with this telescope, and for whatever reason I cannot achieve focus, even after playing around with various adapters (the M42-M48 needed to connect to the telescope, M48-M48, etc.) and filters (tried adding a UV/IR cut filter into the T-ring) - the helical focuser of the Redcat just stops before achieving focus. Is this a back focus issue, a camera issue, or a telescope issue (or all 3)? Do I just need a different adapter (I'm currently repurposing the ones from my ASI533MC Pro, whose sensor is much farther forward)? My current theory is that the sensor is just too far back in the DSLR, but I'm not really sure. Anyway, I'm curious to hear what you guys think, and thanks in advance for any advice.

 

Clear skies,

 

The0s

 

P.S. If this post fits better on the DSLR and mirrorless forum, please feel free to move it

[Quadstronomer]

Yup, backfocus for sure. REDCATs are Petzval designs, which means they are fully color corrected and flattened. But, finding that focal plane is still important, that is to actually find focus on your sensor, just not to affect the "shape" of the field.

[daveco2]

If you turn the helical ring so the camera moves toward the scope; and it looks like you're coming into focus, but the focuser reaches the end of its travel, then you need to a thinner adapter(s) to move the camera closer to the scope. 

Yes, the sensor is too far back in the DSLR.   

[The0s]

Okay, I think I just realized my mistake - yes, it was the back focus, but it was also that I bought a 42 mm T-ring instead of a 48mm. Because of that, I can't get the camera close enough to the the end of the the telescope. So yeah, looks like I'm now going to need to either return the T-ring or buy a short adapter.

 

[Quadstronomer]

Dont send it back, you can never have enough adapters, right!  haha!

[michael8554]

"42 mm T-ring instead of a 48mm"

 

Since the back spacing was too much, 42mm is shorter than 48mm so should be better ?

 

But do you mean T2 Adapters ?

 

The thickness of Canon T2 Adapters should always be 11mm, whether they are 42mm or 48mm diameter.

[unimatrix0]

I don't have the Redcat 51 and I'm aware it's a Petzval design, but I think if the helical focuser runs out of travel before reaching focus (assuming it's been extended and not retracted ) then his camera is too close and needs more distance between the scope and the camera.  

 

I'm just coming from the assumption that regardless if it's a petzval and flat field or not, there still need to have some distance between the sensor and the end of the scope. 

[Yerman]

It’s real easy to check. Take everything off the focuser. Set the focusing ring to about mid point. Point your scope at a distant object in daylight. Hold a piece of white card or paper where you think the camera would be. Move it back and forth until you see a focused image on the paper. Approximate is close enough. Measure the distance and that is the distance you have to come up with to get a focus.