EdgeHD 8's Focal Length 1494 MM, not 1422, dark vignetting in the corners of image - Forums

Topic starter

Oct 7, 2023 · Edited: Oct 7, 2023

So my ASIAIR Plus is reporting my Celestron EdgeHD 8's Focal Length as 1494 MM, not 1422 mm. I know a little variation is normal, but 1494 mm seems WAY off of 1422, isn't it? I'm also getting vignetting of dark in the corners of image. That seems to mean something's off. Aren't Edge HD's are famous for being perfect from corner to corner?

I set up the image train is EXACTLY 105mm as it should be for OAG and .7x reducer. Even measuring from end of reducer, a little inside OAG, as several people tole me here not outside. I factored the correct back focus of my ZWO ASI2600MC camera, the OAG, the Reducer, and the small adapter I have at the end. Collimation is perfect, and image is in focus, reported with ASIAIR and using EAF. plus shot some good footage of the Moon last night.

ASIAIR is good at reporting true focal length. I put on a 500 mm scope before that has no electronics and it set it to 500 mm. It reports my "250 mm" RedCat 51 as 248 mm, which I've heard is the true focal length.

So what gives? My scope was second hand, I can take in-focus pix….BUT, I'm thinking this might be why I'm having issues with OAG seeing correctly.

QUESTION: IS THERE ANY REASON this would happen if everything's correct? I even tried taking the prism out of OAG just to check it wasn't changing anything. Still 1499 mm.

ALSO: would I solve this by adding, or reducing, length to my image train. Any way to figure out how many mm to add or remove? Is it a straight 1:1 (like I'm off by 68 mm I add or remove 68 mm? It can't be that simple, right?

Thank you.

Oct 7, 2023 · Edited: Oct 7, 2023

It's normal. The C8's focal length is actually 2125mm, not the advertised 2032, which was a number devised to make the focal length match the advertised focal ratio of F/10 (It's actually ~F/10.456). The equivalent focal length is actually quite inline with the reduction of the Edge HD 8's Focal Reducer, so nothing out of whack there. My ASIair in particular shows 2148mm. Not too sure what adds the 23mm, likely the correctors of the Edge HD system.

And for the dark corners, the vignetting on the APS-C sized chip is perfectly normal, the issue is with the focal reducers themselves not providing a fully illuminated circle like the F/10 one does.

I've stopped using mine at F/7, since there's not much SNR gains from going from one slow speed to another, and theres a noticeable decrease in sharpness, contrast, and field flatness, and the fact I use a Full-Frame chip.

Heres an example of a Full-Frame flat at F/7, showing the poor size of the fully illuminated area.

Verses a flat at F/10. (Mind I'm using an OAG too, which encroaches on the field, but I simply crop that out.) Note that the vignette is also pushed out- infact I speculate the Edge 8 if it weren't for the M42 adapters and such, would be able to accomodate a full frame chip without vignette, but with the illumination dropoff the F/7 experiences instead of vignette. Both were unfiltered with a very evenly illuminated flat mask.

Helpful Engaging

Michael W. Dean

Topic starter

Oct 7, 2023 · Edited: Oct 7, 2023

It's normal. The C8's focal length is actually 2125mm, not the advertised 2032, which was a number devised to make the focal length match the advertised focal ratio of F/10 (It's actually ~F/10.456). The equivalent focal length is actually quite inline with the reduction of the Edge HD 8's Focal Reducer, so nothing out of whack there. My ASIair in particular shows 2148mm. Not too sure what adds the 23mm, likely the correctors of the Edge HD system.

And for the dark corners, the vignetting on the APS-C sized chip is perfectly normal, the issue is with the focal reducers themselves not providing a fully illuminated circle like the F/10 one does.

I've stopped using mine at F/7, since there's not much SNR gains from going from one slow speed to another, and theres a noticeable decrease in sharpness, contrast, and field flatness, and the fact I use a Full-Frame chip.

You made my day! Thank you. Whew!

Yes, I intend to remove the reducer at some point, and I got a 174 to do solar and Moon, (and to use for OAG), and got the Celestron solar filter for that scope.

I'm just taking my time getting there because the jump from 250 mm to almost 1500 mm is jaw-dropping, comes with its own issues, and learning to columnate is not intuitive for me. But I'm getting it.

I shot this of the Moon the other night, with video and this scope with reducer, made me pretty happy, since I've been amazed by the moon for 54 of my 59 years, and this from my back yard really jazzed me:
Coming In for a Landing on the Moon ( Michael W. Dean ) - AstroBin

Engaging

Oct 7, 2023

Michael W. Dean:
It's normal. The C8's focal length is actually 2125mm, not the advertised 2032, which was a number devised to make the focal length match the advertised focal ratio of F/10 (It's actually ~F/10.456). The equivalent focal length is actually quite inline with the reduction of the Edge HD 8's Focal Reducer, so nothing out of whack there. My ASIair in particular shows 2148mm. Not too sure what adds the 23mm, likely the correctors of the Edge HD system.

And for the dark corners, the vignetting on the APS-C sized chip is perfectly normal, the issue is with the focal reducers themselves not providing a fully illuminated circle like the F/10 one does.

I've stopped using mine at F/7, since there's not much SNR gains from going from one slow speed to another, and theres a noticeable decrease in sharpness, contrast, and field flatness, and the fact I use a Full-Frame chip.

You made my day! Thank you. Whew!

Yes, I intend to remove the reducer at some point, and I got a 174 to do solar and Moon, (and to use for OAG), and got the Celestron solar filter for that scope.

I'm just taking my time getting there because the jump from 250 mm to almost 1500 mm is jaw-dropping, comes with its own issues, and learning to columnate is not intuitive for me. But I'm getting it.

I shot this of the Moon the other night, with video and this scope with reducer, made me pretty happy, since I've been amazed by the moon for 54 of my 59 years, and this from my back yard really jazzed me:
Coming In for a Landing on the Moon ( Michael W. Dean ) - AstroBin

That's actually a really good shot of the moon, looks like pretty decent seeing too.

I wouldn't worry about collimation, the Edge HD's hold collimation for years after purchase. I've had mine for 3 years now, and just finally had to collimate for the first time. (Keep in mind, it wasn't out of collimation, I had to collimate because I added bob's knobs to the setup.

Helpful Concise Supportive

Related discussions

Screen shots of my specific Edge HD 8 Collimation issue

Started new thread with more info and pix that are not in this post below: Something's wrong with my Edge HD 8 - AstroBin =–=-==-=–= Hi. I'm struggling with Collimation on Edge 8HD. Have proper back focus (105mm with .7x reducer f...

Sep 20, 2023

Both posts describe technical problems with the same telescope model, the Celestron EdgeHD 8.