what am i doing wrong

[brrttpaul]

I have a sigma 300mm f2.8 and I also have the zwo asi 533 osc camera, I was thinking buy the adaptor and try the camera on the sigma but when I attach it I can not  achieve focus  i can see the stars but they are way out of focus, is this fixable or doable, it was juust a thought as im hoping to get either the redcat51 or the askar 400 to compliment my ed80/asi1600 mono. I am in limbo ATM deciding what best to go with it keep swopping and changing my mind tbh any input would be helpfull

[PeterCPC]

When you say "I cannot achive focus" do you mean that whereever you screw the focuser it will not focus? If so I think that you need an extension tube added. I ask because with my Samyang and ASI533 the stars only come into focus long before you get anywhere near the infinity mark on the lens.

Edited November 10, 2021 by PeterCPC

[ONIKKINEN]

Sounds like a backfocus issue. Is the image at its best in near or far focus?

If in near, add spacers. If in far, youre out of luck because you cant really remove spacers if there are none. You should have the same backfocus distance to the 533 as the flange distance is in rhe camera your sigma is made for.

[brrttpaul]

thx for the reply, got a feeling i am out of luck,  with it as adding spacers only made it worse

[900SL]

21 hours ago, brrttpaul said:

I have a sigma 300mm f2.8 and I also have the zwo asi 533 osc camera, I was thinking buy the adaptor and try the camera on the sigma but when I attach it I can not  achieve focus  i can see the stars but they are way out of focus, is this fixable or doable, it was juust a thought as im hoping to get either the redcat51 or the askar 400 to compliment my ed80/asi1600 mono. I am in limbo ATM deciding what best to go with it keep swopping and changing my mind tbh any input would be helpfull

Is the Sigma 300mm an F mount for Nikon? How are you connecting the Sigma to the ASI? A photo would help... The backspace is whatever the original lens mount format is. 

 

[michael8554]

If the  Sigma 300mm fits you D7200, then that lense's Back Focus is 46.5mm.

The ASI 533 sensor is 17.5mm in from the M42 adapter.

Leaving 46.5 - 17.5 = 29mm of spacers and adapters. 

With any focusable lens, as far as I remember, if the Back Focus spacing is slightly less than the spec, it will focus at infinity, before the infinity mark.

Conversely, if the Back Focus spacing is more than spec, you cannot reach infinity focus.

So 30mm of spacing probably won't work, 25 to 29mm will reach infinity.

(Modded DSLR's, the sensor is too far away so won't reach infinity focus with matching lens system.  Moving the sensor closer = shorter Flange Distance = infinity focus).

Michael

[brrttpaul]

thank you so much for that,  i think I will have another look at it pretty sure i can get to 29mm with the spacers i got