Hello. I have purchased a ZWO EFW and OAG-L to use with my ASI183mm and 174. I am trying to set up the 183 with the OAG-L prism. The 183 simply screws onto the EFW, so the threads dictate where the sensor is angled with relation to the prism, so one of the corners of the sensor is pointing at the prism. I am not seeing any adapters with the OAG-L or EFW to help with this. What am I doing wrong? OAG-L (with ASI174) –> EFW –> ASI183
Trying to align sensor edge with OAG-L prism
I had a similar problem… different sensor (ASI533 mc) and OAG (Celestron). I used a micrometer to measure from the bottom of the OAG to the top of the sensor. Then used that measurement to set the prism. I left a bit of leeway, but it seemed to work OK.
George
George
Concise Supportive
As you will have to extend your Backfocus by (usually) 0.8mm to take into account your filters you can play with thin distance rings. Put very thin (0.1 or 0.15) spacers between the camera and the filter wheel until your sensor is properly oriented.
With a 0.1mm spacer you can change the final position of your camera by roughly 50 degrees.
When things are roughly parallel then adjust the prism location so that the border of the prism lines up with the top of your sensor.
Do not tighten up the prism you will still need to move it out a little bit.
Put the rest of the required spacers between the OAG and your scope.
Now do the usual stuff you do when you create flat frames, you will see the shadow of the prism on your single flat exposure. Now move out the prism until the shadow is gone.
Then use aberration inspector in Nina to perfectly adjust Backfocus or manually adjust the shape of your corner stars by adding/removing spacers in front of the OAG.
Michael
With a 0.1mm spacer you can change the final position of your camera by roughly 50 degrees.
When things are roughly parallel then adjust the prism location so that the border of the prism lines up with the top of your sensor.
Do not tighten up the prism you will still need to move it out a little bit.
Put the rest of the required spacers between the OAG and your scope.
Now do the usual stuff you do when you create flat frames, you will see the shadow of the prism on your single flat exposure. Now move out the prism until the shadow is gone.
Then use aberration inspector in Nina to perfectly adjust Backfocus or manually adjust the shape of your corner stars by adding/removing spacers in front of the OAG.
Michael
Helpful
Excellent info. Thank you Michael!
you can also try the blue fireball Camera Angle Adjuster (I used one of those when the sensor didn't align after using the M54 thread to attach
https://agenaastro.com/blue-fireball-360-camera-angle-adjuster-rotator-m54-thread-r-06.html
https://agenaastro.com/blue-fireball-360-camera-angle-adjuster-rotator-m54-thread-r-06.html
Oh nice! I didn't know that existed. Thanks Nick!
It sounds like folks believe that the proper alignment for the prism is to align the edge of the prism to the edge of the sensor, but that's not correct. You want the prism to lie outside of the ray height for the sum of the chief and marginal rays at that location in the image train. In simpler terms, that means that the prism should not intercept any of the off-axis ray bundle at the prism position. If you don't achieve that, you'll wind up with a shadow of the prism at the edge of the field. Yes, flats should remove the shadow, BUT that's not good practice. Go ahead and follow the advice here but then back it out until your flats show no shadowing. The amount you have to back it out will depend on the focal ratio of your scope and on the size of your sensor.
John
John
Well Written Helpful Insightful Concise
If your OAG prism is aligned with the corner of your sensor, it's either 45 degrees or 135 degrees off from where you want it to be (long or short edge of the sensor). The thread pitch on your camera is 0.75mm. For 45 degree rotation you need 0.75 * 45 / 360 mm of spacing. That's 0.095mm so a 0.1mm spacer would work. For 135 degree rotation you'd need 3 time that, so 0.3mm.
Well Written Helpful Insightful Concise
Gary Brown:
If your OAG prism is aligned with the corner of your sensor, it's either 45 degrees or 135 degrees off from where you want it to be (long or short edge of the sensor). The thread pitch on your camera is 0.75mm. For 45 degree rotation you need 0.75 * 45 / 360 mm of spacing. That's 0.095mm so a 0.1mm spacer would work. For 135 degree rotation you'd need 3 time that, so 0.3mm.
That makes total sense. Thank you for simplifying that, Gary!
Well Written Respectful
John Hayes:
It sounds like folks believe that the proper alignment for the prism is to align the edge of the prism to the edge of the sensor, but that's not correct. You want the prism to lie outside of the ray height for the sum of the chief and marginal rays at that location in the image train. In simpler terms, that means that the prism should not intercept any of the off-axis ray bundle at the prism position. If you don't achieve that, you'll wind up with a shadow of the prism at the edge of the field. Yes, flats should remove the shadow, BUT that's not good practice. Go ahead and follow the advice here but then back it out until your flats show no shadowing. The amount you have to back it out will depend on the focal ratio of your scope and on the size of your sensor.
John
So if my prism is as low as it can go, but is still not blocking the sensor even when it is on an angle, then I should be ok? No need to worry about making the long edge of the sensor parallel with the bottom edge of the prism?