4 replies to this topic

[lucam]

Last year I put together a DPAC testing rig and used it visually to test my optics. I was never really successful at taking good images, especially at focus. Some of the recent threads in this forum pushed me to give it another try. I am curious to hear your thoughts on the results.

 

The OTA is a Tak FSQ106EDX4 that I use primarily for imaging. The Ronchi grating is an Edmunds Scientific metal on glass grating with 10 line/mm spatial frequency. Please note that this is twice the spatial frequency of the gratings more commonly used in this forum (5 line/mm) and therefore the sensitivity of the test is also doubled.

 

Here is the montage of the RGB LED images and white light LED:

 

 

I have tried to fit the shape of the bands in AOS and my best guess is that the optic is just a hint better than 1/10 wave with the worst correction in red. I have a bit of a light spill from the blue LED eyepiece that I need to improve. This is particularly visible in the inside focus image. The spherochromatism is obvious in the white light images but, again, it is very mild. 

 

I think I captured the at focus images correctly, but I welcome feedback.

 

Cheers,

 

Luca

[joshman]

I have an earlier model fsq106, and i have no idea what this all means, but i do know that my scope is sharp.

 

I'm particularly interested in what the "At focus" image is showing. I've definitely noticed a gradient in my Ha that looks like that when shooting too close to the moon

[RichA]

Last year I put together a DPAC testing rig and used it visually to test my optics. I was never really successful at taking good images, especially at focus. Some of the recent threads in this forum pushed me to give it another try. I am curious to hear your thoughts on the results.

 

The OTA is a Tak FSQ106EDX4 that I use primarily for imaging. The Ronchi grating is an Edmunds Scientific metal on glass grating with 10 line/mm spatial frequency. Please note that this is twice the spatial frequency of the gratings more commonly used in this forum (5 line/mm) and therefore the sensitivity of the test is also doubled.

 

Here is the montage of the RGB LED images and white light LED:

 

FSQ106EDX4_montage.jpg

 

I have tried to fit the shape of the bands in AOS and my best guess is that the optic is just a hint better than 1/10 wave with the worst correction in red. I have a bit of a light spill from the blue LED eyepiece that I need to improve. This is particularly visible in the inside focus image. The spherochromatism is obvious in the white light images but, again, it is very mild. 

 

I think I captured the at focus images correctly, but I welcome feedback.

 

Cheers,

 

Luca

10lpmm is 254 lines per inch.  How does that compare (results) with the typical 150lpin grating?

[lucam]

10lpmm is 254 lines per inch.  How does that compare (results) with the typical 150lpin grating?

My understanding is that the sensitivity of the test is linear in the resolution of the grating (twice the resolution, twice the sensitivity). A simple numerical experiment in AOS shows the point. Compare the Ronchi bands in the two setups below. Both have the same conic constant and lead to the same P to V error and Strehl, in this case 0.982 to match as close as i can the red inside focus image at 10 lpm. If you look at the AOS image at 5 lpm the curvature of the bands is barely perceptible, while it is easy to trace out in the 10 lpm scenario.

 

5 lpm:

 

 

10 lpm:

 

[lucam]

I have an earlier model fsq106, and i have no idea what this all means, but i do know that my scope is sharp.

 

I'm particularly interested in what the "At focus" image is showing. I've definitely noticed a gradient in my Ha that looks like that when shooting too close to the moon

I don't think the gradient you mention has anything to do with the pattern in the in focus image. The in focus image is closely related to the map of the wavefront, which can be used to point out surface polish irregularities, hills and valleys in the glass, and zones. The gradient you mention could be related to reflections between the sensor window glass and the filter at shallow grazing angles, if you get close to the moon but with the moon well outside the field of view.