AO-7 Experiments and Observations

05/22/2003

 

There are some issues that I have uncovered when the AO-7 is used with my ST-8E.  I am documenting them here for your information.  These experiments and data were obtained with a 10" RC truss operating at F/9 with an ST-8E camera and CFW-8A filter wheel.  The effective image scale is 0.8 arc-sec./pixel.  This data was taken with my AO-7 and may or may not be representative of other products.  All recommended modifications have been made to cabling and power to insure sufficient voltage is delivered to the camera.

 

Flat Fields

After taking some images with the AO-7 in the image train, I was unable to get a successful flat field correction of the data.  This occurred whether I used a light box or sky flats.  I became suspicious of the AO-7 as I had never seen this issue when the AO-7 wasn't in the path.  The following images are flat fields taken with the clear filter, no IR reject coating, taken with and without the AO-7.

    

With AO-7                                                            Without AO-7

Note the circular pattern visible in the flat field with the AO-7, as compared to the flat field without it.  The bright ring seemed to not flat out completely during the flat field correction process, even though everything else did.  Similarly, without the AO-7, flat field correction was very acceptable.  The variation across the left image was approximately 7%; the variation across the right image was more like 4%.

Upon examination of the SCT adapter/captain's wheel extension that is used to mount the AO-7 to the OTA, it was obvious that the inside of the adapter was quite reflective.  There were no threads or baffles to upset any reflections.  Brad Ehrhorn tells me that anodized surfaces are very reflective to IR.  This made a lot of sense to me, especially when I didn't see anywhere near the difference with and without the AO-7 with a red filter.  The red filter of course blocks IR.

I added some flocking material to the inside of the captain's wheel with the object of eliminating any reflections from the insider surface of this extension.  Here is the resultant flat field image with the AO-7 and flocking.

AO-7 with flocked extension

It can be seen that the flat field is similar to the non-AO-7 arrangement and is more as expected with a smooth variation of around 4%.  I expect that future images will flat much more accurately.

Based on these experiments, it appears that some reflection correction is needed in the captain's wheel extension.  Alternatives are machining some threads/baffles inside the extension of adding some flocking inside.  Of course, another alternative may be to add an IR-blocking clear filter but that costs a lot of signal.

 

Mirror drift

In preparing for the above experiments, I took some 30 sec. exposures with the mount tracking turned off.  I was surprised with what I saw and repeated the exposure when the AO-7 was out of the optical chain.

  

With AO-7                                                            Without AO-7

In these images, the RA axis is vertical.  Note the horizontal "wiggle" when the AO-7 is in the chain.  Note that there is no AO-7 guiding, as well as no tracking, so this movement is with the AO-7 operating in its unexercised but biased mode.  I combined a star trail from each image and magnified them below.

Upper Trail: With AO-7   Lower Trail: Without AO-7

 

At my image scale, the equivalent DEC variation due to the AO-7 is on the order of 1.6 - 2.4 arc-sec. 

I know that some imagers take LRGB with the L AO-7 guided and the RGB auto guided.  If the AO-7 is left in the camera chain for this auto guided RGB operation, you may see some elongation as a result of this wiggle.  Of course, this is not an issue at all during normal AO-7 guided operation.  In any case, it might be worthwhile to evaluate your AO-7 by this technique.

5/22/2003 Update: Since changing the camera power to a DC-DC converter, which provides proper voltage at the OTA independent of cable lengths, this problem has not been seen again.  Other inputs also indicate this is not seen frequently.  I assume the combination of the ferrite bead removal and power supply corrections as noted here has resolved this problem.

 

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