I assuming that you're using a GSO store-branded RC here. I have an AT-16RCT myself. My understanding of the RC design is that it is only optimized for one design focal length, so yes, you should stress over the 47 mm difference you're measuring. That's a lot. Here's my take on the focal length. If it wasn't important, then GSO wouldn't be quoting the focal length to 4 significant figures. By default, you have to assume that GSO designed these RCs to an F/8 with the specified focal length to within ± 1-2 mm. Ultimately, the accuracy of your plate solving will limit how accurate you can set your focal length. There may be RCs shipped that are out of spec. on the focal length, but you will find out pretty quickly if yours is one of those.
I have read about people adjusting the secondary to change the focal length, with mixed results. I think you would be better off using the three primary mirror adjustment screws in tandem to change your focal length. If you try and adjust the secondary center screw, you run the risk of perturbing the secondary mirror factory centering. The OTA mechanical structure is made using numerical-controlled machining, so the tolerances should be ± .02 -.03 mm., but the screws are just standard tolerance stainless steel metric screws and not precision metric screws. Again, by default, you have to assume that GSO uses the mechanical tolerances of the OTA to locate the secondary near the optical axis of the primary, as there is no way to adjust that in the OTA mechanical design. There is always some small amount of lateral play in the center screw, so if you loosen it there's no guarantee that you will tighten it back in the same position. BTW, the three equally-spaced secondary screws mostly adjust tilt, not centering (offset).
The DSI Method seems to be the consensus cost-effective method to collimate an RC, short of an expensive laser interferometer collimator. The DSI Method is mute on adjusting the focal length, perhaps because the DSI scopes used a movable secondary to focus the image, which is different than the GSO RCs. According to the DSI Method, the secondary tilt adjustments are used to adjust off-axis astigmatism and to correctly balance the image at the corners, while the primary tilt adjustments are used to correct for on-axis coma. The DSI method goes into excruciating detail on why you shouldn't use collimation lasers or a Takahashi collimation scope. It's because the secondary optical axis and the secondary geometric axis (which these devices collimate to) aren't necessarily the same. You don't have this problem with SCTs because of their spherical secondary. I suggest you get Timo I 's advice as well, as he has a lot of experience collimating these RCs. He recently posted here https://www.cloudyni...t +another +gso
Edited by Terry White, 09 June 2020 - 08:51 AM.