ATM part 3 – punching holes

Yesterday I visited fellow amateur astronomer Bert. He has loads of experience with ATMing dobson telescopes and was willing to help me out installing the optics in my dobson.
We used Jan van Gastel’s Excel-sheet to figure out what focuser to use (I had a 2” and 1.25”-only focuser available for this build). Good thing we did too, because it immediately became clear there was no point using a 2” focuser and eyepieces due to the small 42mm secondary mirror. Upgrading it to a 50mm was considered but since I am not currently willing to spend any money on it I decided against that, for now. It is possible to upgrade it later on. First, I want to make sure the primary mirror is any good. I’m hoping it to be at least a .93 Strehl one.

Then we studied the secondary mirror some more. Bert noticed I mounted it dead center on the spider vane but since I am already low on light thanks to it’s small size he suggested to mount it a bit off-center. That’s a trick to gather a bit more light. So I’ll have to peel the mirror off again and mount it off-center. All this is done to maximise the already shaky performance of this telescope J

Then we used Stellafane’s website Newt … to figure out where to mount the spider vane, focuser and primary mirror. Thing is, I don’t know the F-value of the mirror. We decided on assuming (oh dear…) it’s a F6. The calculations then showed we could easily mount the spider vane 10cm below the top of the OTA. With that there should be sufficient wiggle room to mount the primary mirror later on. Figuring out where the primary has to be mounted is going to be a nice little experiment involving me looking through the focuser and someone else moving the primary up and down the lower end of the OTA until we figure out it’s position. Then the whole thing *should* work. If it does I will apply velvet to the inside of the OTA, mount the Telrad and paint all shiny parts inside of the OTA black.
After that it’s time for a serious field test on some objects. If the results are satisfactory I will send the mirror out for an official test and if strehl > 0.93 a new mirror coat will be applied as well.

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