Just wanted to share something I have just learnt about collimation. It’s relevant to all types of reflectors but mostly SCT’s needing in the field collimation. This may help those new to this dark art and save some frustration.
Ever since I replaced the collimation screws after accidentally stripping the heads on my CPC1100 I have not been able to get back to a decent level of collimation. Stars have had a slight flaring to them and being a keen planetary ogler, the images I’ve been getting have been quite underwhelming. I’ve got the out of focus donut as central as I can, but as in-focus collimation will need some pretty good seeing with this size scope, I’ve been trying to work out how I can get it better in the meantime.
After re-reading online tutorials and a lot of internet searching I stumbled across what I think has been the missing step in my procedure. While collimating I’ve noticed a small bright dot appear when moving out of focus to obtain the donut to centralize the central obstruction shadow. I wondered if this dot was something to be noted or something to ignore. This tutorial
http://www.astromart.com/articles/ar...article_id=548 from Astromart is about the only article I can remember where this dot is clearly referenced.
I now know that this dot is called the Poisson point / spot and it does need to be centred. I have not tried it out yet but on my next excursion outside I will centralize this dot as my second step of alignment. This should get me far closer to proper collimation, as from memory, it was a fair way off centre last time I was out. This may be common knowledge to some, but newbies (I may forever be in this category
) may never know as it’s not often / clearly mentioned.
I hope this helps someone out there. An internet search will bring up all sorts of info on it for those interested in the technical side of things. The more experienced here can correct anything I have said or elaborate in any way they see fit.
Brendon.
