Star Testing and "Too Bright" Stars

[MimasDeathStar]

Hello all 

My telescope is great - but my previous (and first ever) one had a fault with it and thats where I learned about star testing to try and ascertain what the problem might be. I'll be honest from the start and say I don't know nearly enough about the practice (or what to look for) to be any good at it, but I do periodically star test my 70mm refractor by going in and out of focus and checking that everything seems more or less as it should - which it always does.

But what I was wondering specifically is that I've seen advice pop up in several places about not picking a star that is "too bright" like Altair or Vega or Sirius. However, I'm not sure why this is the case and have done a bit of googling and I'm still none the wiser! I will admit that trying to do a star test on a really bright star like Vega is sometimes challenging as I occasionally get a bit of a "speckle" going on even at perfect focus and a bit of a red/green smear when the conditions aren't great at really high magnifications - could it be something as simple as that or is there another reason?

Probably isn't important - apologies if you took the time to read all of this and have come to the conclusion its another daft beginners question!

[Peter Drew]

Generally, if the target star is too bright, the glare swamps the finer quality details under test.  Usually, if the out of focus star image looks round either side of focus, a small dot without spikes and no smearing to a side when at focus, you're off to a good start.        😀

[John]

I find Polaris is a good star for star testing. Bright enough but not too bright. Easy to find (from where I am anyway) and it does not move (much) when being observed.

A green filter can be used to better discern the diffraction ring / airy disk pattern.