Dohnuting

[Cdsinuk]

Hi all i saw this word used the other day whilst looking through forum, Dohnuting,  i didnt understand at the time, took some photos of betlegeuse last night and that seems to be all i can capture a big red dohnut.....  is it a focal problem or exsposure problem ? any help please appreciated 

[John]

If you see a star as a donut shape then that generally means that it is not in sharp focus.

What sort of scope and camera are you using ?

[Dave S]

The Doughnut effect as stated by john is usually a focusing issue.  It is common with catadioptric type of lenses/scopes such as SCT's and Maks which aren't properly in focus.

 

Dave...

[Cdsinuk]

Hi John, many thanks for your reply, i am just trying DSLR  canon 600d with 70 300mm lens, on my goto mount, its not the best mount but im only just starting out,  

Chris

 

[Cdsinuk]

8 minutes ago, Dave S said:

The Doughnut effect as stated by john is usually a focusing issue.  It is common with catadioptric type of lenses/scopes such as SCT's and Maks which aren't properly in focus.

 

Dave...

Hi Dave, many thanks for reply, i will try some very fine adjustments tonight if i get a clear sky ,cheers Chris  

[Dave S]

I too use  a canon (550d) with my preferred lens being a Tamron 55-200mm f4/5.4 and I use an alt/az goto mount for speed (particularly considering the current weather)  If the lens is way off focus then I also get doughnuts.  focusing a dslr lens can be tricky but using it in conjunction with BackyardEOS and a little practice it's not too difficult to obtain focus.  I love the combination of the canon and it's lenses and there are some great widefield targets that a dslr can capture.  With my alt/az mount I can consistently achieve 30-60sec subs which for me is fine for the brighter widefield targets.  I gather usually about 60 30-60sec subs and DSS seems to take care of the field rotation quite nicely.

With your kit it's surprising how many objects there are available in the widefield arena.  I've been doing widefield for a few years now and still find more to go for.

The weather however is something that can't be controlled (worst luck).

Dave...