8inch F4 or F5 for imaging

[bmitchell82 (Brendan)]

Most of it is noise.

But firsts first how are you collimating the newt?

Things I can see straight up,
Your spider vanes are not straight. grab a ruler or straight edge and lay it across the aperature from edge to edge where the spider vanes meet the tube. if the vanes don't line up across then adjust till they do. This will stop your diffraction spikes having a off angle secondary spike.

Collimation with newtonians well if you don't have it then forget about it pack up your gear and go and sleep or have a dinner party as its not worth your time to bother for AP. If it scares you don't let it its pretty easy.

Tools to get/forget
Forget Laser collimators of any discription even the high end ones. they make you dumb! just like the old word doing it by hand will give you more reliable results than letting the PC do it for you!

Get
Cheshire eye piece aligns your primary mirror axis to your focuser axis
Auto collimator (no its not automatic ) aligns your secondary axis with the primary and focuser
Sight tube aligns the secondary under your focuser.

the full set of these of quality gear will be approximately the same as a quality laser, difference is your laser will be far less accurate!

Brands that sell personally I use cats eye, but you also have Farpoint and a few others that i cannot remember.

Keep it up and good luck!
BM

[billdan (Bill)]

Thanks Brendan for your reply, I was wondering what to do with those secondary spikes.

The only collimation tool I have is a cheshire, and I collimate every time I use the scope just before dusk. Then later I use a 6.3mm eyepiece and check on a de-focussed star and make sure all the circles are concentric.

It usually looks OK to me, however I suspect something must be moving, because I have to re-adjust every time I carry the OTA out.

Needs further investigating and I need more practise.

Thanks
Bill

[bmitchell82 (Brendan)]

having the cheshire is not enough to get proper collimation, there are 4 different things to correct and for proper collimation you require 2 to be correct.

A plain laser does only 1 of 4 of which the cheshire corrects the same error.

Barlow laser and laser correct two of the four but it just doesn't do it that accurately.

Auto collimator (original type with a central peep hole will do 1 of the 4)
Auto collimator (improved with an offset peep hole as well as a central will do 3 out of the 4)

The sight tube is also important and if you know what your looking for in the actual image you can see if you have it right or not.

Upgrade the springs on the mirror cell, if the mirror cell only has rubber O rings, get springs to fit in that are solid makes a huge difference

Your secondary vanes have to be tight. mine have just started to cause a slight divot into the OTA thats how tight they are. Though i have also upgraded the whole front secondary you can see that here the vanes are 1.7 odd mm instead of 0.7mm I did this for two reasons, visual, i like diffraction spikes it makes the stars interesting. and Strength when collimating.

The 0.7 vanes are more than capable of holding the secondary without flexing about as long as you tension them up but in the rotational direction they will flex like a bugger making collimation quite hard.