The effects of a bad focuser.

[toryglen-boy]

Howdy

I am taking images through a 200mm F5 Skywatcher reflector, and i am hitting a slight issue, my focusing appears to be out, although my collimation is fantastic, PHD is giving me an osscillation index of around 25-30ish, which i am led to believe is quite good. i am taking my time to get focused, while it appears to look great at the time, the stars are kinda egg shaped, and i am wondering if this has to do with the grub screws making the connection with the QHY8 and focuser, get pushed to one side, that is, ill fitting.

What kinda symptoms could one expect with a rubbish focuser, and what would you recommend to combat this? adjust the focuser, or just get a new unit?

thanks in advance.

:)

[AlexN]

Sounds like it could be either the focuser sagging, or the camera being slightly tilted, so that the sensor is not orthogonal with the image plane..

Here is an idea of what focuser sag looks like taken with a GSO 8" RC with the standard focuser. See the stars in the bottom left hand corner are really bad, whilst the stars at the top right, through to about the center of the field are fine... That was caused by the focuser tilting a small way..

The results would look much the same with a tilted camera, although probably less severe..

[toryglen-boy]

Thanks Alex, thats kinda what i am getting, although with longer subs its getting worse, although i know my polar alignment is spot on, sound about right to you?

i am thinking a moonlight with electric focuser is in hte offing ..

;)

[AlexN]

If it gets worse in longer subs, it could be a mixture of tilt and focus shift..

Also, with a newt, as the target gets higher up, the focuser is holding the load out horizontal.. this induces more and more flex on the drawtube...

A motorized moonlite will fix the problem! :) If you're looking at motor options, go for the stepper motor with the mini-moonlite controller... Thats the way to go on an imaging scope. The DC motor will do the job no worries, but it does not have absolute positioning, Ie. The focuser does not know the current position of the drawtube, and as such, you can not save your "focused" position and simply return to that spot night after night.... The Stepper is a little more expensive, but worth the coin!

Alex.

[toryglen-boy]

Thanks fella, its appreciated :thumbsup:


anyone else wanna chip in?

:)

[marki]

You can spend a lot of time trying to fix the dodgey one that came with the scope or just replace it with a quality unit. Like Alex I think the moonlites are a good choice (I own two). The manual two speed will do the job nicely as long as you get the brake fitted and make sure the drawtube has the 2" compression ring with three thumb screws to lock your camera in (0 flex or movement). The stepper drives with the moonlite controller are definately the go if you have the cash. The controller has both USB and serial connection options as well as built in temp sensor for temp compensation. The repeatability of these units is outstanding. The focusers are pretty to look at and built like tanks (I know my refractor fell 7 ft onto the ground last night, glad it landed on the moonlite...not a scratch). I often have my refractor racked out about 100mm and have never had any sag even with filter wheel, OAG, camera and field flattener on board. The drawtube length on a newt is much less so I imagine you could load the thing as much as you wanted without trouble (certainly up to 3.5kg). A good focuser is an excellent investment and you can move it from scope to scope . Trying to fix a substandard unit is just a pain in the behind.

Mark

[Bolts_Tweed]

Gday Duncan

Can I back up the guys here - the best investment you will make is a quality focuser like a moonlite (there are others but I like moonlites).

I advise everyone I talk to that is thinking of starting - the first thing is good mount, then optics (but you can achieve good results with limited expense with optics - you dont need an astrophysics refractor when you can take good images with a fast newt) and then by far the next importance is a good focuser.

I use refractors so I dont have experience with Newts but the beauty of the moonlite stepper with the moonlite controller is autofocus - I am virtually blind in one eye and impaired in the other - smile - thats why I image and dont own an eyepiece. Autofocus with FocusMax (free) gives me repeatable excellent results - so for someone that can see it would be even easier.

Apart fromn that Ron Newman from Moonlite is a pleasure to deal with and is happy to give advise.

My vote anyway - I have 3 and i'll probably put another on the astrophysics - sacrilige to some I know but I like them that much

Mark