Focusing Mirror Shift in a Mak

Regulus (Trevor) · #1 14-08-2014, 05:28 PM

A question on focusing a cassegrain type scope:
I notice a mirror movement in my mak (SK150) everytime I go to focus and wonder if this is standard on all cassegrains with this type of focuser.
Is this right? It is certainly a jarring experience when u first start to refocus to see the image shift slightly.
I'm thinking that if there is a mirror movement (tilt) then there must be some play in the mechanism and this must tilt the mirror away from parallel to the light path. If this is so then there is no 'settling" back into parallel, it has to be at a slight angle all the time.
However, the view seems ok but I really haven't had enough time to fully inspect it for coma, or other distortions.
I really like the scopes contrasty view and would hate to have to give it up, since I think it would make a reasonable beginners astrophotography and general planetary scope.
Some one said that this type of focuser has this slight jarring movement in all cassegrains, but surely this cannot be so.
I am thinking of getting an SCT 2speed focuser for fine focusing anyway, but this isn't going to solve a misaligned mirror problem in the other focuser mechanism.
Can someone with experience of these give me an idea as to the problem and it's resolution?
Thanks in advance - Trev

LightningNZ (Cam) · #2 14-08-2014, 10:57 PM

It's not pleasant is it? The Mak I briefly owned has a secondary Crayford focuser on it to avoid the mirror shift. I still copped it a bit when moving around the sky but it wasn't anywhere near as dramatic as when I had to refocus using the primary mirror.
-Cam

Camelopardalis (Dunk) · #3 15-08-2014, 10:47 AM

As an SCT owner, maybe this trick will work for you...

...wind the focus from one end to the other, and back, a dozen or so times. This redistributes the grease on the baffle tube. When I first got my big cat the shift of focussing was about 1/4 of the FOV at 200x. Having done the above and actually got to use the scope a decent amount, the shift is now much less...at 400x, I can still get fine focus easily.

The other trick is to always perform your focus movements such that the mirror is being pushed up. If you miss the focus point, back it up, and push up again. In my SCT, this is anti-clockwise, towards infinity. But if you wind the focuser a bit you should readily be able to tell which way the mirror is moving.

Hope that helps...

Regulus (Trevor) · #4 15-08-2014, 12:06 PM

Thx, and whoops. I was referring to the mirror being off parallel to the light path instead of perpendicular. Idiot me.

Tip of the day: avoid Cassegrain scopes whose mirrors are parallel to the light path.

Dunk, the tuning of a guitar is the same; start from below the pitch and move up into it. Nice to see that the principle applies here too :-)

It seems that putting an SCT dual speed focuser on is the way to go here, but the same question arises: will the mirror be off perpendicular? I cannot see how it couldn't be if it has this movement, and if the mirror moves like that then the 'axle?' is not entirely secure. Although I see people using them for astrophotography so.... there must be a fix, or I am worrying at nothing.

I had a Vixen VMC110 with a problem like this and had a bush made up for where the focuser screw goes into the pusher-plate and that took the 'play' out of it, but I really, really don't want to pull this apart.

I am going to have to put a field flattener on and take some photos to see, aren't I?

dannat (Daniel) · #5 15-08-2014, 12:39 PM

the mirror focusing is common -you are worried about the small focus movements causing the mirror to be slightly out of alignment with the optical tube correct?

have you used a collimation eyepiece to see how far off it is?

Wiliam optics (among others ) made a bolt on focuser to prevent this, does the scope have a focuser lock?

Camelopardalis (Dunk) · #6 15-08-2014, 01:40 PM

Nice analogy Trevor

I learned early on with my first SCT. It's become so habitual I do it with every focuser I lay my hands on

Is the SK150 the Skywatcher? If so, it's probably overloaded with the same gunk as the Celestrons, the very same 'grease' used to launch large marine vessels

I've found the grease will soften up, or at least redistribute, with the above end-to-end procedure, and it may reduce your mirror shifting significantly. My C11 was horrendous from new but it's better now.
Over time I've heard of a number of folk stripping their scope down and replacing the gunk with something lighter and reporting great results. Just beware with anything that might run in the heat here. But there may also be physical tolerances that are too slack that are not so easily resolved.

Like Cam says you can somewhat remove the mirror moving focuser from the equation by adding a Crayford or such as that's not touching the mirror. Of course, if your mirror is flopping around as you slew around that's not going to solve that problem. Likewise a dual speed focuser still connected to the mirror. I use a Feather Touch on my C11 and it's a great addition, but you still get the shift, although it can be tempered a little with the use of the fine ratio focus. Still doesn't answer whether or not your mirror is correctly aligned...

As for whether or not the mirror is orthogonal with the optical path...you'll probably only know from doing image analysis...

Regulus (Trevor) · #7 15-08-2014, 08:57 PM

Thx Daniel and Dunk.

Quote:

the mirror focusing is common -you are worried about the small focus movements causing the mirror to be slightly out of alignment with the optical tube correct?

- Yep. That is my concern.

Quote:

have you used a collimation eyepiece to see how far off it is?

It's fairly new to hand and I don't yet have the collimation tools for a Cass., but it looks like I will need to get one for the kit since I rather like the Cass' contrast and so I think they may be a permanent part of the kit if I can get past this hiccup.

Quote:

Wiliam optics (among others ) made a bolt on focuser to prevent this, does the scope have a focuser lock?

I did discover the nice looking WO focuser last night while surfing Daniel, and it does look like one of these will be coming to join me sometime soon.
If this is a common 'feature' of the Cass then when using the WO focuser, do u pull the focus right back, or push full forward, and leave it there and just use the WO? Is that how that works? Or do u do a basic focus with the Cass focus and fine tune with the WO?

Do u think an artificial star and the use of the out-of-focus Airy rings in the meantime would give a good enough indication of coma?

It is the SK150 (Older Gold tube type) and doesn't appear to have a focus lock on it.
I havent touched the collimation screws (usual push/pull and lock system) to see if the lock screws are in fact tightened, or one or more isn't adjusted up against the plate. I need to get some appropriate sized allen keys for this, and that is a trip to the shop for tomorrow.

Quote:

Over time I've heard of a number of folk stripping their scope down and replacing the gunk with something lighter and reporting great results.

- Only if I really have to :-)
Dunk, the actual focuser is very smooth and is neither too soft or too hard to turn. Very buttery.
The movement in the mirror is not huge, but it is jarring, and that it is there at all, disappointing.

I will look into a collimating EP and the WO focuser.
Check that the Collimation Lock screws are actually doing the job.
These should be suitably diagnostic, and if I have to live with an occassional 'jar', so be it. As long as the mirror is aligned correctly then all will be well.

Thx guys.

Trev