SCT Focuser upgrade

[Mr Spock]

I don't see the fine focussing problems others have. Mine works exactly as it should.

[Stu]

18 minutes ago, Peter Drew said:

I would be reluctant to move the primary mirror its full extent to create a mirror lock for two reasons. First, on any of my SCT's that would push the focus point well out of the rear cell needing extensions, second, a SCT is opitically optimised at "normal" focus settings, outside of those the correction changes.

Thanks Peter, expert opinion as always ??

[Stu]

10 minutes ago, Mr Spock said:

I don't see the fine focussing problems others have. Mine works exactly as it should.

Do you not get mirror shift when you change focus direction Michael?

[Colinlp]

2 minutes ago, Stu said:

Do you not get mirror shift when you change focus direction Michael?

This is the only time mine moves, never at any other time, no mirror lock

[Mr Spock]

Do you not get mirror shift when you change focus direction Michael? 

No, none at all.

[Stu]

7 minutes ago, Mr Spock said:

 

 

No, none at all.

Yours must be one of the only SCTs existing that doesn't then! Lucky you ?

[Marky1973]

All great stuff again - thanks! Definitely moving towards a crayford I think. Leave the stock focuser for broad focus, then fine tune with the crayford.

Now I just need to get saving....

Although I could start with a hair net from Poundland tomorrow...

[Stu]

59 minutes ago, Marky1973 said:

All great stuff again - thanks! Definitely moving towards a crayford I think. Leave the stock focuser for broad focus, then fine tune with the crayford.

Now I just need to get saving....

Although I could start with a hair net from Poundland tomorrow...

I confess I'm completely at a loss as to what you actually do with the hair net????? ??

[Marky1973]

Keeps the mirrors hair free....

 

Blame Peter

:-)

[Davey-T]

20 minutes ago, Stu said:

I confess I'm completely at a loss as to what you actually do with the hair net????? ??

If someone was to reinstate the # post number system I could show you

Dave

[Colinlp]

1 hour ago, Davey-T said:

If someone was to reinstate the # post number system I could show you

Dave

Some things are for the best then

[Marky1973]

What happens in Poundland, stays in Poundland... 

[iapa]

Not sure gif this is too old...but

I keep reading that the SCT needs a focal reducer (the one commonly quoted seems to be the f6.3 fr) to correct for vignetting?

How would you mount the f6.3 fr and a focuser, and keep the required back focus of the  fr of 110mm given that the focuser will be filling that distance?

Or am I missing the point there?

Or, is the assumption that the focuser will fill the 55mm gap from fr to camera (assuming DSLR which has a back focus of 55mm incl t-ring). Somehow I don't see a focuser being less than 2 and qtr inches

 

[brantuk]

A reducer does the opposite of a barlow and reduces the f-ratio of the scope. So an f-10 sct becomes f-6.3. It attaches to the visual back and then you attach accessories to it (eg dslr, oag, etc). It speeds up the scope so the time to take subs is reduced.

(The focuser for an sct is usually to the side of the visual back - though you can get add on ones for the visual back too - which I would use for observing only)

[iapa]

Thanks, yes, I also knew of the focal reduction (given in the device name LOL) and the decrease in exposure time, I was more concerned with correction of vignetting at this point.

Re last sentence - you say visual back twice, did you mean rear cell in one instance?

I've just bought a 2" crayford SCT focuser (for mounting to the rear cell)

So, the FR fits to the rear cell, then the focuser, and my 2" nosepiece, m48 t-ting and dslr..

Which brings me back to ...

If I need to have 110mm from the FR to the imaging plane (sensor) and the DSLR provided 55mm of that with the t-ring fitted. Surely the focuser will be more than 55mm?

How does that measure up/fit in to the 110mm back focus? My head says I will be way past the 110mm - and that will also increase f-ratio (to nearer f7).

Alternative, perhaps, is Focuser, some sort of 2" to SCT adapter, FR, my 2" - M48 nosepiece and t-ring

[brantuk]

Ahh - I did miss the point - you're using a crayford focuser between the fr and camera. That will use up a considerable portion of your 110mm back focus. It's going to be a close call and will make focusing difficult or impossible. I guess you can only add up the distances of all the parts and see what you get. I've only ever used a crayford on my sct for observing.

For imaging I use fr, oag, filter wheel, and camera (plus extension tubes to get the right back focus where necessary). For fine focus I use a Feather Touch in place of the stock sct focuser. Not sure you can do it your way - maybe someone who has done it will come along and comment. I'll experiment with mine in the morning and see if I can come up with something for you.

[iapa]

Appreciate that.

Always useful to have the info first