150mm GSO secondary offset?

[kalasinman]

Trying to learn about my new scope. Getting used to collimation. Reading Astrobaby page on it there is says that f/5 and faster scopes may have the secondary spider and support offset (with illustration). Says if you don't know, just center as a normal. Ok, so I centered, but my 4mm of back focus disappeared. Unrelated or indicative? Where to go to find proper setting. No paperwork.-thanks

[SkyGibbon]

Make it up with your primary.

[Macavity]

The offset for a "fast" Newtonian is approximated by:

offset = minor axis / (4 * focal ratio) 

[http://web.telia.com/~u41105032/kolli/kolli.html]

For a typical (f5) ~60mm(?) wide secondary: 60 / (4x5) = 3mm

But, as the say in the article:

"...an offset of a few millimeters should have little effect visually"

I intend to try it with my 8" (faster) f/4 GSO photo Newt with a (larger) 75mm

secondary. I'm fairly convinced GSO mirrors are centred on the spider axis!

But there still remain two schools of thoughts as to the worth of this... For more

common f/5 scopes (As noted Above) most people simply adjust the primary

[kalasinman]

Very much appreciate the advice. I tore it down to a bare tube today and chopped off 15mm to make room for my OAG. I'll have to re-do collimation tonight for sure. I'll just center it. I'm at 735mm f/4.9 now.

[Peter Drew]

Chopping 15mm from the tube makes no difference to the focal length, just the position where the optics come to a focus. You still have 750mm F5. 

[russ.will]

Indeed. You may have been able to achieve the same thing by simply screwing the primary mirror forward on it's collimation screws.

Russell

[kalasinman]

I stand corrected. I needed 15mm for an OAG. I don't think there is more than 3-4 mm in the screws. Trying it tonight and camera and guide cam both focus, by a hairs breadth and mirror screwed max forward.

[laser_jock99]

Decent bit of scope modifcation going on here! Chopping some length of brand new scope tube takes some nuts- all hail to you I hope it all goes back together okay!

I would have left the secondary in place if possible as these are notoriously the hardest part to re-set correctly. Good luck with the project.

[kalasinman]

Decent bit of scope modifcation going on here! Chopping some length of brand new scope tube takes some nuts- all hail to you I hope it all goes back together okay!

I would have left the secondary in place if possible as these are notoriously the hardest part to re-set correctly. Good luck with the project.

Yes it seems to be brilliant. Got it assembled and collimated yesterday afternoon. Having never owned a scope before and going right into OAG, I had a lot on my mind last night at dusk.

The imaging scope snapped to focus with 6mm to spare. Fired up the guide cam and PHD2 and got a white screen. Ooops forgot a driver. Once that was in I ran Firecapture and was really thrilled to get a screen sprinkled with focused stars!

Back to PHD2, but won't guide. It gives "star not moved enough":. It seems to be responding in manual guide. I packed it in, then read the sticky elsewhere in SGL. I realized I had the mount guide speed at .5x . Tomorrow is another night.

When I received the scope I measured the spider and it was off so much the dust cap wouldn't go back on. Leaving it alone not an option. I'd had it pretty close before the chop job, and ran the primary all the way forward before re-do yesterday. Surprisingly collimation was reasonably close.

[kalasinman]

[laser_jock99]

Great job, looking forward to seeing some images.

[Moonshane]

Great job. It's simple enough if careful and brave. The offset is to all intents and purposes fixed when the mirror is glued on the stalk so just centralise and forget. You need to check primary and adjust every time a smidge but although checking the secondary every time is recommended you are unlikely to have to adjust regularly. Good luck.

[Macavity]

Heheh. No pedantry: I managed to convince myself (by measurement)

GSO don't include an offset on their F4 Newts secondary mounting.

Aside: No matter how hard I try, I do see elongation of stars to one side of field. I sense the (laser collimated) scope "squints a bit"! An online ray-tracer suggested that the diameter of the OTA is only just adequate. I do sense the view is being slightly cut off... and the scope never QUITE points where my mount thinks it does. 

BUT all this is for a F4 Newt... Things are "2x easier" at F5.

N.B. These "random" discussions are always useful! I ever work on improving my Newt. I just picked it up casually (carelessly!) by the focusser... and discovered the focuser SCREWS have always been perilously loose? Aargh! lol. Recently, I have invested in Baader "clicklocks" all round. I would still like to have a go with a Coma Corrector - Might make things generally better? Ever the challenge, but an interesting one, nevertheless...

</wibble>

[happylimpet]

Regarding 'squinting' laser collimators - and even for perfect ones - use the 'Barlowed' method.  Its very easy to implement and Much more  accurate.  Theres a good doc somewhere online..sure you can find it.

And at f4, unless you're using a tiny sensor, get a coma corrector....theyre not too expensive, considering...

[kalasinman]

Regarding 'squinting' laser collimators - and even for perfect ones - use the 'Barlowed' method.  Its very easy to implement and Much more  accurate.  Theres a good doc somewhere online..sure you can find it.

And at f4, unless you're using a tiny sensor, get a coma corrector....theyre not too expensive, considering...

Agreed, although I don't have a Cheshire yet. I use a cap I made. Between the cap and a collimated laser (mine was way off) I've got a usable collimation.

Indeed. You may have been able to achieve the same thing by simply screwing the primary mirror forward on it's collimation screws.

Russell

Running the mirror all the way forward helped a little, but between that and a 15mm shortening, I've got it all in the train and about 6 mm to spare.