Cutting down Newtonians

Lee · #1 25-09-2006, 05:26 PM

I've decided to take the plunge and cut down my Newt

(well - soon anyway!) for imaging.
If I move the mirror up the OTA up by 35mm, everything will be OK for visual by adding a 35mm extension tube in the focuser - right?

(I realise there could be some vignetting if secondary not quite big enough)

If it all turns to custard visual wise I guess I'd have to get a 12" anyway....

Garyh · #2 25-09-2006, 06:22 PM

Hi Lee,
Your are right there, a extension tube will get it going for visual use. You won`t notice vignetting much for visual but you will see it in your images.
You should go and put a bigger secondary mirror in as well.
How big is the secondary in your newt?.
Cheers Gary

Lee · #3 25-09-2006, 06:34 PM

Have no idea Gary - whatever comes with a standard GSO 8" f/6.... I thought about the secondary, will probably leave it initially, and replace it if vignetting is intrusive.
Happy about the extension tube, from my eyeballing experiments the other night, 35mm movement should bring the SLR to focus easily

EzyStyles (Eric) · #4 25-09-2006, 06:50 PM

Hi Lee. I know for mine GSO 8" F/4 the secondary mirror is 63mm. what about longer collimation screws? also, what adapters do you use to attach the DSLR into the focuser? if it is:

focuser --> prime focus adapter --> t-mount --> DSLR

unscrew the 2" barrel from the prime focus adapter. If it is a standard t-mount ring, there is a small inner ring attached by 3 screws. take this ring out and put in the 2" barrel instead. This will give you more in-focus. make sure the 3 screws are fully tighten.

ausastronomer (John Bambury) · #5 25-09-2006, 07:16 PM

Lee,

That newt has a 50mm secondary which is somewhat oversized anyway, particularly for visual use. Before you go and buy a replacement secondary I would try what you suggest and just use the extension tube for visual work. I think the secondary will be fine as it is. The 10" version of the GSO scopes are similarly fitted with a 63mm secondary in a 10" scope (25% CO) which again is somewhat oversized for visual work in a 10"/F5 newt.

If I get a chance I will run all the numbers and calculate the illuminated field size with the 50mm secondary for you, if I can find all the old tube and focuser specs I had from my old scope. My gut feeling however is that it should be ok.

CS-John B

Lee · #6 25-09-2006, 07:47 PM

Thanks guys for the advice.
Eric - checked the T-ring - it does come apart (3 allen screws), but on the weekend I did an experiment with the SLR body and the scope, still couldn't get focus holding the body against the bare focuser (no adaptors at all) - so I still need more in focus.
I might make an article out of it - might be a warning of what not to do!
Fingers crossed!

EzyStyles (Eric) · #7 25-09-2006, 08:18 PM

are you using a R&P focuser or crayford? the crayfords are lower profile.

Lee · #8 25-09-2006, 08:21 PM

Already got a crayford.....

EzyStyles (Eric) · #9 25-09-2006, 08:42 PM

i guess the only way is to cut poor tubby, JMI low profile focuser or move mirror closer. i can't think of any other options.

ballaratdragons (Ken) · #10 25-09-2006, 08:54 PM

just cut the tube in half and make knockies!

gbeal · #11 26-09-2006, 06:32 AM

Lee,
I would cut the tube only as a last resort. The first things to try are what you already tried, removing the T ring etc to get the body as close to the focuser as you can. I used a lower focuser as well, but this is a cost which you may not want or require. Try longer collimation screws as well, shifting the primary towards the secondary and sometimes this can be all that is needed. I would aim to have infinity focus with the DSLR, and about 2 mm's in focus just in case. Any more is wasted.
Lastly, how does the primary cell attach? Is it with three screws/bolts into the tube, or an annulus ring with numerous screws? If three screws/bolts you could just bore three new ones a little closer, and "patch" the existing ones. If the annulus, at least your rough cut will be hidden.
Gary

Lee · #12 26-09-2006, 07:11 AM

Still contemplating its fate!

One hassle with moving it all "just enough" is that my 20mm Plossl likely won't focus - it currently focuses at the near end of out-focus - at least moving it a bigger amount, and using an equivalent extension solves this.

The primary cell attaches by an annulus ring.... which will cover my *clean* cut

- I'm pretty handy with a pair of tin snips....

GS/Synta - watch this thread - make Newts imaging ready out of the box!

Garyh · #13 26-09-2006, 08:21 AM

Chop it!! Lee.....

Seriously its all a compromise between tube diameter, focuser height, secondary size and how far the tube extends past the secondary etc..
With a few quick calculations your 50mm is too small, if you go up to 63mm which Eric has in his that should give you a much larger field of illumination (about 20mm, with some guess work with your dimensions) at 100% illumination which would not be too bad.
If it was mine and you are going to do a bit of imaging I would just cut it to start and take some long exposures through it to see how bad the vignetting is. Then go from there..
With mine I have a 55mm secondary in my 6" and a low profile focuser which is 45mm high. 100% illumination is nearly 25mm.

Go on Chop the dinosour...

Cheers Gary

rmcpb (Rob) · #14 26-09-2006, 08:57 AM

Lee, get a copy of NEWT to check your calcs BEFORE cutting. Also, it has some great info pages on setting a scope up for visual versus photographic use. You will probably find the standard secondary is OK for your purposes, I use mine for visual and took the 1.8" secondary out and replaced it with a 1.5" to great effect visually but it would be a disaster for photography.

Cheers

P.S. Don't use tinsnips for the cut, a fine hacksaw will do a much more controlled job.

Lee · #15 26-09-2006, 09:05 AM

I will be measuring and calculating a few times before "the chop" - goint to wait until I have a decent moon, and work out where the infinity focus is with a sheet of card - will give me more ideas of just how far the thing needs to move.
Fine hacksaw - will remember that, thanks.....

gbeal · #16 26-09-2006, 09:13 AM

OK, you win, I am with the "CHOP IT" brigade now.
I got some iron cut the other day, and the sheet metal guy used an angle grinder, with a very fine blade/cutter. Worked well.

Geoff45 (Geoff) · #17 26-09-2006, 02:19 PM

Wouldn't it be easier to move the spider and focuser down a bit? Four new (little) holes for the spider and one new (big) hole for the focuser. If you put the focuser on the opposite side to its present position you could have a reasonable interchangeable setup--maybe have 2 focusers and just move the spider up or down.
Geoff

Lee · #18 26-09-2006, 03:18 PM

You are probably right you know!
Hadn't thought of that - the big hole left from the focusers original spot might interfere with the 4 little holes though - will check it out - good idea!

edit - didn't see what you added to your post - might try the rotation too....

Lee · #19 28-09-2006, 10:46 PM

Update - its all clear - some numbers!

Distance from lens ring of D70 to CCD - approx 42mm
Distance from end of Newt focuser (racked fully in) to prime focus - approx 36mm

Adding adaptors likely to add another 10-15mm or so.....

So need to move the mirror by minimum 20mm or so.....

At least I know a 35mm extender is enough!

For future reference - does anyone know the distance to a 350D CCD from its lens ring????

Garyh · #20 29-09-2006, 08:44 AM

Hi Lee,
Not exactly sure but I think its around 44.5mm.
Just work on the distance from the front of the T thread adaptor (side with the screw thread) and add 55mm from that to the ccd sensor. As all T thread adaptors are made so that the focal plane is 55mm back from the front surface. So if your Nikon focuses there should be no probs with other major brand cameras..

Cheers Gary