Moving mirror up on 12" Newt

[Scooter]

Hi Folks,

OK Finally decided to have a go with the Nikon D90 at prime focus on the GSO 12” Newt (been using a 2x Barlow) and now am wondering how far I should move the mirror up the tube?

Basically tonight I did some quick measurements as I was trying to work out how much play I have with the focuser and my EP and Barlows. There is about 35mm (0 – 35) of travel in the focuser and when using a 25mm EP focus point is 19mm out, 15mm EP is 14mm out and 6mm EP is 18mm out. Add in the Barlow and it is then 30mm, 28mm and 24mm respectively. Now my question is if I move the mirror up 25mm or 50mm up the tube does that translate to the focuser needing to go out 25mm and 50mm (i.e. is it a 1:1 relationship) or would a 25mm move add X into the focuser movement?

Reason I ask is would love to be able to use the DSLR and visually so want to move it the optimum distance to facilitate this. I have seen Alchemy’s guide and note he moved his up 50mm as this is a common spacer/extender length (indeed if I used 50mm I could unscrew the lens from my Barlow and just use that as an extension as it is 50mm) however if I moved it up 50mm (will this impact on the light cone?) then if it does just require a 50mm move out required on the focuser then none of my EP will work with the 2X Barlow unless I use the 50mm extension and the Barlow right (so EP’s are going to be a fair way out from the tube?).

Now I don’t know exactly how much in travel I need but I had hoped to get away wit a move on the mirror that allowed the EP’s to work and the DSLR without extensions etc. however if it is a 20mm move up on the mirror translates as needing the EP to go another 20mm out on the focuser then given my focuser only has 35mm travel then going on the figures above (and I did not try all combo’s – was just getting an idea of distances) the most I could move the mirror up would be 16mm and I would still need an extension if I wanted to Barlow right?

Incidentally the DSLR and 2x Barlow focused at about 13mm out on the focuser. So does what I am asking make sense?

Thanks in advance folks :thumbsup:

[Alchemy]

gday scooter

cutting 50mm off the bottom of mine just moved the focal point 50mm further up and out, you can buy a 50mm extender from bintel , plug it straight in to your focuser and it works just as before, the secondary is oversize somoving the light cone up the tube has no effect that ive noticed.


correct,


have you tried using a MPCC from baader ? it will bring the focal point in slightly and give you more flexibilty.

if you are in a dob configuration you will be limited to the moon and planets pretty much

a barlow brings the focal point in with respect to the focuser.



if you then move 16mmm up the tube the barlow will move the same out (16mm) so as long as you have 29mm of travel you are fine.

most dobs are made so that most ep's will focus somewhere within its adjustment range. if you use a standard adapter ie 50mm ( there are others) then once in and cut you are still where you were before.


its a decision that needs great consideration as you would probably devalue it for resale purposes. i cut mine , if i bought another id do the same again.

[Alchemy]

had some more thoughts overnight

the balance of a dob will be affected if you do relocate the mirror, so you either have to add weight or relocate the balance point.

you might just be able to make focus with an MPCC AND a low profile focuser. Note the low profile focuser in my opinion is useless for deep sky imaging as it has too much movement, but could suffice for short exposure moon photography.

hopefully someone else out there can confirm or deny my last point

ive had mine on an eq mount for so long i forgot about the balance issue.

if you use it ( 12 inch ) for deep sky imaging on an equatorial mount there are a host of problems to overcome.... some of which i havent fully nailed yet. iceman uses his 12 inch as a planetary scope using webcam type devices and it works really well for that purpose. ( i dont think he has cut his), as the chip inside a webcam or similar is not so far back in the camera ie dslr might be 50mm whereas a webcam might be 15mm ish so the problem isnt so bad... you could ask him.

clive




cheers clive

[Scooter]

Cheers for the response Clive :thumbsup:

I guess the main issue I wanted clarified was that a 50mm move up of the main mirror = 50mm move out of the focus spot (for whatever was being used). I assumed it was but just wasn’t sure so thanks for that. As I said there was only 35mm of travel in the focuser so even a 10mm move of the mirror is going to mean that I can’t use Barlow and EP combo’s without an extension so I guess the question is did I wasn’t to try and move it the least distance I can so I can still use some EP’s without extenders (to Barlow them I will need an extender) or move it a set amount such as 50mm and accept all visual stuff is done with an extender.

I didn’t bother trying to check the focus distance for the DSLR last night as there is only 10mm extra when I have the camera adapter attached so unless it was within that 10mm (doubtful which is why I didn’t check – but you never know maybe I should have tried holding the camera without the adaptor instead of slotting it into the focuser) then I would need to remove the focuser to find the distance which I didn’t really want to do yesterday.

Guess the main reason I wanted to try the DSLR at prime focus without the Barlow was to increase the field and light coming in and see what I thought but perhaps I might just keep having a play as is (it’s not like I really know what I am doing – hell I haven’t even mastered drift alignment to get some long exposures :P ). Thanks also for commenting on the size of the secondary it’s nice to know if I did move a std distance (like 50mm) that the light cone doesn’t appear to be diminished etc.

Have an EQ6 but it would be nice if the tube could be used both on the dob mount (which is what I used last night for example) and the GEM. I keep reading that speaker magnets are a great way to redistribute weight so should try and source some. As for things like an MPCC etc. nope don’t have access to anything like that so maybe I need to hook up with some of the Brissy based folks from here so I can get a look at some of the (expensive :eyepop:) gear you can get. I keep meaning to try and see if I am available when the do one of the weekends at Astrorons but always have something on (travel and kids etc.). Will get there some day I hope.

Just out of interest (seeing as you are an experienced power user – love your work by the way) any chance you could dot point your “host of problems” with a quick comment which ones were sorted and which ones weren’t? Like I said I am just messing around so no problems continuing as is because I certainly need to get a lot better (am trying to do deepsky and I know if have made if difficult for myself with the 12” on the EQ6 and using the Barlow etc.).

Thanks again for your responses Clive and I will mull over cutting the scope (know I can do it anytime).

Cheers

[Alchemy]

1. size of the scope.
its a heavy beast and extra counterweights were needed. SOLVED
2. Mounting dovetail.
the small mounting on the top of an eq6 probably is a bit small a losmandy interface is bigger. SOLVED
3. Mounting Plate.
need to fix guidescope and other bits SOLVED
4. Rings .
RIngs need to be solid i had some custom made out of stainless steel , not quite rigid enough, have ordered some from orion optics in England... PENDING
5. Flexure in the tube.
the tube is thin and large, it flexes with redistribution of weight as it rotates on mount; hoping new rings will help..PENDING
6. focuser
where focuser bolts to tube, again tube thin and flexible 2mm stainless plate added to make more rigid. plus wire traces with elastic bands adding tension to stop barrel movement in focuser, a better focuser ie feathertouch would be better but havent gone there yet. SOLVED PARTIALLY ???? hard to say as other flexure might be causation so again till new rings are installed.....
7. Dew Heating.
strips needed for rear of tube 12 inch, secondary 2inch, laser or finder2inch, guidescope 4 inch , other finderscope 2 inch. SOLVED
8. Wind.
such a big telescope is soooo subject to wind over a long exposure. build a windscreen or observatory. SOLVED
9.Thermal cooling,
easily solved open observatory 1 hr before using SOLVED
10. Cables
need a computer with lots of USB ports , tie them up as they are a pain, and especially make sure only a short section next to camera... tie off to tube so tension will not pull camera off axis , you may need powered boosters for some components SOLVED
11. WATER
cover your laptop at night with blanket.... i killed one and learned the hard way, dew runs everywhere SOLVED
12. Extra bits apart from your main scope
Mount... (duh), Mounting plates, Guidescope, Guidecamera, PHD guiding program, Dew Heaters, Main Camera, Custom made rings (none off shelf available), shoestring usb adapter for guiding, mpcc( multipurpose coma corrector.... you will need it) that should get you up and running, the list gets bigger if you add filters filter wheels etc etc. SOLVED SOLVED hmmm maybee not , theres always more stufff to buy.


theres probably other things which dont quickly come to mind,

pic of my scope in its current configuration atached

cheers clive