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Prime focus on Meade LXD75 6" Newtonian

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#1 ZeRoY

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Posted 19 November 2012 - 07:35 AM

Hi all,

My first post here. I just bought myself a second hand Meade LXD75 6" Newtonian (762mm f/5 152mm apert.) and I am wondering if Prime focus using a DSLR can be achieved with the stock Focuser. I have a Standard T-Ring + 1.25" nose and as far as I can see I have 2 ways to attach this to focuser; First would be with the 1.25" adapter ring already on the Focuser tube or without (without reduces the distance from DSLR to secondary mirror by 55-60mmm i think)

I done some tests already but the conditions were pretty bad to judge results.

Any advice would be appreciated.

#2 Atl

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Posted 19 November 2012 - 08:15 AM

One way to find out is focus on a terrestrial object. An extension tube may be needed.

#3 ZeRoY

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Posted 19 November 2012 - 08:26 AM

One way to find out is focus on a terrestrial object. An extension tube may be needed.


Thank you for quick reply, how far would this object need to be however?

#4 Atl

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Posted 19 November 2012 - 12:42 PM

A quarter mile probably. That works for me.

#5 jgraham

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Posted 19 November 2012 - 01:27 PM

I've used both the N6 and SN6 and you shouldn't have any trouble reaching the focus with the stock focuser. I used an EOS to T-ring with a T-ring to 1.25" nose piece adapter onto the 1.25" eyepiece adapter on the focuser with the focuser's extension tube removed. To help carry the weight of the DSLR I snugged the lock screw a tad. Also, to minimize the effects of the focuser shift I always set the focus moving the draw tube in the same direction, inwards. If I over-shoot I'll draw the focuser out a bit and try again. Once I've got the focus set I'll mark the draw tube with a pencil. This makes it easier to rough-in the focus and to to help visualize small changes in focus.

This is my SN6 fitted with a DSLR. The geometry of the N6 is exactly the same.

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#6 ZeRoY

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Posted 19 November 2012 - 02:34 PM

Thanks a lot John, I thought as much for the focuser's extension tube removal but didnt try at the time! Can I ask what sort of exposure time max you achieve without guiding on that setup? I dont yet have a Guiding scope and planning to use Drift align with Canon Live View in APT 2.0

#7 jgraham

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Posted 19 November 2012 - 03:03 PM

I used the setup for a couple of years before I added the guide scope. If I recall right I pretty much did everything with 30 second exposures. This gave me a high success rate and allowed me to stack a lot of images (typically 32 to 64). It worked very well.

#8 ZeRoY

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Posted 20 November 2012 - 04:42 AM

Well not sure if its the type of Tring + nose I use or the Canon 1100D but I could not get Prime focus directly :( -- I had to do a little hack job to gain almost 20mm and give me the prime focus, I removed the nose + small ring from the T2 ring and using the 3 existing holes on the T2 I was able to firmly hold the 1.25" adapter that screws onto the focuser, here are some pictures;

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#9 jgraham

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Posted 20 November 2012 - 08:46 AM

Hmmm, another way to draw the focus in on the N6 is to move the mirror back by extending the telescope tube as needed. I made a tube extension for mine with a sheet of aluminum flashing from home improvment store. Flashing comes in rolls that will spring flat if you let it. I cut a length long enough to go 2-3 times around the inside of the N6 tube. I then removed the mirror cell, inserted the roll of flashing, and let it expand until it fit tightly against the inside of the tube. I adjuested its position to give me the length that I wanted and then drilled holes though the original mounting holes for the mirror cell and installed small bolts to hold it in place. I did the same on the end to remount the mirror cell. I took it apart to apply a finish of flat black paint, and then reassmebled it. Voila! A longer telescope tube. In my case I did this to draw the converging light cone back a bit to fully illuminate the field with the original diagonal.

This is a picture if my original N6 configured for visual with a DSX-90 mounted on its back. (This scope eventually became my guide scope for imaging.) If you look closely at the lower end of the telescope tube you can see the extension. You can also see how far in the focuser draw tube is.

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#10 ZeRoY

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Posted 20 November 2012 - 11:01 AM

Thank you John for this new tip, I am currently looking at changing the focuser for a low-profile crayford, If I can get one that is just 20mmm less that the stock it will work. Besides, the Stock focuser as quite a bit of image shift during focusing.

#11 denodan

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Posted 25 November 2012 - 02:05 AM

Had one of these scopes also, worked well, but one problem? The spider and vanes are far to thick.






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