Need some advice

[chrislj74]

I've been considering ways I can raise my mirror in my dob for my Nikon d40x so I can bring it in to focus. So yesterday I went out and bought new longer screws for collaminating my XT6 dob. The longest I could locate were 2" which should have given me about 1" or a little more hieght for my mirror. Well it worked out well but I still don't think I have the primary close enough for good focus. Using the 2" piece that came with my t-ring it can not come to focus at all. But if I attach directly to my focuser it appears to be close. I wanted to try on the moon but had to settle I on a brigth star since the moon was no where to be seen at the time for focus but was unable to tell if it was focusing correctly since my camera does not do live view and I had to rely on the viewfinder which is not easy to say the least. I'd also like to mention I can still easily see my whole mirror (clips and all) when peering through the focuser. I'm guessing I could still raise my mirror atleast and inch possibly more without losing any performance. So my question is without causing permanent damage, should I cut my secondary down? I figured I'd start with an inch removed while still using the longer collamination screws and see where that gets me. I'd like to not lose any visual performance and this seems the least likely to cause any permanent damage that can't be undone. So if anyone has any advice please share. Thanks

[ronin]

If you mean what you say "cutting the secondary down" then that will not alter the optical path at all, it will still come to a focus in exactly the same place. If reduced in size light from the mirror may miss the secondary and head back to whatever you are looking at.

However cutting the secondary down doesn't make much sense, have we a misunderstanding of what you mean?

If the Dobsonian is not driven then in reality imaging anything other then the moon is not a realistic option. Even if it is driven then I would expect it to be an Alt/Az configuration and would be usable on planets but not for long exposures for DSO's.

[chrislj74]

Yes a misunderstanding lol. I meant cutting down the tube. I see I typed secondary, not sure why since that doesn't make since. I want to shorten the bottom of the tube to bring the primary closer to the secondary.

As for the being driven part, currently it is not. I know as it is as a dob the moon is pretty much my only target with it being a still camera but I do have the spc900 and point and shoot I use in video mode that has proven to be effective for astrophotography of the planets and such as it sits. But I already have a suitable mount in mind that I want to convert to and I wanted to go ahead and get my telescope ready for when I get it.

[chrislj74]

Well I decided to go ahead and take off 1" from the bottom of the tube. That should have moved my primary up the tube about 2" when taking into account the longer collamination screws too. I can still easily see the entire primary mirror and clips while looking through the focuser so I didn't lose any aperture. Even after raising the mirror cell 2" I still can not bring my d40x into focus using the t-ring and adapter. Now if I remove the tube from the t-ring and attach directly to the focuser I can achieve focus on the dslr. I do however have to use an extension for eyepieces which I took care of with a spare barlow I have with the lens removed.

The xt 6 series dobs from Orion must have very oversized secondaries because even after moving the mirror up 2" I can still see quite a bit of the tube around the primary and easily the whole mirrror. While I had everything out I happened to measure the secondary and it's minor is 1.35". I'd venture to say I could raise it an inch or more and still be fine but I'm not really inclined to do so for the risk of losing any aperture if I'm wrong. I would however like to be able to use the adapter with my t-ring because attaching it directly to the focuser sets the camera at a weird diagonal angle. I may need to find a better ring or an adapter that attaches that will fully insert into the focuser.

Anyway, below is a picture from my dslr on the xt 6 dob with no barlow or eyepiece. It's kind of fuzzy in my opinion because I don't think it was in perfect focus which as I've said before is difficult with no live view and relying on the viewfinder. But atleast I can bring it to focus which is what I was after.

[mdstuart]

It sounds like that worked for you.

I have also shortened a tube on a Newtonian before with no major issues.

A tip for focusing. Use the pole star which does not move..it makes it easy to experiment until you get a good focus with a web cam etc and then move the dob to the moon / saturn to take a shot..

Mark

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[chrislj74]

I think I may leave it alone now and not worry about a t adapter that will completely fit inside the focuser without adding anymore height between the camera and focuser. The adapter I have is a celestron #93625. Upon looking at it closer where it threads to the t ring I noticed 3 tiny allen screws around the edge. Loosing those up and repositioning it I am able to get my camera in the desired position without it being at that weird diagonal angle. I also opted to leave the t ring adapter attached to the focuser to avoid having to screw it on everytime I want to use it so as to reduce the possibilty of dropping my camera.

On a seperate note I just got my flocking paper in today and plan on flocking the tube soon. It maybe overkill for a 6" scope but I figured what the hey. Afterall it only set me back about $30.