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When does in-focusing length matter


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Reading in various posts about people not having enough in-focusing for certain optics. 

As I am considering a Moonlite focuser for my dob Newtonian, I see the single speed has an option of 70mm tube length. 

While extension tubes for out-focusing can be added, I am wondering if the 70mm would be a good choice to cover any in-focusing need. I have never read of extra tube length being added for in-focusing so advice would be greatly appreciated. 

I hope to get bino viewers in the future as well as a 2 inch or lower power barlow for the dob's. 

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As Dave says, it can occur when you need to focus on a sensor, a classic case for me was trying to image with a Lunt 50 solar scope.  That lacked sufficient in-focus for my chip.  I overcame it by using eyepiece projection.  I have never come across a lack of in-focus (or out-focus for that matter) when doing visual observation.  I have never used bino viewers though.

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I have moonlite on my Dob.  It has the standard drawtube length, and has two spacers fitted.  I cannot get focus with BVs even with all the spacers removed, but it will focus with camera prime focus with the spacers present (as it is normally set up).  If I use a 2x barlow I can get focus with BVs.  Otherwise it works ok with all EPs that I have tried.

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It all depends on where the focal plane of the scope is. It's outside the tube wall, but by how much ?

Also, having a longer drawtube does not give you more inwards focus. The profile (ie: low or high) of the focuser itself and it's positioning with respect to the focal plane of the scope is what matters.

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Find where your telescope focal plane resides by placing tissue paper or tracing vellum over the opening of your focuser after focusing on the moon.  Bring the moon to focus on the paper and note how far out your tube is extended.  For my Dob, that is about 20 to 25mm above being racked all the way in.  Thus, if I have an eyepiece with its focal plane more than 20m to 25mm above the shoulder, I can't reach focus with it.  For 1.25" eyepieces, there are ultra-low profile adapters for 2" focusers to place the eyepiece about 1/2" below the shoulder.  I actually had less in-focus before I racked my mirror out as far as I dared on its spring loaded collimation bolts.

If an imager or camera has greater than a 25mm film to flange distance, I can't reach focus.  The ubiquitous T-mount needs 55mm of in-focus.  I have to use my GSO coma corrector directly screwed onto the T-mount to reach focus with a DSLR.

Binoviewers need about 100mm of in-focus.  I have to use a 2x barlow nosepiece to reach focus with my binoviewers in my Dob yielding somewhat over 3x.

Forget about using filter wheels with a Dob.  You need to use a 2" filter slide installed on the underside of the focuser inside the tube.

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Some eyepieces coming from terrestrial observing, e.g. for spotting scope, can require more inward travel when used in astronomy. This is often due to long adapters to make them compatible with 2" or 1.25" formats. Both my zoom eyepieces would require a fair amount of extra inward travel when used with their in-house-built adapters. Thankfully, Baader built a low profile adapter for the Zeiss diascope eyepiece (it still needs some manual work to make it fully effective). On the other hand Dennis, here on SGL and on CN, indicated how to modify the Nikon zoom to reduce inward travel of about 2cm. So, in the end, they can operate like any other eyepiece I have own with some work now. :) 

If your eyepieces require more inward travel, you can play try to solve your problem with a low profile focuser (as John said previously). If they require more outward travel, you need some extension tube. Clearly, between the two issues, the former can be tricky to solve. 

On a Newton telescope, one could gain more travel but moving the primary mirror up. This can be done depending on your primary mirror cell. On a refractor, it could be impossible. 

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5 hours ago, Piero said:

On a refractor, it could be impossible.

Several folks cut out part of the main tube and use an extension in the focuser tube to reach focus when not using binoviewers.  This can be particularly effective with large diameter focusers where large diameter extension tubes won't vignette the image.  Borg makes a line of modular refractor tubes than can be configured to various lengths so no cutting is necessary for them.

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8 minutes ago, Piero said:

Yes, sure, one can do that, but this is a drastic solution.

I would never cut the tubes of my TV60 or Tak! :crybaby2:

Indeed, a scary prospect! I do wonder why more manufacturers don't produce scopes with removeable sections to make them bino friendly. The Tak seems quite ok in this respect anyway, but much harder with a newtonianian, I guess it would have to be specifically designed to be optimised for binoviewers. Moving the primary up is fine but you have to to sure you are not over filling the secondary as the whole light cone moves up.

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A refractor tube with sections would also be very handy for travelling. Most refractors would fit in a hand luggage.

 

You are right about the secondary mirror. A solution to that is to replace the secondary mirror with a larger one. 

Similarly to moving the mirror up, one could cut the poles on a truss dobson.

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46 minutes ago, John said:

APM include a telescopic section about 100mm long between the end of the scope tube and the focuser on some of their refractors. 

 

 

Yes, that's right. Very handy it is too. I had a Tak FC-76DC with split tube for a while and it was very easy for travel

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