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Mega-mod thread for Zhumell dobs

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#1 Tim L

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Posted 28 January 2010 - 10:49 PM

When I decided to replace my 60mm refractor with a larger dob, I turned to the internet to compare scopes. That was how I discovered the Zhumell line. And further searching on Google turned up this thread called "New Zhumell owners upgrade mega list thread." That was my introduction to Cloudy Nights.

Once I got a Z10, I returned to that thread over and over for help and new ideas for improvements to make. Since that thread is now in the archives, and the Zhumell line has gone through some changes since then (different alt and az bearings), I thought it would be a good idea to collect suggestions and links people have that will help new Z owners get the most out of their scopes.

So, here are the mods I'll start us off with:
  • Improvements for secondary mirror adjustment
  • Change the screws! Why? Using a screwdriver is a hassle, and has the potential of being dropped down the tube. Many people use Bob's Knobs, which have large plastic heads on them. I used allen-wrench screws, which have a barrel on the end that can be tightened with the fingers. These can be found cheap at the local hardware store.
  • Insert a washer (or two!). Jason D brought this great idea up in this post in the "squirrely secondary mirror adjustments" thread (the thread is a great read by itself). The basic idea is to cut out a washer out of a plastic milk jug to make rotating your secondary easier and allow adjustment of just one screw without having to also tweak the other two. I believe Jason now advocates stacking two of these washers together for greater improvement.

    Improve your laser
    Getting a laser with your scope--how cool is that! But, here's how to make it better...
  • Collimate your laser. Someone please locate a good thread for this with pictures! The adjustments aren't too hard, once you understand what you're trying to do. Use a jig, or two sets of nails in "V" formation in a block, or anything else that lets you rotate the laser freely while pointing at a paper taped to the wall. Use the three tiny allen screws at the back of the laser to adjust the beam until it rotates without inscribing a circle.
  • Add an aperture stop to your laser. Described in good detail in this thread. You can get your laser down to a dot instead of a "bar" (making it easier to collimate, and collimate with). You'll get diffraction rings that will cast a shadow of your primary donut back onto the target face of the laser for more accurate collimation (using a barlow is even better).

    Optical improvements
  • Flocking! There are lots of threads and articles on CN on how to do it. I went with a black velvet from Joanne Fabrics since it was highly rated in this test. You can choose to flock a little or a lot, but I went with the whole tube--turns it into a nice black hole for light to get sucked into!
  • Blacken all the mirror edges and shiny parts. Use a sharpie to darken all reflective edges and surfaces. Be VERY CAREFUL around the mirrors!
  • Check the primary mirror clips. They might be too tight. Back them off until there's a slight gap (enough to slide a business card in) to avoid "pinching" the optics.
  • Check your center spot. It might be accurate, then again, it might not. Mine was off a couple millimeters. Catseye tools has a template you can use to put a new spot in place, or you can make your own template.
  • Change your primary springs? Many people do. You can find stiffer ones at a local hardware store. I replaced mine, but then went back to the originals so I could get the mirror closer to the bottom of the tube.
  • Add a dew/light shield. I made mine out of a long strip of felt 3.5' long by 9" wide. I glued three 9x11" pieces of craft foam to one side, leaving about 1/4" between the pieces. I added velcro tabs to the ends, and now I have a cover I can wrap around the end of the tube. The foam gives it rigidity, it rests against the secondary holder screws, and it can be folded up for storage. :grin:
  • Dew/lightshields for your finderscope. For this, I cut a piece of craft foam in half, and added velcro to make a simple wrap for the end of my finderscope. Not pretty, but functional. I also made a small "sleeve" of black felt that sits around the eyepiece end of the finder. In dewy conditions, I can raise the sleeve to give some protection to the eyepiece. Easier than putting the cap on and off.

    Other additions
  • Add a Rigel Quikfinder or Telrad. Telrad seems to have the market share around here, but I went with a Rigel, because it fits perfectly between the focuser and the finderscope! Either one of these tools will be a benefit to your star-hopping.
  • Rotate your focuser. Why? In its stock position, the focus knobs are never horizontal unless you're looking at the horizon. Got this idea from Don Pensack: There are two tiny grub screws on the side of the focuser base--one toward the mirror, one toward the tube opening. Loosen them, and rotate the focuser 45° so that the focus knobs are level when the scope points at 45°. This mod's more of a personal preference, but I think it makes the scope more comfortable to use.
  • Improve your airflow. Adding a baffle to the back of the tube will direct more air up the pipe. Check out this sample posted by Hudson Yak.
  • Keep all things in balance... The adjustable hubs are a great "coarse adjustment" for balance, but a counterweight also helps. Numerous threads on this topic, but I recommend a heavy magnet like this bad boy. I wrapped mine in felt to protect the tube, added a little duct tape for friction, and put it on the belly just below the hubs, on the side opposite the focuser. When changing to different weight EPs, I just push the weight up or down the tube a bit. This saves the rubber in your alt brakes for light duty work only, and makes the motion smoother without the "bounce back" if the brakes are too tight.
  • Get a shower cap--to seal the bottom of the tube against spiders, etc., when stored. Very cheap, can be found anywhere.
  • Align your finderscope crosshairs. Somehow, for me it's just easier if the up-down and side-to-side motion of the scope corresponds to the crosshairs in the finderscope. You can change the orientation of the hairs by rotating the finder in the holder, or if you're adventurous, you can unscrew the EP and CAREFULLY rotate the ring the crosshairs are in.

So, what mods have you made? Feel free to add to this list, or include some links to help illustrate mods already mentioned. I hope this will be a help to others like the original thread helped me! :rainbow:

#2 FLYcrash

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Posted 28 January 2010 - 11:47 PM

Thanks for updating and newly posting this info! :) I was recently looking for the archived upgrade thread and had trouble finding it.

#3 rinalmj

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Posted 29 January 2010 - 11:16 AM

Tim, can you post a picture of your scope with all the mods?

#4 Tim L

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Posted 29 January 2010 - 02:23 PM

Okay, this is my first attempt to attach a photo with a post...

This is the dew/lightshield folded up. It can also just be wrapped around the "waist" of the scope for storage.

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#5 Tim L

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Posted 29 January 2010 - 02:24 PM

Here's the lightshield in place, showing how the wrap attaches with velcro. Felt on the inside, foam on the outside for stiffness.

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#6 Tim L

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Posted 29 January 2010 - 02:26 PM

Photo of the rotated focuser and Rigel placement

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#7 Tim L

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Posted 29 January 2010 - 02:29 PM

Close-up of the sleeve for the finder eyepiece. This is made from enough felt to wrap around 2x, held together with a band of electrical tape. Nice thing is, I never have to take it off. It slides down far enough to replace the cap when I'm done viewing.

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#8 Tim L

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Posted 29 January 2010 - 02:30 PM

Dew/light shield for the finder--can it get any simpler than this?

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#9 Tim L

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Posted 29 January 2010 - 02:31 PM

Finderscope dewshield in place

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#10 Tim L

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Posted 29 January 2010 - 02:48 PM

Here's a shot of the unbarlowed laser after the aperture stop has been installed. Notice the diffraction shows the silhouette of the primary mirror donut. My primary collimation is slightly off here.

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#11 Tim L

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Posted 29 January 2010 - 02:50 PM

Here's the laser barlowed. I threw the collimation way out so you could see the curve of the center spot. It's very large and "mushy." (Sorry about the focus issue here).

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#12 Tim L

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Posted 29 January 2010 - 02:52 PM

When collimating this way, the primary donut is so large, it's hardly a good gauge of accuracy. However, there are some concentric diffraction rings that show up within the center of the donut that make an excellent "target," as shown here:

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#13 Tim L

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Posted 29 January 2010 - 02:53 PM

Here's one showing where I placed the counterweight magnet for optimum balance:

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#14 Tim L

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Posted 29 January 2010 - 02:54 PM

One more very simple mod I should mention with the laser--put a piece of scotch tape around the barrel to help it fit a little more snugly in the focuser. :waytogo:

#15 Jason D

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Posted 29 January 2010 - 03:30 PM

Here's one showing where I placed the counterweight magnet for optimum balance:


Refer to my post. I described how I built my magnet-based counter-weight in such a way to yield maximum grasp.
Jason

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#16 Jason D

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Posted 29 January 2010 - 03:44 PM

However, there are some concentric diffraction rings that show up within the center of the donut that make an excellent "target," as shown here:

I prefer this method over the aperture stop method. See attachment. The method with the barlow is less sensitive to tilts because the barlow lens and the laser window will move in opposite directions cancelling each other -- unlike the aperture stop method.
The negative lens of the barlowed laser method magnifies the returned center spot shadow – that is why it appears larger. Of course, such a problem will not exist with Glatter’s TuBlug.
Jason

EDIT: I updated the attachment:
1- Top figure represents a barlowed laser setup regardless whether the laser has an aperture stop or not.
2- Bottom figure represents a laser with an aperture stop where the center spot shadow is aligned without the aide of a barlow.

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#17 Jason D

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Posted 29 January 2010 - 03:59 PM

Use a jig, or two sets of nails in "V" formation in a block, or anything else that lets you rotate the laser freely while pointing at a paper taped to the wall. Use the three tiny allen screws at the back of the laser to adjust the beam until it rotates without inscribing a circle.


I came up with a jigless laser collimator collimation setup few years ago. An easy alternative method.
Jason

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#18 Jason D

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Posted 29 January 2010 - 04:01 PM

Improve your airflow. Adding a baffle to the back of the tube will direct more air up the pipe. Check out this sample posted by Hudson Yak.

Many are happy with this method -- not me. It did not work for me as expected. I ended up removing it sometime ago.
Jason

#19 Tim L

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Posted 29 January 2010 - 04:06 PM

Here's one showing where I placed the counterweight magnet for optimum balance:


Refer to my post. I described how I built my magnet-based counter-weight is such a way to yield maximum grasp.
Jason


Thanks for adding that link, Jason! Later in that same thread that you included some excellent diagrams helping to show visually how weights affect the center of gravity of a scope. Someone wrestling with this for the first time should definitely check it out! :waytogo:

#20 Jason D

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Posted 29 January 2010 - 04:06 PM

Insert a washer (or two!). Jason D brought this great idea up in this post in the "squirrely secondary mirror adjustments" thread (the thread is a great read by itself). The basic idea is to cut out a washer out of a plastic milk jug to make rotating your secondary easier and allow adjustment of just one screw without having to also tweak the other two. I believe Jason now advocates stacking two of these washers together for greater improvement.

Nice recommendation ;)
My XT10 has two stacked Milk jug washers which I believe is working better for me
Jason

#21 Jason D

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Posted 29 January 2010 - 04:07 PM

By the way great thread, Tim :waytogo:
Jason

#22 Tim L

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Posted 29 January 2010 - 04:18 PM

However, there are some concentric diffraction rings that show up within the center of the donut that make an excellent "target," as shown here:

I prefer this method over the aperture stop method. See attachment. The method with the barlow is less sensitive to tilts because the barlow lens and the laser window will move in opposite directions cancelling each other -- unlike the aperture stop method.
The negative lens of the barlowed laser method magnifies the returned center spot shadow – that is why it appears larger. Of course, such a problem will not exist with Glatter’s TuBlug.
Jason


Hi Jason,

Hey, help clarify this for me (and other newbies who might also need it). When you said "I prefer this method" I assume you're referring to the barlowed method, right? Will use of a barlow without the aperture stop also produce the inner "target rings" for collimation?

Right now I'm using both. The aperture stop gives me a nice dot for tweaking the secondary alignment, then I pop in the barlow for the primary mirror alignment.

Thanks for your input!

#23 Jason D

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Posted 29 January 2010 - 04:32 PM

I meant the method of using the combination of a barlow and a laser collimator is more accurate regradless of whether an aperture stop is used or not. If the aperture stop introduces more rings with the barlow, then I can see how the additional rings would be useful as references.
Jason

#24 Tim L

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Posted 30 January 2010 - 04:22 PM

I'd like to get some more links or other info in this thread. Anyone have a good link for collimating the laser? A good "how-to" flocking link (with photos) would be a good addition as well.

Thanks!

#25 Tim L

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Posted 30 January 2010 - 04:46 PM

Here's one I found for how to flock your scope. It's got great pictures, shows clearly how to strip the tube and how to apply the flocking material. If you have been thinking about flocking your dob, this is a great place to start.






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