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Equipment Discussions >> Reflectors

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Jason D
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Reged: 10/21/06

Loc: California
Re: Mega-mod thread for Zhumell dobs new [Re: sawacs]
      #4331187 - 01/22/11 01:37 AM

Quote:

When I take the CatsEye Teletube and place it in the focuser in the correct position for a F5 scope, I do see the clips. However, if I back it off to where the actual eyepiece would be, I do not see the clips? Am I confused?



Then I think you are OK. It is natural to rack out the draw-tube for eyepieces since the field stop (leveled with the focal plane) is below the drawtube shoulder. When you use Catseye Teletube you will lower the drawtube to level the its shoulder with the focal plane.
Bottom line, I would trust the Teletube assuming you are using it correctly.
Do not worry...
Jason


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: Jason D]
      #4331709 - 01/22/11 10:54 AM

Jason,

Quote:

Rack the focuser's drawtube 1/2 way then insert a collimation cap or a cheshire. Can you see all primary mirror clips simultaneously? If no, your secondary mirror is too small.
Jason




Yes, I can still see the mirror clips, but just barely, which apparently agrees with what the WinNewt program is telling me. In WinNewt, if I set the secondary ma just 1mm less, it will show "Diagonal too small to admit 100% ray: YES." So I have about the minimal-size diagonal that will not set off the red flags in that program.

On the other hand, I intend to use my Z8 here at home for the planets and Moon only, no deep sky. This Z8 originally had a 25.5% CO, obviously intended for general observing. I have a 10" Newt with a 23.2% CO which I use for trips to dark sites for DSO. I don't need the Z8 for deep sky.

For planetary/lunar, is the 100% illumination really that important? Isn't it OK to be right on the edge of not having 100% illumination or even to have a diagonal ma which is narrow enough to allow the scope to fall a little below that? True, you might lose a little aperture, but you could have a much larger reduction in CO% that would more than offset the loss of aperture. For planetary work, an increase in contrast can allow more details to be seen that might not be visible in a larger aperture with less contrast. Contrast isn't everything, but for planets it is a big something.

I read your explanation above about not seeing the entire aperture if the entire primary is not seen at the focuser. That makes sense to me, but I'm still not at ease. For one thing, for planetary observation you use higher magnification and shorter focal length eyepieces, or even a binoviewer, so you would probably tend to rack the focuser in more than for deep sky. That might have a bearing on judging 100% illumination by racking the focuser only half-way in when you eye-ball it.

I think it's probably better to gather accurate measurements of the specific telescope, feed that data into WinNewt, and look at the results. Or you could also rack the focuser in farther and see how much of the primary you can see.

Here's something else to think about: If the diagonal does not see the outer 1/4" or so of the primary, it does not see edge errors such as TDE. That would be a good thing.

Mike

Edited by Sarkikos (01/22/11 11:32 AM)


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: VikingRaider]
      #4331718 - 01/22/11 10:59 AM

Steve,

Quote:

Here's my homemade base built of scrap 2x4s and other bits of wood I had laying around. Cost me $3 in paint (contractor 99 cents a can paint at Depot), $18 in casters (two 3-inch casters $3 ea., and two 3-inch swivel casters $6 ea.) and $2 for a pair of hinges. Has a little table on a hinge that lowers when it's stored in the garage out of the way. My wife thinks it looks like a black hot water heater in the corner of the garage (I store OTA vertical). Crude but it serves it's purpose. I'm fixin to upgrade the hard plastic casters with some rubber or pneumatic wheels from Harbor Freight by Christmas. The casters tend to give a rougher ride than I thought on smooth concrete and like to get stuck in the grooves of my driveway. No big deal, but a minor annoyance that's easy to fix so why not?




Do you observe with the Z8 on that table? Why would you want it that high? It's better to sit when you're observing.

Mike


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: sawacs]
      #4331727 - 01/22/11 11:03 AM

Shawn,

Quote:

Small tip..

You can easily reduce your secondary mirror size by using another brand secondary holder but you have to be willing to drill your 4 vane hub out

Using a drill press, I successfully "wollered" the hole out enough to accommodate the 2.14" Protostar secondary mirror and mount. In fact, the aftermarket collimation screws many of us use for our Orion and Zhumell scopes fit perfectly in the Protostar dimples on the secondary.

Although I do have the 3 vane spider from Protostar, I tried this out just for kicks:)




You have to be careful that the cross-section of your old hub is not wider than the minor axis of your new secondary mirror or the effort is futile. You won't be decreasing the size of your Central Obstruction. A smaller secondary is irrelevent if the CO% does not decrease.

Mike


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Jason D
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Reged: 10/21/06

Loc: California
Re: Mega-mod thread for Zhumell dobs new [Re: Sarkikos]
      #4332357 - 01/22/11 03:18 PM

Quote:

For planetary/lunar, is the 100% illumination really that important?



It is not the 100% illumination as much as getting the full-resolution of your primary mirror. In addition, secondary mirror imperfection close to its edge might reduce contrast. That is, whatever minute contrast you gain with a smaller secondary mirror might be lost with the secondary mirror imperfection. Of course, a quality secondary mirror might not have this issue.

Quote:

so you would probably tend to rack the focuser in more than for deep sky. That might have a bearing on judging 100% illumination by racking the focuser only half-way in when you eye-ball it.



100% illumination calculation has nothing to do with the drawtube racking but rather with the focal plane location. When you rack the focuser for difference eyepieces, all you are doing is positioning the field stop of various EPs at the focal plane of the primary mirror – same location for all.
I suggested the ½ drawtube position as a reasonable estimate for the location of the focal plane.

Quote:

Here's something else to think about: If the diagonal does not see the outer 1/4" or so of the primary, it does not see edge errors such as TDE. That would be a good thing.



That depends in your primary mirror. If it suffers from TDE, then I would rather place an aperture mark on the top of the primary mirror. Do not count is a smaller secondary to do that for you because of three reasons:
1- Your secondary mirror will have to be positioned perfectly to reduce TDE effect
2- TDE will scatter light which means your secondary mirror will still pick up some of the scattered light
3- Off-center objects will still see the primary mirror edge
Jason


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: Jason D]
      #4332563 - 01/22/11 05:12 PM

Jason,

Quote:

It is not the 100% illumination as much as getting the full-resolution of your primary mirror.




As I said, for planetary observation, greater contrast can allow finer detail to be seen at the eyepiece. Getting the full-resolution in a theorectical sense will not matter if you cannot actually see the detail because of poor contrast.

Quote:

In addition, secondary mirror imperfection close to its edge might reduce contrast. That is, whatever minute contrast you gain with a smaller secondary mirror might be lost with the secondary mirror imperfection. Of course, a quality secondary mirror might not have this issue.




Yes, I was aware of this. Ideally if a minimal-size secondary is used, it should be of high quality. What I'm doing now with my Z8 is an experiment to see if in my case with my equipment, there will be an improvement in planetary/lunar. If there isn't, I can always bring back the over-sized diagonal or buy a better narrow-diagonal.

Quote:

100% illumination calculation has nothing to do with the drawtube racking but rather with the focal plane location. When you rack the focuser for difference eyepieces, all you are doing is positioning the field stop of various EPs at the focal plane of the primary mirror – same location for all. I suggested the ½ drawtube position as a reasonable estimate for the location of the focal plane.




Yes, agreed. The collimation cap or sight tube or just your eye looking into the focuser need to be at the focal plane location to get an accurate idea of how much of the primary will actually be seen. By the same token, in order that their field stops will be at the focal plane of the primary, shorter focal length eps and binoviewers will be farther down in the focuser than are longer focal length eyepieces. But they all need to be at that focal plane.

Mike

Edited by Sarkikos (01/22/11 09:20 PM)


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: Jason D]
      #4332588 - 01/22/11 05:21 PM

Quote:

Quote:

Here's something else to think about: If the diagonal does not see the outer 1/4" or so of the primary, it does not see edge errors such as TDE. That would be a good thing.




That depends in your primary mirror. If it suffers from TDE, then I would rather place an aperture mark on the top of the primary mirror. Do not count is a smaller secondary to do that for you because of three reasons:
1- Your secondary mirror will have to be positioned perfectly to reduce TDE effect
2- TDE will scatter light which means your secondary mirror will still pick up some of the scattered light
3- Off-center objects will still see the primary mirror edge
Jason




All good points to keep in mind. The first thing is to perform a star test to see if the primary does have TDE or other edge errors. From what you say, it would make more sense to hide those errors by a narrow aperture ring above the primary. If you have a fan blowing onto the back of primary, such a ring could also direct the air onto the surface of the primary to help break up a boundary layer.

Mike


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sawacs
sage


Reged: 03/15/06

Loc: KOKC Area
Re: Mega-mod thread for Zhumell dobs new [Re: Jason D]
      #4333402 - 01/23/11 12:47 AM

Here are a few photos of the Protostar secondary mirror and holder coupled to the Zhumell hub...

Okay, I give up on trying to get the images to stick so here is a link:)

Zhumell Spider With Protostar Secondary

Cheers, Shawn


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: sawacs]
      #4333423 - 01/23/11 01:02 AM

Shawn,

I don't see the photos.

Mike


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Jason D
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Reged: 10/21/06

Loc: California
Re: Mega-mod thread for Zhumell dobs new [Re: Sarkikos]
      #4333458 - 01/23/11 01:36 AM

Quote:

All good points to keep in mind. The first thing is to perform a star test to see if the primary does have TDE or other edge errors. From what you say, it would make more sense to hide those errors by a narrow aperture ring above the primary.




That is correct. Instead of letting the light be scatter by the TDE then try to deal with it, place the mask above the primary to prevent light from getting there in the first place.

Quote:

If you have a fan blowing onto the back of primary, such a ring could also direct the air onto the surface of the primary to help break up a boundary layer.




Yes, this is another known advantage for the mask.

There is also a 3rd advantage which is to prevent accidental small rolling objects down the tube from hitting the mirror.

Jason


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: Jason D]
      #4333999 - 01/23/11 10:58 AM

Jason,

Yes, even if there are no edge errors on the primary, if a fan is blowing onto its back, an annular baffle directly above the mirror is a good mod to direct air over the surface to break up a boundary layer. If the primary has edge errors, the baffle should be constructed so that it extends over the edge of the primary about 1/4".

I haven't made a combination air flow/TDE baffle for any of my mirrors. From what I can tell through star tests, neither my 8" or 10" have TDE or other edge errors, so when I get around to making baffles, I would set it up just to direct air over the mirror surface. I have an observing buddy who made a baffle from steel (or had it made for him) to mount over his mirror in its rocker box. He had the ring cover the edge of the mirror, assuming it had a TDE. He claims it has made a big difference in his views.

All my Newts are solid tube, so a steel baffle would not be a good solution for me. I would instead make it from two rings of black foam core with another material - such as ABS plastic or stiff cardboard - sandwiched between to create a sharp edge. (A sharp edge would be more important, though, if the baffle is serving to cover a TDE.) The baffle would be snug tight to fit inside the OTA. I'd measure where I should place it, mark that around with a circle, and nudge the ring up into position. I would not bother drilling holes and screwing it onto the tube. I avoid that type of business whenever possible. I would find some other way to position it securely, if necessary.

Mike


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: skizoid]
      #4334104 - 01/23/11 11:30 AM Attachment (67 downloads)

skizoid,

Quote:

Long as we're on the subject of light, I'm also having second thoughts about a flock shield I stuck around the focuser inside the OTA.




You stuck a tube of flocking around the focuser tube? If that's what you did, I don't think that's the best solution. It's better to directly attach the flocking paper to the outside of the focuser tube, so that it is retracted when the focuser retracts. Cut out sections of flocking and attach them so they will not be bound up with the focusing mechanism when you move the focuser tube in and out. Use black marking pen to go over the areas that you cannot cover with flocking.

Here's how the focuser for 10" Newt looked after I covered it with Protostar and blackened some areas with black marking pen.

Mike


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skizoid
member


Reged: 01/12/11

Loc: 43.223N / -85.551W
Re: Mega-mod thread for Zhumell dobs new [Re: Sarkikos]
      #4335309 - 01/23/11 07:42 PM

Gotcha - Didn't take long to notice the shield was partially blocking the mirror when I got around to adjusting the optics. The flock board isn't going to work in small patches I don't think - it's kinda stiff for that. Maybe use velcro - the fuzzy side? Got a bunch of that laying about. I'll come up with something. Either way, I'll cut the shield down just proud of the flocking and leave it in place. That's part of the sealing system for the airgap between the flocking and the tube itself.

Gave the Dub Dob (I like that) it's first field test tonite. 8F ... perfect viewing weather! Clear sky anyway. Gives new meaning to the phrase "crisp air". Set it up about an hour before sunset, then went out to play. Jupiter was an easy target, so I used that to align the finders with the OTA. That Telrad is sweet for quick 'n dirty! I could see Uranus, but wasn't getting any decent detail, so I'm gonna have another go at fine tuning the mirrors. Wasn't too long after sunset, so I still had quite a bit of heat distortion to deal with. I'm happy enough that I could see anything at all first time out. Speaking of first time out ...



Was going to spend some more time out eyeballing the nether regions, but I could feel MY nether regions going numb, so called it a day. Thinking I may thaw out enough by midnite to take a quick peek at the moon. That's when it's supposed to rise according to Stellarium. Or not - I don't imagine it'll be warming up by then. This arctic blast is only supposed to last another week, so I think I'll wait a bit before I get into some serious work. That'll also give me some time to work on a "Denver Chair" - my back ain't what it usta be.

Speaking of significant shrinkage - my first surprise was the OTA cap. That's usually a tight fit. After some time outside, I could spin it around in the opening. Least it didn't fall thru ... <G>


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skizoid
member


Reged: 01/12/11

Loc: 43.223N / -85.551W
Re: Mega-mod thread for Zhumell dobs new [Re: skizoid]
      #4335493 - 01/23/11 09:05 PM

I've heard Zhumell's documentation is ... less than stellar. Their PDF manual sez ...

4. Loosen 3 locking screws on bottom of optical tube (the thin ones that stick farther out from the back of the mirror).

5. Align the laser to the center of the reticle by adjusting the 3 primary mirror adjustment screws (the thick screws in the back of the mirror). Do not over-turn these screws (max of ¼ turn at a time).

Not seeing any thick or thin screws, I assUme the white ones are the locks. Loosen those up so they're out of the way, then adjust the tilt with the black ones. Right? Seems to work that way anyway.


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: skizoid]
      #4335600 - 01/23/11 10:10 PM

Skizoid,

Quote:

Gotcha - Didn't take long to notice the shield was partially blocking the mirror when I got around to adjusting the optics. The flock board isn't going to work in small patches I don't think - it's kinda stiff for that. Maybe use velcro - the fuzzy side? Got a bunch of that laying about.




You bought the flock board and not the flocking in a flexible roll with an adhesive backing? For doing smaller parts like the focuser, the adhesive-backed flocking would be easier to work with. Maybe you could just get a small section or two.

Mike


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Mega-mod thread for Zhumell dobs new [Re: skizoid]
      #4335609 - 01/23/11 10:14 PM

Skizoid,

Quote:

I could see Uranus, but wasn't getting any decent detail, so I'm gonna have another go at fine tuning the mirrors. Wasn't too long after sunset, so I still had quite a bit of heat distortion to deal with. I'm happy enough that I could see anything at all first time out. Speaking of first time out ...




Chances are you won't see much detail on Uranus in a 10" scope. Jupiter and Saturn are better tests.

Mike


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skizoid
member


Reged: 01/12/11

Loc: 43.223N / -85.551W
Re: Mega-mod thread for Zhumell dobs [Re: GrafikDihzahyn]
      #4339832 - 01/25/11 05:17 PM

OK - finished the fan shield.

Thing is, all the pics I've seen show the fan mounted INSIDE the shield with just the grill showing. No way I was going to fit mine in there - it's at least a quarter inch too tall. I ended up mounting it to the shiny new shield instead of the original location. May be a bonus - there's more space for the air to develop velocity, so I'd think better flow around the edges of the primary. This way too, I'll be able to add a box around the fan for a filter. Maybe. Still thinking on that.



They change the fan, or am I just having a Polish moment? Not complaining if they did - a deeper fan would have more pitch to the blades and be able to move a lot more air at lower rpm. I did consider replacing the 80mm stock fan with a 120mm fan that's half the height. Leftover from another project, but I already had the shield cut, so there.

Oh. Saw a few posts about the fan being mounted backwards. Rule of thumb, air blows towards the label on the hub.


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skizoid
member


Reged: 01/12/11

Loc: 43.223N / -85.551W
Re: Mega-mod thread for Zhumell dobs [Re: skizoid]
      #4340161 - 01/25/11 07:57 PM

Applied the KISS (Keep It Simple Stupid) principle to the diverter ring for the fan. Closed cell foam tape, 1/4" tall, 3/4" wide. Just set a gauge and stuck it on so that it clears the top of the primary by about 1/4" ...



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skizoid
member


Reged: 01/12/11

Loc: 43.223N / -85.551W
Re: Mega-mod thread for Zhumell dobs [Re: skizoid]
      #4341889 - 01/26/11 03:32 PM

... and with this, I do believe I'm about ready to hit the road.





I did pick up on a cheepie golf bag caddy to haul the OTA about. Good size wheels, and it folds up small. Have to rework the base a bit, add a couple padded straps, maybe some baby moon hubcaps ...

... a modding we will go! Hi ho the derry oh ...


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graffias79
sage


Reged: 10/01/10

Loc: Madison, WI
Re: Mega-mod thread for Zhumell dobs [Re: skizoid]
      #4358570 - 02/02/11 04:19 PM Attachment (82 downloads)

I don't like the idea of permanently applying or modifying my telescope. I like to keep things completely reversible, at least for a while after I've had them. Having said that, I had an idea. I have been working on PCs for years and have amassed quite a collection of dead hard drives. A few months ago I decided to remove all of the rare earth magnets from them and throw the rest of the parts away. I am glad I did because the magnets come in handy:

For the Telrad, I sandwiched 2 hard drive magnets between some double sided tape and some craft store foam:


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