My 12" Zhumell telescope has problems

[Robstargazer15]

My Z12 dob doesn't respond well to magnification. And it's collimated perfectly. I don't know whats wrong with it, My Celestron 4se responds better to magnification, but the 12"dobsonian don't outperform the 4se. Jupiter is very nice through the 4se, but the Z12 dont even show the great red spot or main belts and zones on its surface. Whats wrong????

[crashtestdummy]

What the focal ratio of the scope and what eyepieces have you got?

[knobby]

Hi Rob, are you cooling the scope to match the outside temperature? 12" is a lot of glass and will probably take a good few hours to cool.

[RikM]

One problem with larger aperture telescopes is that along with being able to resolve more detail in an object, they also resolve more detail in the turbulent atmosphere. They are more affected by rough seeing conditions than smaller telescopes. I am geussing here but I expect your Dob is f/5 or faster. This isn't going to give you as much contrast as the 4se at f/10. Assuming the mirror coatings and figure are in good knick, I doubt there is anything wrong with your Dob, it is just not the right tool for looking at planets. Though for deep sky objects like galaxies and nebulae, it should eat the 4se alive.

[Mr Fiery Jack]

One problem with larger aperture telescopes is that along with being able to resolve more detail in an object, they also resolve more detail in the turbulent atmosphere. They are more affected by rough seeing conditions than smaller telescopes. I am geussing here but I expect your Dob is f/5 or faster. This isn't going to give you as much contrast as the 4se at f/10. Assuming the mirror coatings and figure are in good knick, I doubt there is anything wrong with your Dob, it is just not the right tool for looking at planets. Though for deep sky objects like galaxies and nebulae, it should eat the 4se alive.

Yep, mine is just the same. Its horses for courses I'm afraid. You wouldn't expect a refractor to be the ideal for DSO, you can expect a dob to perform as well on planets as your SCT.

[Mr Fiery Jack]

That should be can't expect.

[John]

The Zhumell 12" is the same as the Revelation and Telescope Services 12" F/5 dobsonians. GSO optics and mirror cells. If it's cooled properly and the seeing conditions are not awful it should show good detail on Jupiter. What test have you done to show that that the collimation is accurate ?

[Moonshane]

My Z12 dob doesn't respond well to magnification. And it's collimated perfectly. I don't know whats wrong with it, My Celestron 4se responds better to magnification, but the 12"dobsonian don't outperform the 4se. Jupiter is very nice through the 4se, but the Z12 dont even show the great red spot or main belts and zones on its surface. Whats wrong????

what are you using to collimate? lasers are notorious for being out of kilter.

I'd disagree completely with the view that there should be not as much detail in a 12" newt as a 4" SCT. perhaps not as much contrast but the substantial extra detail will be there in the periods of good seeing.

at the risk of sounding like a broken record, I get more detailed images through my 6" f11 newt than through any refractor I have looked through and more detail through my 16" f4 with an aperture mask at 170mm (giving f11). I can only comment on what I see through my own scopes but it speaks for itself.

for Jupiter I do tend to use a Baader Neodymium filter as this enhances contrast (to my eyes at least). if you have a light pollution filter this would have a similar effect.

[Moonshane]

just by way of example of what's possible to see with an 8" newt....

[Moonshane]

sorry, I'm getting (d)obsessive again aren't I?

[John]

Sounds like the scope is not showing any detail on Jupiter - "....dont even show the great red spot or main belts and zones on its surface....".

Something not right there. I don't think I've used a scope, no matter how small, that won't show at least the main 2/3 cloud belts. I wonder if the scope is in focus ? - an out of focus Jupiter will show a featureless disk.

[Moonshane]

I suppose cooling is most likely but even my big dob shows the main belts straight out of a hot house at lowest power (about 70x).

[Naemeth]

Sounds like the scope is not showing any detail on Jupiter - "....dont even show the great red spot or main belts and zones on its surface....".

Something not right there. I don't think I've used a scope, no matter how small, that won't show at least the main 2/3 cloud belts. I wonder if the scope is in focus ? - an out of focus Jupiter will show a featureless disk.

Agreed.

My 130P can show the main belts with my 26mm (25x), although they are hard to spot as Jupiter is bright at that magnification!

What magnification were you viewing at, because if you went too high the atmosphere would have completely spoiled the view.

[Mr Fiery Jack]

If the seeing was the same as it was here last night my perfectly collimated and cooled dob showed little more than a milky disc with occasional detail of cloud bands, my refractors performance was almost entirely unaffected.

[estwing]

just by way of example of what's possible to see with an 8" newt....

http://stargazerslou...-superb-seeing/

..never had views like that,my jup's a very bright disc with detail..perhaps i might use a moon filter next time it's clear..hmmm.

[RikM]

If the seeing was the same as it was here last night my perfectly collimated and cooled dob showed little more than a milky disc with occasional detail of cloud bands, my refractors performance was almost entirely unaffected.

Same for me last night. With my 10" I could really only make out the 2 main bands and a few other smudges. The seeing was weird, rather than big swimmy movements it was very small scale shimmery stuff that just blurred everything.

Paul Abel uses an apodizing screen with his Newtonian. This changes the diffraction pattern giving much better detail in a fast instrument similar to what you would get in a planetary scope like Shane's f/11.

[Mr Fiery Jack]

Same for me last night. With my 10" I could really only make out the 2 main bands and a few other smudges. The seeing was weird, rather than big swimmy movements it was very small scale shimmery stuff that just blurred everything.

Paul Abel uses an apodizing screen with his Newtonian. This changes the diffraction pattern giving much better detail in a fast instrument similar to what you would get in a planetary scope like Shane's f/11.

Now that does sound interesting, any links ?

[RikM]

Now that does sound interesting, any links ?

http://www.astronomyhints.com/apodize.html

[Damo636]

My 12" f5 (same optics as the Z12) newt has shown more detail on Jupiter than any other scope I have owned or looked through. If your Z12 is properly cooled to ambient temperature & reasonably well collimated, it should spank the 4se in almost every department! I have used mags up to 428x while hunting for & sucessfully spotting the central star in M57. I have also on occasion, when seeing permitted, had really sharp views of the lunar surface at this mag. It is strange however that you didn't see any detail whatsoever? I wonder did the eyepiece you were using come properly to focus?

[Mr Fiery Jack]

http://www.astronomy...om/apodize.html

Ta

[Capricorn]

If the seeing was the same as it was here last night my perfectly collimated and cooled dob showed little more than a milky disc with occasional detail of cloud bands, my refractors performance was almost entirely unaffected.

Seriously doubt that the seeing was the same, the OP is in Alabama, you are in Yorkshire. :eek:

[Mr Fiery Jack]

Seriously doubt that the seeing was the same, the OP is in Alabama, you are in Yorkshire. :eek:

I didn't see that, me all over...

[cathalferris]

I've never seen more detail in a smaller aperture on the same night in a side-by-side. The larger scope would often appear to be shimmering more, but the details were there. The smaller scope would have less movement for hte same atmospherics conditions, but would never show more detail. The smaller scope may appear to be more aesthetically pleasing due to the smaller movement range, but would always see less.

Regarding the OP: What do highly magnified in-focus star images look like? I'd suggest getting as high a magnification as possible on Polaris and seeing if a) the star pops into focus or if it a mushy region of focus; b; if there are any diffraction rings visible at all and if there are, are they symmetrical about the centre of the focused star. It may be possible that the seeing was just too poor to see anything, or the collimation is way off, or there are problems with the optical surfaces.

Were there ever good images seen with this scope?

[michael.h.f.wilkinson]

One problem with larger aperture telescopes is that along with being able to resolve more detail in an object, they also resolve more detail in the turbulent atmosphere. They are more affected by rough seeing conditions than smaller telescopes. I am geussing here but I expect your Dob is f/5 or faster. This isn't going to give you as much contrast as the 4se at f/10. Assuming the mirror coatings and figure are in good knick, I doubt there is anything wrong with your Dob, it is just not the right tool for looking at planets. Though for deep sky objects like galaxies and nebulae, it should eat the 4se alive.

That penultimate bit is not quite right. A slower Newtonian is only better at planets than a faster one because of the smaller central obstruction of fast newts, leading to more contrast. An SCT, or even a Mak (the 4SE is the latter, AFAIK) has more central obstruction than even a fast Newt. Under good conditions, a well-figured 12" dob should take the 4" Mak to the cleaners on planets (just look at the images by Neil Philips on SGL). The issue of EP quality is particularly relevant, however: fast scopes (dobs or refractors) need good EPs.

[Mr Fiery Jack]

Under good conditions, a well-figured 12" dob should take the 4" Mak to the cleaners on planets

I could agree with Michael more. On those 2 or three nights a year a good Newton will take a Mak or Refractor apart, that's why they call them Apo kilkers