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Why an EQ?

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#26 groz


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Posted 22 February 2009 - 01:27 PM

Ok, lots of info, let me add my _first hand_ experience. First, if you want to image, and do long exposures, some sort of polar aligned mount is essential, be-it a fork on a wedge, some variant of a split ring, a dob on a platform, or something else mounted on an eq mount. The eq mount is by far the most cost effective way to get a polar aligned mount, simply because they are mass produced, and you benefit from the economies of production scale.

Now, on the issue of 'perfect polar alignment'. We read a lot about it in various imaging forums, but, there's a huge discrepancy in what folks consider 'perfect' and what is 'good enough', and what is intolerable. If you are shooting 30 to 60 minute exposures on film, then, 'good enough' is within a couple arcseconds of 'dead on'. On the other hand, if you are shooting 1 minute exposures using a large pixel camera on a short focal length telescope, then 'good enough' is probably within 15 arcminutes of the true ncp. For the film guys, this level of error on the polar access is unacceptable.

I'm shooting with a canon 350, attached to an 8 inch SCT at prime focus, which results in a pixel scale on the order of 0.62 arcseconds per pixel. I polar align my EQ6 by 'lookiing thru the polar scope', takes me about 2 minutes usually, I have a good view of polaris from here. My polar scope is aligned pretty good, and, with my setup, that alignment is 'good enough' to do 5 minute exposures. I guide my system using a QHY5 in an ST80. The guiding is NOT to correct for polar alignment, but, to correct for periodic error in the worm drive system. I'm shooting at a long focal length, with a small pixel camera, so, periodic error is a big source of smear on my shots, with 11 arcseconds of periodic error, that translates to 18 pixels of smear over a full worm rotation, so guiding out that error is essential. A lot of folks starting out make the mistake of thinking the guider is a 'quick fix' for polar alignment errors, but it's not. At the same time, if you are going to shoot at a pixel resolution much smaller than the PE of the mount you purchase, you will need to guide it on longer exposures. No amount of fussing with setup, balancing, and all the other stuff we read about, can fix the PE on a mass produced mount. Sure, there are folks out there that will charge you a big chunk of money to 'hypertune' it, but, even if you do that, plan on spending a few bucks on a guider system to work PE out of your mount eventually. A $400 guide system sitting atop a $1000 mount will make that mount track as good as, or better than, the $12,000 mount. Been there, done that, got that t-shirt here to prove it.

By all accounts, my setup would be considered 'demanding' on the polar alignment, and, when I set the mount up the first time, it was a bit overwhelming to get it all set. I've had a bit of practise since then, and, today, it takes me about 5 minutes from 'turn on the power' to 'polar aligned good enough'. Here's what I do. Power up the system with the gps plugged into the hand controller, then, head off to refill the coffee. From a cold start, the gps usually takes anywhere from 1 to 3 minutes to get 'locked in'. Once the gps has a fix, hit enter on the hand paddle a couple of times, and it tells me the correct position for polaris in the polar scope. Open up the polar scope, take a peek, it's usually very close (I set up in the same location every time), make some minor adjustments, then cap off the polar scope again. Normally takes about a minute to do the adjustments.

BUT, on your first time out with the eq, trying to align in the polar scope, will be VERY frustrating. Just getting polaris into the field of view, and recognizing it is indeed polaris, well, that was a huge obstacle for us early on. _If_ I had a few years of 'using a finderscope' under my belt, it probably would have been a non issue.

Some of the mounts have a 'helper' routine for polar alignment in the hand controller, run an alignment, hc calculates the polar alignment error, then gives some indication of how to adjust the mount to remove that error. That's useful, but, I wouldn't make the decision on which mount over that feature alone, simply because I know for a fact that the feature does not exist in my hc firmware today, but it will in the next 3 months. I've been told this by the person responsible for the firmware development on that system. Even so, we have no problem getting polar alignment 'good enough' without it, but I am looking forward to an easier way to do more accurate alignment.

But, the bottom line of my experience with all this, getting the eq mount polar aligned 'good enough' for my photography setup was a daunting task 2 years ago. Today, it's just 5 minutes of the setup process, not difficult at all. I dont normally fuss with drift align, because my polar scope gets me 'close enough' it's not necessary. I will say one more thing about the polar scope, we have 3 mounts, 2 have the 'illuminated' polar scope (EQ6-Pro and HEQ5-Pro), while the third (NEQ3) is not illuminated with the little red led. The lack of illumination on the neq3 makes the polar scope borderline useless in the dark, you cannot see the reticle, and end up fussing with red flashlights to try get a view of the reticle for polars placement. So, in that respect, my opinion says, there are two types of polar scopes in eq mounts, those that are illuminated, and, those that are useless. Your mileage may vary some, but, that's my opinion, non illuminated polar scope is only _marginally_ more useful than 'no polar scope'. And that's based on my experience with the 3 we have.

The other issue you seem perplexed about, is balance. Again, for balance, I have probably the 'worst case' setup, 8 inch sct mounted side by side with the ST80 used to guide it, in a dual saddle adapter. I end up balancing by moving counterwieghts in one plane, sliding the dual saddle adapter in another plane, and shifting telescopes forward and aft in the third plane. It's a process that takes about two minutes, ie, not at all difficult. But, eyepiece location comes hand in hand with 'complex balancing'. If you look at 1 or 2 arm fork systems, they achieve balance by using offsets on the fork arms and placing the scope at the center of gravity for the system as a whole. Point my balanced system 'strait up', and i do get an awkward eyepiece position, but, at least I can look in that direction. One or two arm forks, cannot point strait up with a refractor, the eyepiece end bumps into the supporting tripod. Now, use an alt/az that has an offset mount, and you can indeed point strait up, but, you get the awkward eyepiece position, and you either get counterwieghts to deal with, or, a system that addresses balance by 'ignoring it', ie just let the drive motors deal with the added load, there are no counterwieghts sticking out the other side to balance the scope. The ONLY reason fork systems dont have a reputation for 'awkwark eyepiece position' is that they cant point in the directions that give an awkward eyepiece position. When your refractor is pointed 'strait up', it doesn't really matter what kind of mount is holding it there, the eyepiece will still be 'strait down'. This is not a function of the type of mount holding the refractor, it's a function of 'where can my mount actually point'. There's a bunch of them that cannot point strait up.

So, after this long tirade, this much I can say, all from first hand expereience. If you want to look thru an eyepiece, then the alt/az mount is simple, and good. The fork arm variety means you wont be looking 'strait up' anytime soon. But, if taking photos is in your future, some sort of polar aligned mount is also in your future, the eq is the most common, and, the learning curve on it is not nearly as bad as one thinks after reading endless horror stories here on CN. Chris and I can start from scratch today, mount and telescopes all in cases just pulled out of the van at a remote location, assuming polaris is visible in the twilight, we can have both kits assembled, balanced, and polar aligned in roughly 10 minutes, with part of that time being 'wait for her to be finished with the gps'.

I will also admit, after we are set up and polar aligned, we spend another 10 minutes fussing with cables to get all the stuff needed for a photography session attached, powered, and running. But again, we have the 'worst case' scenario to deal with. Both kits have an eq mount supporting two scopes, one for shooting, one for guiding. Both use a dslr, so there's a cable for the camera, and a cable for the shutter release. There's another cable from the guide camera to the computer, and yet another one from the guide camera to the mount. We use motor focus, yet more cables. We live in a humid area, so, next we attach dew straps to both tubes. But even with all this added extra stuff, we can _still_ go from 'pile of things in cases and boxes' to 'full setup with side by side astrophotography kit', fully balanced, aligned, and ready to shoot, in 15 minutes if we are rushing, a little longer if we are taking our time and socializing with other folks along the way. There are lots of folks that confuse our 'fussing with cables', and consider it 'fussing with an eq setup', but that's not the case. It's fussing with our camera gear, we have a LOT of it attached to these systems.

The final bottom line, an eq mount is no more difficult than an alt/az mount. Think it thru, an alt/az mount is indeed, the same thing, except its very poorly polar aligned, somebody confused 'strait up' with 'the north celestial pole' when designing an alt/as mount. Later somebody else realized the error, and figured out that a wedge can fix the alignment problem. The real difference is, how flexible are they. Typical EQ mounts can point anywhere in the sky, and typical alt/az mounts are restricted in where they can point, and that is the real difference between them.

#27 BobH



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Posted 22 February 2009 - 02:58 PM

Just as a follow up, I did pick up a non-goto, entry level GEM.

After all my agonizing, I was amazed at how easy it is to polar align. I take pains to level the mount and sometimes use a compass to get roughly pointed toward polaris before dark. Nine time out of ten, once ole Polaris is visable, it's in the polar scope field of view. Takes about a minute after that to get it "semi-accurately" aligned.

It's great to know that if you find an object, then walk away for 15 or 20 minutes, when you come back to the scope all you have to do is turn the RA knob and, Bingo!, there's the object. Can't do that with an alt-az.

For a goto, I'm still debating, but probably going to go with an alt-az. For manual, I'm convinced GEM is the way to go.



#28 astrokido


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Posted 22 February 2009 - 03:27 PM

There's anoher advantage an EQ mount gives you in manual mode that no other mount does. Once it's polar aligned, the RA and DEC knobs will follow gridlines on charts with an equatorial grid. This makes it easy to get to any point in the sky. It helps to align with a recognized star first, then go to another object by knowing the difference in RA/DE to it.

Knowing the gear tooth counts for each axis, you can also use the turn of each knob for measuring arc movements. That why I love the 144/72 RA/DE teeth of my old Vixen Polaris, which provide 2.5° and 5.0° per turn of each knob respectively. That's usually good enough to get an object into a wide FOV, then narrow in with higher power EPs. It's simpler than it sounds, and gets really easy with an RA tracking motor.

#29 BobH



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Posted 22 February 2009 - 05:19 PM

As a new "star hopper" I forgot to mention that advantage. Yes sir, turning a knob and having the scope follow the grid lines is a huge help. Your advice about calibrating the turn of a knob with a certain arc movement is something that I never thought of. That's another big help.

Thanks! :bow:

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