Gear Meshing: Gap Free vs Zero Backlash

[CosmicBuffalo]

I am new here and not quite savvy with accessing older/archived threads, so if this is addressed fairly well elsewhere, please point or link me there.

(background:) I currently observe with a large aperture Dobsonian in a mid-to-high light polluted location and would like to buy a high quality portable set-up mount for astrophotography. I am still in the research phase and I am currently looking at mounts, especially German Equatorial mounts. I want to learn what advantages have a good return on the dollar, since dollars are always limited.

(The question):Can someone link to a good comparison of Gap Free and Zero Backlash gearing?

I know that zero backlash is better, but I'm not familiar with the details of the mechanics.

I am also interested in learning about worms, too.

All advice is welcome. Thank you!

[ollypenrice]

Conventional amateur GEMs usually have two sets of gears per axis, a set of spur gears and then a worm and wheel. Both can be sources of backlash. The EQ8 does away with the primary gears altogether and the ALT-EQ6 and many home modded NEQ6 mounts replace the primary drive with a toothed belt system. Some worms are fixed, some are spring or even magnitically loaded into mesh to reduce backlash. Professional grade worm  wheels are sometimes machined in two halves which are spring loaded apart so they take up any slack in their mesh with the worm. I've only seen this once in an old Zeiss instrument.

There are three alternatives.

Mesu and Gemini make roller (friction) drive mounts. These are 100% backlash free. I have a Mesu and it is extremely accurate and painless, the best mount I've ever used.

Avalon make mounts with tensioned toothed belts running on toothed pulleys like cambelt systems in cars. The PE is low and smooth but they are more elastic than other systems. I have one of these as well and like it.

ASA and some professioanl makers use a literally direct drive system in which the armature of the RA motor is the mount's RA axis and it rotates at the sidereal rate under the control of feedback from very high res encoders. These work once they have been persuaded to work...

There is nothing inherently wrong with worm drives but they do seem to work best on the expensive mounts. I have two non-worm mounts because most of the guiding hassle I've had (and, like most people, I've had a fair bit!) has come from backlash. The antidote is usually to run slightly east heavy to keep the mesh pushing and slightly polar misaligned so you only correct one way in Dec.

The favourite budget mount remains the NEQ6 or, for smaller scopes, the HEQ5. Celestron have similar products too. After that you pays your money and you takes your choice...

Olly

[nmoushon]

With being in the US your choices are going to be a bit different because of brand availability. It mostly depends on your budget and what you are trying to achieve with AP. If you are on a tight budgets the very common starting point is an Orion Sirius/Atlas (same as SW HEQ5/NEQ6) with a small ED refractor usually in the size of about 80mm with a DSLR attached for your imaging source. This is what most start with and will be a good stepping stone into AP. If you have a bit bigger budget then you can add a guiding system into the mix. This would require a guide scope and a guide cam. I use a modded 9x50 finder scope with a QHY5-II mono. I've gotten 15min subs very easily with this set up and whats in my sig and I'm sure I could get longer subs. Also if you plan on imaging in your LP skies make sure to budget for a CLS filter for the DSLR as this really really improves your capabilities. You can also step up to a modded DSLR or a CCD camera if your budget allows it....as you can see a growing trend here budget is everything lol. Now your LP is really really bad this will limit you to narrowband imaging, which we can get into more detail later but is an added expence.

Now if your budget is much bigger there are a wide variety of options for both mounts and scopes and cameras and then the specifics change depending on what you are wanting to image. Galxies and other small objects or do you want to do wide field or planetary/lunar imaging? There are lots of options that require different setups with different price points. Some setups are good for multiple things while other are very strong in only one area. So need a bit more feed back from you then we can really help you narrow it down as to what you should be looking for.

[CosmicBuffalo]

Thanks for the advice and tips.

As for budget, I'm looking at somwhere between $800-$1200. I had considered buying "gently used" and getting an iOptro ieq45 or a similar value mount in that class.

I have a fairly nice DSLR in my Nikon d5200.

Ideally I would like to photograph both planets, nebulae and DSO with the fewest future upgrades. However nebula and faint objects are certainly my goal.

Thanks again for the advice.

[Owmuchonomy]

The mount advice given above is of course excellent. The mention of portable in your original post may put some of the suggestions beyond comfort. I run the HEQ5 pro and have spent 6 months or so reducing the effect of backlash as much as I can by various means. I personally wouldn't claim this mount is particularly portable. It's fine lugging around my garden but I wouldn't want to trek to a dark site with it. Olly has done 3 excellent reviews of the Avalon which is physically lighter and has the belt drive system I believe if you are considering portability.

[StarryEyed]

For details of the mechanics for backlash free worms look at the following link.

http://www.google.com/patents/US20090314114

It explains the approch in principle that applies to many gap and backlash free worms. It is not however a comparison.

Thete are lots of papers at the american institute of mechanical engineers but these are mostly theoretical of acedemic papers.

Patents are always a good source for understanding as are acedemic papers.

Google scholar will nearly always throw up tons of information about stuff that businesses rarely if ever publish.

Regards Kevin.

[nmoushon]

With your budget I would suggest getting a second hand Orion Sirius. Those run about $750-850. (I havent used a ieq45 but have heard some good things about them.)This is a great beginner mount and is what I started with and still using. This will allow you some money left over to buy some extras that you will need to start imaging. You will need a T-adaptor for the DSLR to attach to a telescope. I would suggest a clip-in CLS filter for your specific camera to help limit the LP in your area. Unless you are planning to travel to a dark site every time you want to image. You will either need a laptop to control your camera or a remote timer(this is a much cheaper option).  You havent mentioned a telescope yet but with the mount budget your are limiting yourself to either a small refractor in the 80-120mm range or a fast newtonian of either 6" or 8" max. Not sure if the scope is in a seperate budget or not. I would also suggest finding some books that explain the ins and outs of AP and all that is needed and theres more than just money and equipement. It would be a lot easier to explain it all through a book with good pictures/illustrations than trying to get a description on here. I would highly suggest this first before buying anything. Theres a lot that goes into AP than meets the eye.

[MartinB]

No need to be scared of backlash for long exposure AP.  If significant backlash is present you do need to move the mount a decent amount of time when calibrating so that taking up the backlash in dec only accounts for a small proportion of the movement time.  I try to get a very close PA.  If it is spot on you can turn dec corrections off.  I never quite get to this and after about 10 mins can normally see which way the drift is going.  You then just turn off corrections in that direction if backlash is a problem.   The only major downside to doing this is that you can't "dither" exposures but that's hardly the end of the world. 

[CosmicBuffalo]

Thanks again.

I have stuff to learn for sure.

That was my mount budget. My scope budget is separate, but still finite.