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I need a different mount?


CKemu
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Just been browsing a few online stores, despite my telescope being out of action and undergoing repairs, and I stumbled across this in an advert for a wedge.

Meade Equatorial Wedge 8" LX90

Consists of a standard Meade equatorial wedge, plus an adapter plate to accomodate the Meade LX90 drive base. Allows mounting an LX90 in a polar mode on the scope tripod for long exposure deep space astrophotography (which is otherwise not possible when the scope in its default altazimuth mode). The wedge is not needed for short exposure lunar and planetary photography.

This suprised me, as I've yet to see this "requirement" meantioned in forums etc.

So I do a little wikipedia search and find this snippet:

It has often proved more convenient to build a simple alt-azimuth mount and use a computer to manipulate both axes to track an object, than to build a more mechanically complex equatorial mount that employs only a single motor. When astrophotography is involved, a further motor has to be used to rotate the camera to match the field of view for long exposure photographs.

Is this something I can bypass, or am I really going to have to fork out another small fortune on a wedge to attach on my telescopes tripod in order to do DSO, and other longer exposure astro imaging!
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You can achieve deep sky imaging with an Alt Az mount that's not wedged but you will be limited in exposure time... up to about 2 minutes low in the east/west coming down to about 30 seconds at the zenith and north or south. Field rotation becomes a problem beyond this sort of exposure length. A wedge mounted AltAz mount, in EQ mode, doesn't have the field rotation problem and therefore, if you're tracking/guiding is up to it, the skies the limit in terms of exposure time.

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Okay, well I will probably play with M31/M42 once I get my scope back, and the weather cheers up here in 'sunny' England! Just to see if I can photograph them with enough detail prior to making this investment.

Just out of curiosity, will I need any kind of counter weight with the Eq wedge on my tripod if I do get one, the last thing I want is it tipping over after its just been repaired!

Edited by CKemu
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I always chip into threads at this point to say that I found the wedge route entirely unsatisfactory. I found it a nightmare to align (Observatory only for sure. You couldn't go through it in the field). The scope is hard to balance (you do need a system of sliding counterweights) and the tracking accuracy of many Meade mounts is not up to the focal length involved. Then you need a reducer-flattener and a sensible way of getting focus (aftermarket Crayford). The cost is rising and rising and... will it work when you have stopped spending? For some it does, for some it doesn't. Look at the DS imaging section regularly and see how staggeringly absent all those thousands of fork SCTs are...

I would just say, Don't go there! But please ignore me, I would just feel lousy if I didn't say my bit.

Olly

Edited by ollypenrice
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Not a fan then Olly!

I have never used a fork mount but interesting thread this. At what focal length etc can this 30sec maximum zenith exposure be achieved? I assume a c11 would show more rotation than a c6 all other thimgs being equal.

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From everything I read whilst imaging with my nexstar the focal length has no direct bearing on sub length. Of course it makes the tracking tougher all round.

Certainly right as far as the optics and camera are concerned. Sub length is determined by f ratio. But, as I think we agree, the problem with long focal length is that it needs accurate tracking. That is why so many shy away from it or buy very expensive mounts when they feel up for going after the little galaxies and planetaries. It is a shame because good long FL scopes are not particularly expensive. The mounts to carry them, on the other hand... Ouch.

Olly

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Well the telescope is still under repair, frak knows what went wrong, but after three weeks of actually owning it, not a good start!

What exactly is a c6 and c11, are we talking relative magnitude?

In various videos I've watched, the wedge seems to place the meade SCT's weight fairly central to the mount/tripod, so surely balance isn't that much of an issue?

The biggest issue I have with getting a wedge is that I live in the UK and Meade equipment for the 8" LX90 is like gold dust, no shop I have contacted have one in stock and for that matter the UK distributer doesn't have any either... Heck finding the Orion solar filter to fit my telescope is also a challenge.

Does an after market Crayfound make ALOT of difference, I suspect it might as getting a "respectible" focus on our Moon proves challenging, a challenge the on-telescope focusing knob doesn't seem up to.

Also, just incase I am putting too much strain on the telescope - in terms of balance, would the Meade balance system (weights slung under the scope which are adjustable in terms of position and overall weight) be a useful thing to have, eg worth the £80-90 asking price in the long run.

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I found it a nightmare to align (Observatory only for sure. You couldn't go through it in the field). The scope is hard to balance (you do need a system of sliding counterweights) and the tracking accuracy of many Meade mounts is not up to the focal length involved. Then you need a reducer-flattener and a sensible way of getting focus (aftermarket Crayford). The cost is rising and rising and...

Have you been looking my dome Olly!?!

I'd agree with all of those points -- though they are all solvable.

will it work when you have stopped spending?

Hmmm, sort of. It works, but it's not as good as I'd like... images definitely limited by the telescope and not the site...

For CKemu -- I'd recommend you try a little bit with it in alt-az mode and see how you get on. As you get a bit more experience, you'll probably form a better idea of what it is you want to achieve long term, and what's the best way to get there. To be honest, unless you're doing H-alpha or other narrowband work, having an ~2 minute limit on subs isn't actually that big a deal (unless you live somewhere really dark).

At what focal length etc can this 30sec maximum zenith exposure be achieved? I assume a c11 would show more rotation than a c6 all other thimgs being equal.

C6 and C11 are telescopes (Celestron 6-inch SCT and 11-inch SCT; similar to the Meades). The limit on exposure time before field rotation becomes a problem is actually a function of field of view, rather than focal length. If you measure it in fractions of a pixel (i.e. "I can tolerate a field rotation of 2 pixels before it becomes too much"), then the time limit is independent of focal length (given the same camera), because field of view decreases as focal length increases. Bit counter-intuitive, but true I believe (took me a while to convince myself of this) Edited by FraserClarke
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Well the telescope is still under repair, frak knows what went wrong, but after three weeks of actually owning it, not a good start!

What exactly is a c6 and c11, are we talking relative magnitude?

In various videos I've watched, the wedge seems to place the meade SCT's weight fairly central to the mount/tripod, so surely balance isn't that much of an issue?

The biggest issue I have with getting a wedge is that I live in the UK and Meade equipment for the 8" LX90 is like gold dust, no shop I have contacted have one in stock and for that matter the UK distributer doesn't have any either... Heck finding the Orion solar filter to fit my telescope is also a challenge.

Does an after market Crayfound make ALOT of difference, I suspect it might as getting a "respectible" focus on our Moon proves challenging, a challenge the on-telescope focusing knob doesn't seem up to.

Also, just incase I am putting too much strain on the telescope - in terms of balance, would the Meade balance system (weights slung under the scope which are adjustable in terms of position and overall weight) be a useful thing to have, eg worth the £80-90 asking price in the long run.

Balance is INCREDIBLY important, alas. The tiniest adjustment, even on premium mounts, makes an astonishing difference to tracking accuracy. You have to remember that imagers are working right at the limit of what is possible within 'thinkable' budgets. I am looking for an average tracking error of 0.05 pixel and beyond 0.1 I go to bed in a huff! Now pixels are not very big. (I guide with an FL of 400mm on both mounts.)

balance is mainly critical in that it controls backlash in the gears. The general wisdom is to run a little heavy on the east side so the mount is always 'pushing' and the worm meshes always on the same side. Maybe also run camera end heavy for the same reason.

The killer with forks mounts is getting the dynamic balance right so that the weighting to the east does not vary or reverse during the run.

Focus is fiendishly important. It not only brings sharpness (resolution) but reduces noise. Dspite using computer aid I say to myself and guests that it is worth spending 10 minutes on focus even with premium Feathertouch and Takahashi focus mechanisms.

The moving primary method is awful, full stop. It is hard to know when the mirror has stopped moving, it may continue to move after you have stopped turning the knob, etc. On small webcam chips at extended focal length the planet skips off the chip when you make a small adjustment. Mercifully the budget Revelation Crayford is not bad at all but the Moonlite is much better and moves the focal reducer as it focusses so the critical reducer to chip distance is preserved. You can make things better by always making the last turn of the knob a 'push' - ie pushng the mirror up the tube with the mechanism so as to take up play.

Meade don't tell you all this, do they. Well, they ceertainly didn't tell me.

Olly

PS Please excuse typos. French keyboard. Grrr.

Edited by ollypenrice
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One thing to remember is that unless you autoguide then an equatorial mount (or wedge) is not going to increase your maximum exposure time that much over what you can do in alt-az, due to tracking errors inherent in sub-loads-of-money mounts (OK, so this depends on focal length, but we are talking a Meade LX90 here).

NigelM

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I went through this thinking process.

Crappily made* Celestron Wedge + Upgrade kit = £430

Or go for a super-duper Orbit wedge.....£750

You would then end up with a compromise. Plus you are limited to using the SCT as nothing else would fit. And you have all the disadvantages of SCT for DSO (long focal length, slow f speed, need for pinpoint accurate tracking).

On the other hand...an EQ6 Syntrek is £786. You will get a "better" mount. You can mount different telescopes. it will (potentially) outlive the OTA. And you can opt for a small fast refractor.

I guess that's why I am shelling out for an EQ6. To invest nearly the same amount in a wedge and have to put up with a compromise would end up annoying me. Better to keep the fork mounted SCT for visual and planetary imaging. Plus, it stays grab-and-go, which a wedge-mounted SCT certainly wouldn't be.

All IMHO, of course. Your mileage may vary.

*I haven't seen one in the flesh, but the picture don't look great...lots of room for flexing.

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Im really surprised that you can use an unwedged alt-az for imaging deepsky, i never tried it as i didnt think it would produce any decent results. If it works will need to give it a go! Does anyone have any results of photos taken with the lx90 in alt-az mode?

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@ Olly: Thankyou for the superb feedback there, getting my hands on a Revelation Crayford and some weights. I shall keep your feedback in mind the next time I set up the scope.

Going to stick to Alt Az for awhile, mostly to try out DSOs before spending a fortune on what is effectively a lump of metal with a few knobs attached! (The wedge)

For anything more distant than..well our Moon, I tend to autoguide, though after the dec motor failed, I am without telescope, yet alone autoguiding!

Is it me, or does Meade have a "not-so-good" reputation with tracking from most peoples opinions?

I can't afford to really change the telescope I've invested in, so for better or for worse, I'm going to make it work!

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I quite understand and don't get me wrong, people do get them to work.

Fork SCTs are generally considered to be poor on tracking at the level of deep sky imaging. They are brilliant visually and for webcamming the planets. If I were going to use a large SCT for DS imaging I would first buy a mount costing much more than the entire fork mounted SCT. That is the problem. Making mounts for long focal length imaging is very difficult and very expensive.

Olly

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