Wedge plate not quite level....

[Astroboffin]

Hi all,

I have just acquired a good secondhand Meade wedge from a guy on ABS, I have fitted it to my Meade tripod and got the base perfectly level, by use of the built in bubble level, but I have noticed that even though the base of the wedge is absolutely spot on level, the angled plate that the scope sits on is very slightly out of level one side to the other, around 1mm out.

So my question is, how bigger difference this will make when imaging, a lot or none?

I can't get my head around whether it matters or not, obviously I can PA by making adjustments, but because of the slightly out of level plate, will it cause my guiding to be making alot more corrections?

I know I can alter the tripod to get this angled plate level, but then the wedge base will be slightly out.

Also I do autoguide, so how will it affect that?

Help greatly appreciated

Regards

AB

[Pan Euro]

It won't make any difference. I have never used the bubble on the tripod I could never get it correct. I used a small 6 inch spirit level to set it up. As for 1mm not worth worrying about the scope setup will take care of that.

[Astroboffin]

It won't make any difference. I have never used the bubble on the tripod I could never get it correct. I used a small 6 inch spirit level to set it up. As for 1mm not worth worrying about the scope setup will take care of that.

So even though the tilt plate is not level, it won't matter?.

I thought it needed to be perfectly level for tracking to be accurate, it is the angles tilt plate I mean and not the base plate where the bubble level is....

Regards

AB

[Pan Euro]

Sorry wrong end of stick. I'm not sure why it should be 1mm out any chance of a picture of the wedge?

[Astroboffin]

If you look at the wedge straight on, the tilt plate where the scope bolts too, is 1mm lower on the right side, it is obviously a slight variation in the fastening points.

So to compensate the dec and east west has to be altered slightly to get good PA.

but my concern is the tracking and how it will affect this.

Hope that makes sense, I will try and put an image together to show what I mean

Regards

AB

[michael8554]

As I understand it, for GoTo alignment the scope does not need to be level, the aligment process calculates the error in level - but it does help to start off level.

For good tracking, you drift align, and adjust the wedge angle and azimuth at two points, pointing south, and pointing east or west.

Note there is no mention of how level the scope is to start with in that statement.

Lets say you drift align and then go back to pointing at a southern star at high Dec. Then swing down in Dec to another star at the same RA according to a star chart. 

If your scope is tilted as you describe I don't see how that star will be in your eyepiece.

And I believe that error will also be apparent in your RA tracking at other points in the sky to the two points used for drift alignment.

So I think you should adjust your tripod to make your wedge horizontal in an east/west direction when pointed at the NCP.

Michael

[Peter Drew]

I would slot one of the holes that support the top plate to make it level. 

[Astroboffin]

I would slot one of the holes that support the top plate to make it level. 

Yes that would work, I must admit I had thought of that, but as there is a fixed swivel point at the top, and the bottomed where I would need to slot the hole is where the Dec sliding scale is, so there is already a very long slot !!

the problem I think is that the actual plate is very slightly out of line, so I think the best way forward is either to pack under one side of the base plate between tripod top and wedge, or alter the legs on the tripod to get the tilt plate level as Michael suggested above.

Thanks for the reply.

Regards

AB

[michael8554]

The tripod top and wedge mating surfaces need to be as flat and smooth as possible to aid the Az adjustment, so I wouldn't put anything there.

The problem with Meade wedges is that you make an adjustment, and then it changes when you tighten the knobs.

So widening the wedge slot on one side is okay until you tighten the lock knob, then there is even more room for the plate to wander to.

But I appreciate the difficulty of adjusting your tripod legs to remove a 1mm difference.

Michael

[Peter Drew]

As mentioned earlier in this thread, the base plate can be tilted to correct the top plate. But if a tilted base plate does not matter then it seems to me that a tilted top plate would also not matter, so best left alone. If possible, a picture might help alternative suggestions. 

[Astroboffin]

As mentioned earlier in this thread, the base plate can be tilted to correct the top plate. But if a tilted base plate does not matter then it seems to me that a tilted top plate would also not matter, so best left alone. If possible, a picture might help alternative suggestions. 

I don't think,it matters about the base plate being slightly off, it only matters where the base of the scope sits, on the tilt plate, that is the bit that needs to be in perfect alignment, as that is where the scope will rotate from....

Well that is the way my logic sees it anyway...

I will try and get a picture together today to show exactly what I mean

Regards

AB

[ngwillym]

As long as your polar axis is pointing at the north celestial pole - it doesn't matter one jot what angle the tripod or any other part is level or not. Everything rotates about the polar axis, so as long as that is pointing in the right direction its actually irrelevant as to what is holding it in that position.

[Astroboffin]

As long as your polar axis is pointing at the north celestial pole - it doesn't matter one jot what angle the tripod or any other part is level or not. Everything rotates about the polar axis, so as long as that is pointing in the right direction its actually irrelevant as to what is holding it in that position.

Well, that is one thought I kept having, but wanted somebody to clarify, I kept telling myself that it could not be that easy, but you make a lot of sense, because if the tilt plate is out, which mine is, then the way I see it, I would have to alter the dec and east west adjustment slightly to compensate to get the scope pointing at the true pole, once this is done then all should be good.

The thing that kept coming into my head was that when tracking would the extra adjustment I had to make, alter the tracking accuracy...

Because the tilt plate would not be totally parallel to the pole due to the slight mis alignment.....

It's all getting complicated, but thanks for that reply, you do make sense to me

Regards

AB

[Davey-T]

While being level doesn't matter to an EQ mount on a fork / wedge mount the East West level of the the wedge needs to be level as the fork rotates around this point, so while it may be PA  in one place it won't be when rotated.

If you aim the forks at Polaris and centre it in an eyepiece with a FOV to match the circle of Polaris you can spin the mount in RA to see if Polaris goes round in a circle in the FOV.

I spent a lot of time messing about with my permanently LX200 to get it spot on in E / W, this is only relevant to long exposure imaging which I found impossible mounted on the tripod.

Dave

[Astroboffin]

While being level doesn't matter to an EQ mount on a fork / wedge mount the East West level of the the wedge needs to be level as the fork rotates around this point, so while it may be PA  in one place it won't be when rotated.

If you aim the forks at Polaris and centre it in an eyepiece with a FOV to match the circle of Polaris you can spin the mount in RA to see if Polaris goes round in a circle in the FOV.

I spent a lot of time messing about with my permanently LX200 to get it spot on in E / W, this is only relevant to long exposure imaging which I found impossible mounted on the tripod.

Dave

The east west of the wedge base plate is spot on level, but the plate that the scopes sits on is 1mm out from level one side to the other, so if I pack under one side of the base plate this will level up the tilt plate, would this be a way around my problem, I am guessing the wedge must have been slightly out of line from when it was made.

Or have I got a dud wedge??

Regards

AB

[Davey-T]

Meade wedges are not renowned for their engineering excellence which is why other manufacturers make them.

1mm may not seem a lot but as this is presumably over a distance around 300mm then it is quite significant.

If you loosen the four screws holding the base plate to the side plate you can put a thin aluminium shim in to level it.

The built  bubble level is not particularly useful.

As I said all of this is only relevant for longer than about 10 min's photography.

Dave

[Astroboffin]

Meade wedges are not renowned for their engineering excellence which is why other manufacturers make them.

1mm may not seem a lot but as this is presumably over a distance around 300mm then it is quite significant.

If you loosen the four screws holding the base plate to the side plate you can put a thin aluminium shim in to level it.

The built bubble level is not particularly useful.

As I said all of this is only relevant for longer than about 10 min's photography.

Dave

Hi,

Thanks for that advice

I have the tilt plate off ATM where would I put the shims?,there is nowhere to put shims that would lift it ant all, the top two bolt holes are just slightly over size to allow bolts through, and the bottom two are the long arced slot with degree scale on, what I have noticed is a gap on both sides between tilt plate and the side walls of the wedge, so when it is tightens it actually pulls the side walls in very slightly, is this normal or should there be washers in there to fill this gap, which is I would say about 1/2mm on each side top and bottom, so 1mm overall.

I can't see what is actually causing the out of level tilt plate, could it be the top two pivot holes, one slightly lower that the other??

Regards

And sorry for all the questions, I just want to get it right, and then it's done for good.....hopefully

Regards

AB

[Davey-T]

I was thinking something like this, don't think I would open up any holes.

Dave

 

[Astroboffin]

I was thinking something like this, don't think I would open up any holes.

Dave

Meade-wedge.png

Meade-wedge-2.png

hi,

yes your wedge is a bit different from mine, mine is the Meade standard wedge, i dont have those holes.

but i could pack under the one side of the base plate, that would work wouldnt it?

so you don't think  it is a good idea to maybe elongate one of the holes at the top of the wedge, to allow it to level itself under weight?

regards

AB

[Davey-T]

Does it screw to the tripod with three bolts or just a centre bolt ? if it's got three bolts you could try sticking some penny washers under the appropriate bolt.

Dave

[Astroboffin]

Does it screw to the tripod with three bolts or just a centre bolt ? if it's got three bolts you could try sticking some penny washers under the appropriate bolt.

Dave

Yes it has the three bolts, and a centre bolt, but only really need to use the three

Regards

AB

[michael8554]

I've just read ngwillym's post #12, and having digested that I have to retract my post #6.

I can now see that because of the geometries involved (tripod top is level, tilt-plate slightly not level), a small adjustment in Az will bring the fork arms around to point directly at the NCP.

In other words, the error will be removed during drift alignment.

So no packing required.

Michael

[Astroboffin]

I've just read ngwillym's post #12, and having digested that I have to retract my post #6.

I can now see that because of the geometries involved (tripod top is level, tilt-plate slightly not level), a small adjustment in Az will bring the fork arms around to point directly at the NCP.

In other words, the error will be removed during drift alignment.

So no packing required.

Michael

Now this is what I want to believe, and logic tells me too, but I just and get my head around it, I totally agree that if the tilt plate is slightly out, then an Az adjustment will bring it back, but what about during tracking, because of the slight misalignment won't the tracking slowly drift out.....this is what is confusing me.

Logic tells me that if it is pointing at exactly the right place to start with, then that's all that matters......!, it doesn't matter how out of align anything is, as long as it is firm and pointing at the NCP.

What it will mean is that my wedge set up in Dec and east west would show different numbers from someone with exactly the same set up, and all in perfect alignment, in say the garden next door, but so what.....

I need to get it right because I will be doing long exposure DSO imaging.

Thanks for all the points of view, appreciate it

AB

[Davey-T]

Speaking from experience although logic would seem to indicate otherwise I found that the wedge needed to be level in the East / West direction especially for long exposure. ie 30mins NB imaging.

What appeared to happen was although it guided OK  and stars looked round as individual frames downloaded the image would gradually move out of register with the next one, this is obvious when using something like Artemis Capture as you can see the image jump as it downloads.

Checking PA with PHD showed no drift over 20mins, so although images looked OK something was making them "jump" could just be mechanical faults in the mount but if you can get the wedge level at least if you have any problems you will know it's not the wedge causing it.

Long exposure imaging ( and you need as long as you can manage for narrow band imaging ) is hard enough at long focal lengths without introducing any unnecessary problems.

As previously mentioned if you aim the forks at Polaris and spin the mount in RA Polaris should rotate in the FOV depending on the eyepiece to do this the wedge needs to be level as no amount of twiddling with the adjusters will correct it.

Dave