Clearance Issue

[Back40]

Howdy. I have a Celestron 8se and installed a 2" diagonal. If I get close to Zenith the diagonal will hit the base. I am thinking about purchasing an eq wedge. Will the wedge change the position of the ota enough to clear the base? I would greatly appreciate your help before I purchase the wedge. Thanks all 

[johninderby]

Simply move the OTA forward a bit to gain clearance. A wedge fits underneath the mount so has no effect on clearance.

Edited May 31, 2020 by johninderby

[Back40]

I have the ota as far forward as it will go. 

[johninderby]

Then it looks like the diagonal is the problem. You need to use an SCT fitting 2” diagonal like the  William Optics SCT diagonal or similar. should give more clearance. 

https://www.rothervalleyoptics.co.uk/william-optics-dura-bright-2-dielectric-sct-diagonal.html

Edited May 31, 2020 by johninderby

[Cosmic Geoff]

Yes, it does look from the photo that the tube is as far forward as it will go.

What are you planning to do with the wedge? The C8 SE is a visual scope outfit, and putting it on a wedge will not transform the mount into a precision deep-sky imaging instrument. Anything else, e.g. planetary imaging, can be attempted with the alt-az GoTo mount. If deep-sky imaging with long exposures is your aim, it would be less frustrating in the long run to put the cost of the wedge toward buying a proper equatorial mount.

[SimM]

You might find this review of the Celestron  2” XLT Diagonal. Or search for Slymin and XLT on YouTube:

https://youtu.be/twfgNDf1Qoo

It’s worth looking at the whole thing because it shows the same SCT on an equatorial Mount. He then puts the diagonal on the Alt/Az and to see the “problem” and the ”solution” look at 6:15 thru’ 7:30.

Notice that there is also clearance to the focuser and your face is kept away from the SCT. The clearance with this diagonal is 1cm when looking at the zenith.

There is also also a few words about why would you fit a guide port on an Alt/Az Mount. The Evolution scopes have a well engineered (and expensive) wedge that also fits the 8SE. But why?

However, just because you can, doesn’t mean you should. No one telescope or mount does it all. On the mount with the 8SE you couldn’t fit a bigger scope and you couldn’t use a Refractor whereas with an Equatorial all these things are possible.

The wedge doesn’t make it perfect, only just about useable. Alt/Az has a simplicity, but an Equatorial with GOTO isn’t far behind and you don’t suffer field rotation (the 500’s rule). AVX Mount is about twice the cost of the wedge but significantly better and certainly good enough for visual use.

There’s a second “follow on” review by Slymin on the 8SE. Worth a look because unlike many reviews (which are adverts for stuff they got free) he does say what’s good or bad after years of owning it.

One thing that’s not mentioned is any play in the gears. When any mount reverses the direction, it takes some time to switch direction. When an Equatorial Mount is balanced, the RA is always driven so any slight play or backlash is always taken up. If you were guiding, then movements in Decl. are +/- to make corrections compared to + or stop or 2x plus etc. in RA. With Alt/Az the Mount is never stationary in both directions e.g. always stepping up and across. So in both axis it’s like RA on Equatorial and the tolerances on the Alt/Az do not need to be perfect.

Adding a wedge to an Alt/Az probably exposes you to all these issues and for AP all these things become more critical. So using the wedge (£400) isn’t a good idea IMHO. Also, not all Equatorial Mounts are designed for AP, some are just good enough for visual.

So like all purchases, you have to know your eventual goal to not make too many costly mistakes along the way.

Simon

Edited June 1, 2020 by SimM

[michael8554]

Whichever solution you opt for, with the wedge you will reach the zenith in all compass directions - I just tried it (51 degrees north).

For visual you don't need anything fancy - plenty of online designs for fixed angle wooden wedges.

Michael

[SimM]

Reaching the Zenith is OK. There is in front and behind the EW line too.

How will you be PA on the wedge and “travelling” back along the Meridian to face due South?

With a wedge you now have to rotate the diagonal as you move in RA to keep the EP   “up” without hitting the single arm.

Simon

[Cosmic Geoff]

1 hour ago, michael8554 said:

For visual you don't need anything fancy - plenty of online designs for fixed angle wooden wedges.

For visual use I do not see that a wedge serves any useful purpose.  And it will complicate the alignment process compared with an alt-az GoTo.

Edited June 1, 2020 by Cosmic Geoff

[Physopto]

Wedges are a problem waiting to bite you in the rear end. On top of this they make the whole mount very shaky because of the extra weight on the already poor legs. At least that is what I found with my Celestron 5SE. Mind you it was a wedge built in to the mount. But I also had a wedge for my LX200  12" Meade and was never happy with that either. Then there is alignment to mess with.

Nice scope but legs a problem!

Derek