Help Me Understand a Wedge

[rabbithutch]

I'm having a great deal of trouble visualizing how a wedge works. Maybe, I also need a better understanding of how latitude is involved, too.

As I understand it, a wedge is a compensator to make the center axis of your scope parallel with the polar axis of Earth. This allows the field of view in your scope to revolve around the polar axis instead of its own axis WRT a line drawn to the center of Earth.

If this is incorrect, all else probably is as well.

I live at approximately 31° 6" North latitude. As I understand it, this is measured from the equator; so if I moved to Oxford, England, the measure would increase numerically. I've been reading this thread - http://stargazerslounge.com/diy-astronomer/67614-how-equatorialize-dobsonian.html - and trying to understand what the angle of the base WRT the horizon he built the wedge plane.

If EA2007 uses his dob at 54° N, my mind wants to make the compensatory angle 36° in the wedge. By extension, my location at 32° N, my wedge plane angle would have to be 58°. Clearly this would not work for me because it would be too top heavy.

I'm trying to figure out whether I can do a similar EQ adaptation on a dob and possibly automate RA tracking and use the scope for AP. I just can't seem to wrap my mind around how paralleling the earth's polar axis gives the same orientation to the sky.

Clearly, I do not understand this subject. If someone could point me to a web page that would help me - or would like to take on the challenge - I will be very grateful.

[pvaz]

If you stood exactly on the north pole then you would point straight up.

If you travel for 1º straight south, you will be in paralellel 89º and you need to compensate for 1º.

If you travel for 2º straight south, you will be in paralellel 88º and you need to compensate for 2º, and so on.

Maybe you are not visualizing how the EQ works. Try watching this:

Edit: Link update. Found a better video, with a wedge, It explains the whole aligning process.

[rabbithutch]

Hello Paulo!

Thanks for responding!

Your case of stepping away from the pole makes sense to me assuming that you are keeping the central axis of your scope on the same point in the sky and starting with it pointed straight up at the pole. In order to maintain the center of the scope axis from where I sit I would have to compensate about 58°. That assumes that the scope remains at 90°, right?

If I'm visualizing that correctly, I would set a wedge beneath the base of the dob with the hinged side aligned toward the polar axis. The base would then pivot on that hinge rising to an angle of 58°. It seems to me that the center of gravity of the scope would make it impossible to make such a wedge for use in my location.

??

[pvaz]

Yeah I suspect it will be a problem.

It also forces you to align the scope with the pole which won't be as practical as putting it on the ground and just point it wherever you want. It's a trade off between the simplicity of using an Alt-Az mount which no special setup, for a bit easier way to track objects.

If your goal is to use the equatorial coordinate system to find objects, then I would suggest getting an equatorial mount instead of a Dobson mount.

[rabbithutch]

I see your point, but finances are going to limit me. I'm leaning toward a dobsonian because of the lower entry cost for an 8" aperture and because of the simplicity. The big trade off is that without tracking, a dob won't stretch to AP if I ever feel that urge (or at least that is what I understand from what little I've learned so far).

Ideally, I would be able to purchase into a middle of the range of apertures (I consider the 8" to be that) and get a platform that I could grow as my knowledge and interests grow while not having to reinvest multiple times, losing value in past purchases. I might be the only American around who drives his vehicles for 20 or more years, wearing them out before buying new ones. That admission should give you some idea of my idea of values vs. vogue.

[pvaz]

Well a dobsonian is a newtonian scope on a dobson mount. You can, at a later time get an equatorial mount and use the newtonian scope on it. That way you'll only have spent extra money on the dobson mount which is very cheap.

I understand your perspective too well as I also want to get into astrophotography at some point. This is the cheapest solution available to me. Still it's not exactly cheap. For a decent mount capable enough to carry an 8" most experienced people recommend at least an HEQ5 mount for decent results. Those go for around 930€ here, including motors and goto. Then I need a powerpack, a modified SLR and a few other bits to make things click.

Tracking is also essential. Most astrophotographers here use a more complex setup with 2 scopes on the mount, one haves a cheaper camera connected to the PC that analyses the image and accurately instructs the mount compensating for tracking errors. Then the other scope haves a camera that actually takes a huge amount of pics through the night. After collecting the data you need several hours with 2 or 3 different softwares combining and tunning those pics to make a single final image.

I been studying my options and I think it's better for me to just get 2 separate setups. I'll get an HEQ5 and a refractor, probably the 80ED, as it's the cheap and I see people getting excellent results with it. With a refractor I'll avoid the collimation hassles. I'll start unguided (1 scope only) taking shorter exposures. While the camera shoots through the night I can use that time to observe with the dob.

[rabbithutch]

Good thinking, Paulo!

I suspect that if my interest continues past casual viewing with the dob that I hope to get, that I, too, will get a refractor and EQ mount. I was just trying to think through whether I could get my feet wet in AP with just the dob. The fork mounted Mak-Cass with EQ look good too. I wonder about that arrangement over a refractor?

[vlebo]

[pvaz]

LOL to vlebo

As far as I know it can be very hard to get good enough tracking for long exposure with a fork on a wedge. Or so I read as my astro photography experience equals to none.

So then you are limited to shorter exposures, which forces you to take many more combined hours of individual pics (the pros seam to call this "subs" ), to get the same results you can achieve with longer exposure subs. I'm also not sure if there is software capable of autoguiding with a fork/wedge, if you want to evolve to that at some point.

[OzDave]

One thing you might just get away with on a dob is afocal coupling to take a photo. Basically this is holding your camera up to the eyepiece and taking a snapshot. You can get kit to hold the camera in place for you.

This will only really work for short exposures though. But should make for some decent moon shots.

David

[rabbithutch]

vlebo,

Got your post but don't see it here. We call that a 'wedgie'.