Imaging on a Forked Mount with a wedge

[Gerhard]

Hi all!

as suggested by Carastro, starting a new thread, hoping to catch the eye of someone with specific experience.

As a starting astrophile, I still have no telescope, and depend on a friend of mine who has an old (> 20 yrs)  celestron C8 on a Forked Mount, with a wedge.

http://upload.wikimedia.org/wikipedia/commons/8/8e/C8_smaller.jpg

It is motorized and provides therefore a fixed image. 

As I'm very interested in AP (yup, a bit of a masochist), I already got my T2 adapter and ring to start shooting. 

The issue is, I'm not really sure as to how to use this scope for AP, ie, how long can subs be on this setup, etc.

My friend, before the digital era, shot some pretty nice photos of mainly planets with it, but I doubt he remembers exposure times, etc... He's just getting enthousiastic again because I kept pestering him about my "new" hobby!! :-)

So, any pointers would be very much appreciated!!

Gerhard.

[ronin]

Planets are OK with a fork mount and no wedge,

You get a webcam, remove the webcam lens (leaving the filter in there), and reassemble. Then the webcam goes in where the eyepiece usually goes and you adjust/refocus to get an image on the sensor then simply take a video of the planet, say about 2000 frames.

You then process this video with something like Registax.

It/you select the best 400 or so and tell Registax to stack them and then you play with the "Wavelets"  (No idea what or why Wavelets) to improve the image. Simple "improvement" is to move all 7 Wavelets sliders from 0 to half way and see what comes out. :eek:  Not exactly a technical explanation I suppose.

That is the short lunatic guide to planetary imaging. :grin:

Give that a try on Jupiter if possible.

There is a nice webcam in a circular housing that is about 1.25" dia, try to find one as that makes fitting easier.

For DSO type images you will need a wedge or an Equitorial mount, you are also likely to need a shorter and faster scope then the C8. The implication here is you could want a different scope and a different mount. Which doe no leave much of the C8 and the fork mount.

Try the planetary first and see how it goes.

[Gerhard]

The mount has a wedge, so DSO should be doable... When we did some viewing two weeks ago, we managed to see M31, 32 and 33... And a very faint glimpse of the ring nebula. My friend told me he used to be able to see it with more definition, but the scope is something like 30 years old, so the mirror might have correded somewhat...

the problem with the planets is they are not above the horizon in the evening, these days. Only Uranus and Neptune... Jupiter rises early morning, Saturn only after the sun...

can I not just use the video capture of my 500D, instead of the webcam?

I was recommended Autostakkert for stacking videos... But will give it a try, when the next good clear sky presents itself. Next week all rain.... :-(

[steppenwolf]

You'll be fine for lunar and planetary imaging but deep sky will be 'challenging' because of the long focal length and lack of guiding for the long exposures required.

Sent from my iPhone from somewhere dark .....

[carastro]

Gerhard:

Steve - Steppenwolf is the author of Making Every Photon Count - the book that many people recommended to you.  

lack of guiding for the long exposures required.

 I never realised that Steve, I thought there was a way to do it.  

Carole 

[steppenwolf]

 I never realised that Steve, I thought there was a way to do it.

Hi Carole - not quite sure what you are alluding to here so it could be I have caused some confusion! Just to clarify, an Alt/Az mount on a wedge can, of course, be guided but from the OP's description of this particular equipment, it seems pretty basic and doesn't have any guiding gear included.

[DaveS]

If it is 30 years old then probably pre-autoguiding so wouldn't have the necessary ports.

[carastro]

an Alt/Az mount on a wedge can, of course, be guided but from the OP's description of this particular equipment, it seems pretty basic and doesn't have any guiding gear included.

Ah, that explains it. thanks for the clarification Steve.  

Carole 

[ollypenrice]

The wedges are fine in theory but rather less than fine in practice. Exposures will have to be short but try some bright targets like the Trapeziulm in M42.

Olly

[Gerhard]

Hi Carole - not quite sure what you are alluding to here so it could be I have caused some confusion! Just to clarify, an Alt/Az mount on a wedge can, of course, be guided but from the OP's description of this particular equipment, it seems pretty basic and doesn't have any guiding gear included.

In fact the scope is quite simple: the only thing it has is the motor for tracking. no guide scope, or any other advanced stuff like that. As far as I know no ports in fact for attaching other electronics, being an old scope. 

Next time we'll have a go with this scope, we'll try for andromeda, I think, because Orion will be too low in the evening. Maybe if we can figure out how to transport his scope from his home... :-)

[schmeah]

The wedges are fine in theory but rather less than fine in practice. Exposures will have to be short but try some bright targets like the Trapeziulm in M42.

Olly

Until recently, I imaged for years with a fork on a wedge (LX200R) and was able to get decent shots (even an APOD) with FL as long as 2500 and subs as long as twenty minutes. Granted, it took a lot of effort to get to that point and required a quality wedge (Milburn) and accurate guiding. The wedge and fork in the OPs picture seem pretty flimsy though, and without guiding, subexposures beyond a couple of minutes would not likely be possible. The simplicity of a well performing fork/wedge system, however, is a beautiful thing.

Derek

[SkySurfer]

Hi,

I have a fork mounted SCT on a wedge, and would not swap it, I do auto-guide and can get subs at up to 15mims with nice round stars.

The secret is balance, balance and more balance, you need the mount perfectly balanced on all axis, with a dedicated weight balance set up, then trust me when I say it works a treat

I have a permanent pier with wedge, which is pretty well polar aligned, and with no guiding can get around 2 min subs at 1260mm fl as I use a focal reducer to take from f10 to f6.3, I don't think I have ever tried without the reducer, but don't see what it would not give me as good results.

But when autoguided then I have had up to 15min subs as i said before.

If you done have any balance on the scope then forget it, it will put too much strain on the gears and will jump all over the place, especially if you have a heavy camera on the back and a guide scope on top, I found out the hard way.

Hope that helps

:)

SS

[IanL]

Well I started out with something very similar, a Meade LX10.  The OTA itself is fine and I still use it (as good as anything else Meade made at the time optically).  It had all sorts of deficiencies which I had to overcome.  Firstly I used the thing off the bat with no changes for a fair bit of lunar and planetary imaging (with or without a Barlow).  This is what SCTs are realty good at and I suggest you give it a go - all you need is a camera and the moon, and if you're feeling flash maybe some capture software (e.g. APT or BackYardEOS) to capture AVIs and stack them in Registax, etc.  Hours of fun and some great images.  For example here is a lunar mosaic I did by stacking a bunch of AVIs through the LX10 and 500D:

If you want to go deep sky, here is what I had to do:

a) The motors were simple DC battery motors.  The Alt motor drove a tangent arm which meant it had a fairly limited range of motion and once you reached the end of the travel you had to reverse the motor to centre the tangent arm again and then unlock the clutch to point the scope at the target.  The Az motor supposed had sidereal tracking set at the factory, but the drive rate would vary depending on the condition of the battery and the temperature.  The first mod I did was to replace the internal potentiometer that sets the drive rate with an externally accessible one so that it could be adjusted by eye to match the motion of the stars.

b ) The wedge adjustment was pretty poor.  I picked up an after-market Alt adjuster which greatly improved that bit, but there was no adjusting mechanism for the Az motion and I never found a solution.  Therefore polar alignment (essential with such a poor mount) was very difficult to say the least!

c) There was a simple pushbutton hand controller to drive the mount in Alt and Az.  On the Meade LX10 Yahoo group there are a bunch of designs for converting this to a rudimentary autoguiding port.  Basically you need an Arduino board and an add-on board to 'press' the hand control buttons - various designs using an off-the shelf Arduino relay board or self-built transitor boards are possible.  There is some code available that makes the Arduino act like a Meade autoguider and so you can use the appropriate Meade ASCOM driver with PHD guiding or similar.  I don't know if your mount has a hand-control port or not - if so the wiring will probably be different than the Meade, but the basic principle is the same.

d) I used a piggy-back mounted ST80 guidescope and a webcam for guiding and imaged using my Canon 500D.  This is what I managed to get after a lot of work:

e) I also piggy-backed the camera with a lens and used the LX10 to guide it:

After that I de-forked the OTA and got myself an NEQ6 - I never had that much luck doing deep sky with it and quickly switched over to using an ED80.  This is probably one of the better efforts with the LX10 on the NEQ6:

It isn't easy, but if your budget is limited you can certainly get something out of the kit you have.

[Gerhard]

hm. very complex. Especially for me, as I am an absolute beginner.... 

I think with this C8 we will have to limit ourselves to planetary and lunar. I don't think my friend will want to lose much time and money on modding his C8, also because as said, it's quite old, and probably needs the mirror cleaned of rust, as he is suggesting.

He's kind of a DIY person, but at the moment is trying to make something out of an old parabolic antenna, to try and get some image out of that, just for fun... :-) put mylar on it, place an eypiece on the arm, etc... will be interesting to see how far he gets.... :-)

anyway, as said, I don't think we'll do a lot of modifying on this scope... also because doing good AP requires some extras (guidescope, webcam, etc), that are maybe not worth it for this old C8... 

So, next time we'll have an opportunity, I'll just put on my camera, hope the balancing and the motor are not TOO bad, and try for some nice objects, whatever is available at the moment, be it planetary, or DSO... play around a bit, get everything wrong, and learn from these experiences how NOT to do AP! :-DD

in the meantime I am still undecided about my first buy... whether to go ahead and have some initial visual fun with a 130P(DS) EQ3-2, or to not "waste" that money and stock up for an HEQ5 / EQ6 to start seriously with AP, but after a potentially much longer wait.... but that's another thread... :-)

[ollypenrice]

It's good to read of people succeeding with the American fork and wedge systems and my objection to them is not that they cannot be made to work but that they are often inordinately difficult to get working to the level demanded by the focal length. For the very technically-minded they may even present a satisfying challenge. But in my view production equipment marketed as Meade and Celestron market their products should make a closer approach to 'just working' than it does. I'm full of admiration for those who get them to work because I know many people who, like me, failed to do so.

The basic fork mount idea is great. No doubt about that. If my OTA would fit on an Avalon M Uno I'd have one in a trice.

Olly

[michaelmorris]

From the set up you've described, there are a number of relatively simple imaging options available to you.

• Planetary/lunar/solar imaging - use a modern DSLR to take either stills or video
• Deep sky wide field imaging - piggy-back a camera and lens on the back of the scope.  You will hopefully be able to 5 minute+ sub-exposures with a 50mm lens.  You should  even be able to go up as high as 200mm lens (depending on how near the celestial equator the target is.  The near the celestial pole the target is, the lower the angle it moves in the sky (arcsec/minute).
• Through-the-scope deep sky imaging - you'd only be able to get very short sub-exposures due to the high magnification.

I hope this helps

[Gerhard]

very helpful! :-)

maybe piggybacking could be an option! :-) will discuss this with the owner of the scope! :-)

concerning the through the scope DSO images: how short would these subs have to be? just to limit the errors I will be making a little.... :-)

btw: these days it's rain rain rain over here, so no joy.... :-/