ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
|
|
I hear this question often. For those newbies, who may wonder what the big deal is, I will try to explain why you need an equatorial mounted scope (either a German Equatorial Mount - GEM, a Fork mount on a wedge, or an equatorial platform for a DOB).
First.. you do not NEED an equatorially mounted scope for very short exposures, such as the planets, Sun or Moon. A simple ALT/AZ mount will do. But when you want long exposures, you really need to have the scope on an equatorial platform, here's why.
An ALT/AZ scope moves n/s, e/w that is, Up/Down and East/West.
And you can certainly track objects that way all day long. But what you may not realize is that while the scope is tracking those objects, they are slowly rotating in the eyepiece.
Consider you are looking at the constellation Cassiopeia in the Northern Sky
Note the angle that Cassiopeai is. Now watch what happens after 2 hours.
And again, another two more hours...
You can see that the constellation moves across the sky.. but more then that, it has ROTATED too. This natural ARC movement can be clearly seen if you add an equatorial grid on the sky.
You can see that the whole sky does not simply move left/right/up/down but revolves. If you were to take a long photo of Cassiopeia the photo would show the effect of this rotation. Even if you tracked perfectly in ALT/AZ, the object rotates within the frame. Here's an example of 3 shots taken at 3 different times in the night, with the camera tracking in ALT/AZ only.
The whole constellation rotates within the frame. This would show up as a very blurred image indeed!
To correctly compensate for this movement, an equatorial mount is designed to rotate the scope and camera along the same axis as the Earth is revolving. This means instead of moving the scope in tiny left/right and up/down movements, it is simply rotating about one single axis... called the Right Ascension Axis. No other movement is needed and the photograph will no longer rotate!
I hope this sheds some light on this subject!
CF
--------------------
Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
|
c131frdave
Post Laureate
Reged: 01/17/05
Posts: 4376
|
|
That's pretty cool, man. Nice job. 
--------------------
Tak NJP
Various sizes and shapes of formed glass
|
Ben Ritchie
Lost in Space
Reged: 01/31/05
Posts: 4389
Loc: Bosham, UK
|
|
Nice explanation, CF!
--------------------
Observatory:- Astro-Physics 130EDT StarFire, Baader SkySurfer V, EQ6/pro
Outside:- TeleVue 76is/Vixen Porta II
Eyepieces:- 2-4mm Nagler zoom, 5mm & 6mm Radians, 9mm Nagler, 13mm Ethos, 24mm & 34mm Meade SWAs
|
Rammysherriff
Carpal Tunnel
Reged: 03/26/04
Posts: 1967
Loc: Lancs, UK.
|
|
Yup - gold star for CF plse Moderator!
--------------------
Simon.
One man and his shed: http://s211.photobucket.com/albums/bb288/Astroshed/
|
Spaced
Post Laureate
Reged: 03/01/05
Posts: 3608
Loc: Tacoma, Washington, USA
|
|
Peter, that is a most well-presented, lucid and (dare I say?) graphic explanation! It raises a question I haven't come right out and asked before, although I've been curious about it for some time:
It's my understanding that equatorial platforms make dobs equivalent to un-wedged fork-mounted scopes, in the sense that they track but don't deal with field rotation. Assuming that my understanding is correct, can an image stacking program such as Registax accurately deal with field rotation, and successfully combine video shots through a dob on an eq platform, thus allowing long exposures?
Thanks!
Mike
--------------------
Mike
"Once in a while you can be shown the light
In the strangest of places if you look at it right"
- Robt. Hunter
_____________________________
Webster 14.5" f/4.5 "Sugaree"
Megrez II 80 ED Triplet APO "Punk"
Siebert Black Night BVs
8 X 42 Celestron Regals
|
c131frdave
Post Laureate
Reged: 01/17/05
Posts: 4376
|
|
Mike, yes and no. Yes, you can stack with registax with an alt/az mount (which is what a dob is), but no, you still can't do long exposures with round stars from edge to edge. With field rotation comes star trailing at the edge of your fov.
If you want to shoot planetary, I think your dob would be perfect. But you need to take an .avi of the planet and stack in registax. Typically this is hundreds of exposures from the video file.
edit- reread your post. A dob on a eq platform is the same as a fork ON a wedge. This will give you pretty round stars with no rotation at all. Assuming your eq platform is polar aligned well, you can indeed take long exposures.
Quote:
It's my understanding that equatorial platforms make dobs equivalent to un-wedged fork-mounted scopes
This is incorrect. EQ platforms or wedges do the same thing- they put the mount in EQ configuration the same as a GEM. This will not give any field rotation at all- the purpose of an eq platform.
--------------------
Tak NJP
Various sizes and shapes of formed glass
Edited by c131frdave (03/15/06 01:48 PM)
|
Spaced
Post Laureate
Reged: 03/01/05
Posts: 3608
Loc: Tacoma, Washington, USA
|
|
Quote:
This is incorrect. EQ platforms or wedges do the same thing- they put the mount in EQ configuration the same as a GEM. This will not give any field rotation at all- the purpose of an eq platform.
Ahhhhhhh. First Light!
Mike
--------------------
Mike
"Once in a while you can be shown the light
In the strangest of places if you look at it right"
- Robt. Hunter
_____________________________
Webster 14.5" f/4.5 "Sugaree"
Megrez II 80 ED Triplet APO "Punk"
Siebert Black Night BVs
8 X 42 Celestron Regals
|
ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
|
|
Now you've got it!
But field rotation effects a lot more than just the edges.
The whole image is rotated around your guidestar., not the center of the image (unless that's where your guidestar was!)
So while objects closer to the guidestar have less blurring, all stars are still trailed.
Here's an example of a photo with a miss-aligned Polar Mount but ACCURATE guiding. The whole image is rotated around the guidestar. This is also what you would see with an ALT/AZ mount, which is the same effect as a grossly miss-aligned equatorial mount. Can you see which star was used to guide on?
CF
--------------------
Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
|
ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6254
Loc: Baghdad, Iraq
|
|
By the way... With a poorly aligned equatorialmount, there is an easy way to estimate just how much Field Rotation you have accumulated while taking a long photo. It's the total amount of Declination correction you make.
While there are other causes of declination error in tracking, the vast majority is poor polar alignment. This in turn causes rotation. Thus, while you are guiding a long exposure add-up the total amount of DEC error you keep "correcting" and that's how much ROTATION you just added. This is why Polar Alignment is so important. If you have to keep making DEC "corrections" you are allowing your image to be blurred. This is also why I use quotes around the word "corrections" when talking about Declination drift. It's not a correction.. it's a ROTATION.
CF
--------------------
Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
|
Spaced
Post Laureate
Reged: 03/01/05
Posts: 3608
Loc: Tacoma, Washington, USA
|
|
Quote:
Can you see which star was used to guide on?
Yes! It's either El Paso or Juarez! 
Thanks again,
Mike
--------------------
Mike
"Once in a while you can be shown the light
In the strangest of places if you look at it right"
- Robt. Hunter
_____________________________
Webster 14.5" f/4.5 "Sugaree"
Megrez II 80 ED Triplet APO "Punk"
Siebert Black Night BVs
8 X 42 Celestron Regals
|
Kenny2004
Vendor - Icodome
Reged: 08/17/04
Posts: 1688
Loc: Toronto, Canada
|
|
Clownfish: Great explanation and great graphics!! Thank you for such a great post
--------------------
http://www.icodome.com
|
syed
member
Reged: 04/11/08
Posts: 75
|
|
Hello guys
I was wondering if an equatorial platform can perform exactly like an equatorial mount (apart from 1 hour tracking limt which is more than enough for astrophotography) then why invest many times more on an equatorial mount.
You may get the same performance on an equatorial platform that is much cheaper.
Any answers?
|
Carlos Milovic
sage
Reged: 09/07/04
Posts: 283
Loc: Santiago, Chile
|
|
syed, 1 hour for astrophotography is not enough!
Let's just skip some details. To have enough signal to noise ratio you have to accumulate a huge amount of total exposure time, taking as long pictures as possible. For example, I'm doing from 8 to 12 subexposures of 15 minutes with a DSLR.
Of course, you can rewind the platformeach time, move the scope and try to match the framing field... but, this is so paintfully tedious.
From another point of view, for moderate to long focal lengts, an EQ mount will be much more accurate, easier to align; and at the end, it will be the only way to "mount" your equipment.
By the way, some 8 years ago I made a barn door tracker for 50mm wide field shots. I worked pretty well. Now, that I'm working with a DSLR and 135mm, I would not recommend anything "inferior" to a Vixen GP (and clones) for good quality results. And just because I don't know of any eq platform that includes an autoguiding port, I would discard them instantly 
--------------------
Regards,
Carlos Milovic F.
// Astro & Photo - CMF
// http://www.astrophoto.vze.com
|
pdlpsher
member
Reged: 04/30/08
Posts: 47
|
|
I have some questions. Thanks in advance.
- I have a CPC 800. Which wedge will work with my scope? And why are they so expensive?
- I have seen a device called a field de-rotator. Will it be cheaper to invest in one of these as opposed to a wedge?
- I've read that Polaris is not truly north. When you guys speak of polar alignment are you referring to a Polaris alignment or a true 'polar' alignment?
|
Grouptele
sage
Reged: 08/11/07
Posts: 334
Loc: Qcy, IL
|
|
CM,
Celestron's HD wedge, including altitude adjustor, will work fine with the CPC800. I used one with a CPC800 I previously owned - make sure that it is the latest model with a flat/wide base to properly mount the CPC.
A wedge eliminates the need for a [typically] expensive field de-rotator. I never have used a FD; thus, just passing along comments made by others.
Polar alignment is made to the celestial polar axis, which happens to be about 1degree off Polaris.
BTW, you can [CCD] image with your CPC800 on alt-azth mounting by limiting captures to 20-30sec and stacking/aligning them with commercially available software. See attached sample of M20, made with CPC800.
Victor.
--------------------
C8-XLT, G5, AT66
EQ-G
Starshoot color CCD
Canon XS, HapMod
SPC900NC webcam
|
Jared
Post Laureate
Reged: 10/11/05
Posts: 4554
Loc: Piedmont, California, U.S.
|
|
Quote:
Hello guys
I was wondering if an equatorial platform can perform exactly like an equatorial mount (apart from 1 hour tracking limt which is more than enough for astrophotography) then why invest many times more on an equatorial mount.
You may get the same performance on an equatorial platform that is much cheaper.
Any answers?
Unfortunately, you don't get the same performance in most cases. Typically, platforms are used to allow telescopes with fairly long focal lengths to track the night sky, i.e., large Dobsonians. At focal lengths over 1,500mm you need really accurate tracking and really smooth tracking to allow sharp subexposures to be taken. Generally speaking, platforms are not up to this task. They are great for visual use where only rough tracking is required, but very few of them allow the level of precision required for photography.
--------------------
- Jared Willson
- TV 60iS refractor
- 80mm SV/LOMO refractor
- A-P 130 EDFS refractor
- Planewave 12.5" CDK
- A-P 900 mount
- A-P Mach1 GTO mount
- Takahashi Teegul SP travel mount
- SBIG STL-11000 C1 camera with AO-L
- Some heavily light polluted skies
|
pdlpsher
member
Reged: 04/30/08
Posts: 47
|
|
Thank you Victor. That's a nice picture. May I ask which camera did you use?
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
