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field rotation FORK versus GEM

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#1 Guest_**DONOTDELETE**_*

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Posted 22 October 2005 - 05:06 PM

If I want to do ANY CCD based imaging of DSO and I intend to image digitally and stack DO I HAVE TO worry about field rotation on a fork mount : what time scales?

GEM would be better but not sure I can get a good enough low PEC mount/drive to make it worth while.

How many people here use Nexstar/Celestron/Orion on fork mounts WITHOUT wedges and get decent DSO pictures?

What bout the MEade de-rotator?

Thanks
Ivan

#2 NeoDinian

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Posted 22 October 2005 - 05:08 PM

GEM would allow for long exposures... If you are doing short (few minutes) exposures and stacking, you can do so with the fork.

#3 JerryWise

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Posted 22 October 2005 - 07:25 PM


I think this brings up an interesting subject. Using forks, I seem to get more field rotation in different parts of the sky. The rotation rates seem different on M81/82 than on say M13 or near the pole vs ecliptic. Also seems the longer the Focal Length, the more rotation will show. I say this in a questioning format because I don't have a fork mount right now to test it with. Can someone please validate or refute this?

Also, about 2 minutes is about all I expose on a fork. Rotation is visible when the image is magnified.

#4 jrcrilly

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Posted 22 October 2005 - 08:05 PM

I think this brings up an interesting subject. Using forks, I seem to get more field rotation in different parts of the sky. The rotation rates seem different on M81/82 than on say M13 or near the pole vs ecliptic. Also seems the longer the Focal Length, the more rotation will show.


Correct on both counts. The magnitude of field rotation varies with sky region and the effect of a given amount of rotation varies with image scale. There's software available from tech2000 that will calculate the maximum usable exposure based on those two parameters.

#5 Guest_**DONOTDELETE**_*

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Posted 22 October 2005 - 11:00 PM

This is why I am wondering do I NEED a GEM at all?
If I limit the stacking exposures so field rotation is minimised on a form mounted cpc 1100 say, do I need the GEM at all? A low PE still good obviously.

Beginning to think perhaps I forget wedge and GEM and go for fork and large aperture, keep exposures short enough and then stack in software. Making field rotation minimal.

????

thanks
Ivan

#6 Guest_**DONOTDELETE**_*

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Posted 22 October 2005 - 11:03 PM

will look at th tech2000 software... get an idea of how bad field rotation effects will be.... then I'll KNOW whether or not I'll need a GEM based on the sky location/Fratio I'm using I guess a s a funtion of t.
thanks!
Ivan

PS will help clinch fork versus GEM mount

#7 NeoDinian

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Posted 22 October 2005 - 11:08 PM

Getting a quality GEM and OTA would probably be about the same cost as a comprable aperature fork.

I think with short exposures the fork would be fine. Even without the de-rotator, since the software doing the stack would compensate for that anyways. (Am I wrong?)

Most of the "Stacks" I've seen are all less than 30 seconds anyways. I know you can do longer... But that would also depend on the mag you do, or you would chance getting rotation.

#8 Bob Pasken

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Posted 23 October 2005 - 01:09 AM

You need to be careful about your reference to a fork versus a GEM. The common fork-mounted SCT's can be used in ALT-AZ mode where you will have field rotation or in POLAR mode where there will be no field rotation as it has become a fork equatorial mount (FEM). A GEM point away from the pole becomes an ALT-AZ mount and would have field rotation. ALL AlT-AZ mounts have field rotation including Dobs place on Poncet platforms.

#9 JerryWise

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Posted 23 October 2005 - 06:24 AM


A lot of experimentation has pretty well convinced me that if you have a good fork mounted SCT you have the best of both worlds. You can have an Alt/Az platform with solid goto and ease of use. Or, if you want to grow into long exposure it will only cost the price of a wedge to get there. The PE may be a little more put that can be programmed and guided out. The forks are just easier to view with and have a much better goto for me (and I use EQ mounts {CGE and LXD650} right now).

#10 JAT Observatory

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Posted 23 October 2005 - 07:55 AM

This is why I am wondering do I NEED a GEM at all?
If I limit the stacking exposures so field rotation is minimised on a form mounted cpc 1100 say, do I need the GEM at all? A low PE still good obviously.

That is one way to solve the field rotation problem. But as pointed out in other posts the rotation time varies. Depending on what part of the sky you image you may see rotation with a 30 second exposure.

Beginning to think perhaps I forget wedge and GEM and go for fork and large aperture, keep exposures short enough and then stack in software. Making field rotation minimal.

The problem with short exposures is the signal to noise ratio is higher. My feeling is if you are really serious about imaging you are going to have to get away from a ALT/AZ mount and use some type of polar mounted scope.

It is also important to note there both GEM and fork mounts can suffer from PE and rotation. The good news here is PE is generally only an issue on the RA axis as that is the only axis that is tracking on a properly alignment polar mounted scope. But it can be mitigrated with PEC training the scope and guiding.

The bad news is a mis-aligned polar mounted scope will show star trails on long exposures.

#11 jrcrilly

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Posted 23 October 2005 - 10:02 AM

The problem with short exposures is the signal to noise ratio is higher. My feeling is if you are really serious about imaging you are going to have to get away from a ALT/AZ mount and use some type of polar mounted scope.


Yes. Shooting at the focal length of a C11 is very demanding so exposure time in alt/az will be pretty limited. On one hand, there are lots of objects that will yield very nice images with 30 second exposures. On the other hand, most of them are globulars and they tend to look alike after a while. There are quite a few other objects from which a usable image can be coaxed with short exposures and a lot of work - but a far greater number which simply cannot be imaged successfully without substantial exposure time.

It'd take a while to run out of interesting objects in alt/az so he could surely get his feet wet - but if his interest lasts he'll most likely find himself switching an EQ arrangement.

#12 LLEEGE

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Posted 23 October 2005 - 11:36 AM

Ivan, welcome to CN! You would be able to extend your exposure times a tad, by using a focal reducer. Celestron makes a .63 reducer that will make your CPC11 an f/6.3. It will also flatten the field and expand the FOV. But, in AL/AZ you will be limited still with the reducer. If you are serious about imaging, you will need an well polar aligned mount. This will eliminate field rotation all together. Yet, anything over 5 min. will need to be guided no matter how good the PEC training is. But, part of the joy of this hobby is the learning curve. Every session is a new experience.

#13 Bob Pasken

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Posted 24 October 2005 - 10:29 PM

I agree with you Jerry! If I were buying a SCT these days I'd buy a fork mounted scope. You get the best of both worlds; Put the scope into Alt-Az mode or Equatorial mode when you need it. The only problem is that most wedges these days aren't as good as they should be

#14 TK6411

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Posted 25 October 2005 - 10:40 AM

Ivan,

Good questions...the Meade Field De-Rotator is very good at eliminating field rotation on an Alt/Az Fork Mounted Scope. All things being equal in terms of expense and other factors it really doesn't matter whether you choose a Fork Mount or GEM design for Astrophotography...both will yield excellent results when properly setup and used. GEM's do offer greater flexability in being able to change out OTA's at the drop of a hat without overloading the main OTA as some come close to doing with Alt/Az Fork mounted scopes. I think it comes down to personal preference...I prefer GEM's myself but see no problem with Alt/Az Fork mounts either on a wedge or equiped with a Field De-rotator if you plan on doing any astrophotography.

Jim

#15 JerryWise

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Posted 25 October 2005 - 01:27 PM


I do like my CGE and LXD mounts and most likely will continue to use them. That RCX looks so good though I am going to really try and make it work. When mine comes back from Meade service I have a Ultra Wedge and derotator waiting. One or the other will work well. I would like the derotator to work out. One thing that did surprise me on the derotator is you loose the opportunity to program out PE with it. PE correction only works with wedge mount.

#16 imjeffp

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Posted 30 October 2005 - 09:18 PM

A derotator will not work with a guide scope, either.


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