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Atlas EQ-G, imaging C-11, 80mm ST, PHD Guiding

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#76 yg1968

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Posted 24 November 2008 - 02:14 PM

Peter, I also have an Orion ST-80. I will probably use that instead of my Orion 80ED. It saves me about 3.5 pounds (6 pounds vs 2.5 pounds). Another option would be to get a Stellarvue F50 finder and use it to guide.

But I do not see much field rotation on my frames. I see stars shift in between frames. This looks more like flexure issue to me.


Perhaps, you have both problems. But I doubt that you will be able to image longer than 10 minutes without doing a drift alignement. Anyways, your results are better than mine, so I am not sure that I should be giving you advice...

#77 AlexN

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Posted 24 November 2008 - 03:07 PM

My C11 takes 4 counter weights with the ST80, cameras etc for deep sky photography.. for visual or planetary imaging, its happy with only 3...

Im currently looking into a good finder scope that I can guide with, or an OAG to bring the weight of the system down...

#78 Peter in Reno

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Posted 24 November 2008 - 05:51 PM

Perhaps, you have both problems. But I doubt that you will be able to image longer than 10 minutes without doing a drift alignement. Anyways, your results are better than mine, so I am not sure that I should be giving you advice...


Thanks to Deep Sky Stacker and Nebulosity for making my images look good but that's cheating!!!! :cool:

Peter

#79 AlexDJ30

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Posted 24 November 2008 - 05:56 PM

I think the problem relies on the C11 weight pklus all the accesories on it, the Atlas is a a very good and i have seen people take nic epictures with C11, but probably in paper i wouldnt try that even on an atlas, i would step down the aperture to something about C9.25 or a C8 so the system should have plenty of left over. I think with the C11 and accesories you are almost in the peak of their quality performance.

#80 yg1968

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Posted 24 November 2008 - 06:57 PM

Personally I also have a C6 in addition to the C11. But the problem is that the time of exposure must be much longer with the C6 than the C11. The C6 is only 10 pounds but with a (somewhat heavy) dual saddle, it ends up being much heavier than that. Also the C6 with a larger camera or DSLR shows some vignetting which is not the case with the C11. The C11 is a light bucket and there is something to be said for that. My best images have been taken with the C11. So I am partial to keep it and using it. Although I also like my C6. It's nice to have options!

#81 Strgazr27

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Posted 24 November 2008 - 08:33 PM

The C11 is a light bucket and there is something to be said for that. My best images have been taken with the C11. So I am partial to keep it and using it. Although I also like my C6. It's nice to have options!



Try shooting with the 11 at f/6.3 and compare your background levels. I think you'll be surprised to see the 6.3 images show a better SNR than at f/10. Your spreading more of the light over a given pixel. Yes the image scale will be a bit smaller but your guiding will be better and your exposure length may be able to be shortened up a bit compared to f/10. If the guiding is better than the 6.3 image can easily be cropped to bring up the image scale.

#82 Peter in Reno

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Posted 24 November 2008 - 09:52 PM

I spoke to Celestron about the dent on my scope and they authorized a return for repair/replacement. I also told them that the scope looked out of collimation and I tried to re-collimate but was unsuccessfull probably because of the dent on the aluminum tube. They told me never touch the collimation screws because it's already collimnated from the factory. I find that odd since the instruction manual mentioned collimation instructions.

Peter

#83 Peter in Reno

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Posted 24 November 2008 - 09:56 PM

Personally I also have a C6 in addition to the C11. But the problem is that the time of exposure must be much longer with the C6 than the C11.


Both C6 and C11 have same F/10 focal ratio and the exposure times should be similar. The C-11 has more light gathering power and reveal dimmer images but I believe the exposure should be the same as C-6 due to same focal ratio. Correct me if I'm wrong.

Peter

#84 Peter in Reno

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Posted 24 November 2008 - 10:19 PM

This is still bugging me since I started this thread. I have a very rock solid equipment including ADM Losmandy dovetail, saddle, mini-dovetail and clam shell tube rings for guide scope. Where the hell is the flex coming from?

We have not mentioned the durability of Celestron aluminum tubes. How thick are these tubes? What if the flexure is coming from C-11 aluminum tubes? The front bezel and rear cell are pretty heavy and can put some stress on aluminum tubes. Can top and bottom dovetail plates which connect front bezel and rear cell cause stress on aluminum tubes? Would tube rings supporting C-11 aluminum tubes be better if they are available?

So many variables. :foreheadslap:

Peter

#85 AlexDJ30

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Posted 24 November 2008 - 10:35 PM

I still think the atlas yes is for 40 lbs payload, but like many said its for visual use, photography is another ball and like many said you should lower ther weight on the atlas. Maybe you can try with a lower OTA (If you have a C6 try with it) and compare the results if the flexture is still there then is something on the mount or how you balance or something else, if the flexture is gone now you know that is the C11 weight plus all the accesories.

#86 freestar8n

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Posted 24 November 2008 - 11:49 PM

I have a very rock solid equipment including ADM Losmandy dovetail, saddle, mini-dovetail and clam shell tube rings for guide scope. Where the hell is the flex coming from?



Refractors have long aluminum tubes and they do ok with guidescopes. SCT's and RC's have two mirrors on supports, and they have flexure issues. The primary mirror is big and heavy, and it only has to tilt by microns or less to show a shift on the scale of pixels at f/10. Even with mirror locks it's hard to make a big mirror remain perfectly oriented to that level of precision as the OTA tracks the sky and the direction of gravity changes. For a c11 with no mirror locks, it will definitely have issues.

With OAG - it's just not an issue anymore.

Frank

#87 Peter in Reno

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Posted 25 November 2008 - 12:10 AM

Refractors have long aluminum tubes and they do ok with guidescopes. SCT's and RC's have two mirrors on supports, and they have flexure issues. The primary mirror is big and heavy, and it only has to tilt by microns or less to show a shift on the scale of pixels at f/10. Even with mirror locks it's hard to make a big mirror remain perfectly oriented to that level of precision as the OTA tracks the sky and the direction of gravity changes. For a c11 with no mirror locks, it will definitely have issues.

With OAG - it's just not an issue anymore.

Frank


Makes sense.

Thanks,
Peter

#88 AlexN

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Posted 25 November 2008 - 02:45 AM

Both C6 and C11 have same F/10 focal ratio and the exposure times should be similar. The C-11 has more light gathering power and reveal dimmer images but I believe the exposure should be the same as C-6 due to same focal ratio. Correct me if I'm wrong.

Peter


Nope, not true...

An 11" Scope at F/10 will capture more light than a 6" F/10 on the same target. Not only will the 11" image scale be bigger, but the image will be brighter as well..

I tested my 102mm F/6.9 APO vs my C11 @ F/7 The C11 captured much more nebulosity in M42 in a 5 minute exposure than the refractor.

That being said, the entire experience with the refractor was easier, And I routinely run 30 minute exposures with the refractor, the idea of a 30min sub with the C11 seems a bit ridiculous... Its just not going to happen.

Horses for courses. I am a narrow field junkie, So i've persevered with my setup to be able to achieve round stars in 5-7 minute exposures. (Even managed 2 minutes unguided at F/6.3) However its taken a LOT of tweaking, tuning and at least an hour or so to get the balance perfect...

#89 freestar8n

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Posted 25 November 2008 - 10:17 AM

An 11" Scope at F/10 will capture more light than a 6" F/10 on the same target. Not only will the 11" image scale be bigger, but the image will be brighter as well..


There is a lot of confusion on this stuff, but a simple way to describe it is: For nebulosity, the S/N per pixel depends only on f/ratio, but the S/N per square arc-second depends only on aperture. If you see "more nebulosity" with more aperture at same f/ratio on a per-pixel basis I'd like to see the comparison. In this case, the smaller aperture image will be smaller than the larger one - but just as 'bright.' The nebular detail may show better in the large aperture, but that's due to finer structure at the larger image scale - not greater brightness due to the aperture.

the idea of a 30min sub with the C11 seems a bit ridiculous... Its just not going to happen.



With OAG the impossible becomes possible, and the weight is reduced so that a smaller mount can be more nimble. I have done 20m and could do longer with no problem - but I prefer having more images to stack. Here is a study of 20m subs with c11 at f/10 on cge, using OAG.

Galaxy pair 20m subs

Here is one with 15m subs:

Bubble 15m subs

Of course - I'm using different guiding software and video, which also helps.

Frank

#90 yg1968

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Posted 25 November 2008 - 10:24 AM

Alex,

I agree with you. There was a long debate on a number of threads about this issue on the CCD forum (focal ratio vs aperture). I will spare you the details but the conclusion was that aperture rules (if your mount is up to the task).

Other thread

But reducing the focal ratio (with a focal reducer) helps in terms of getting stars that appear to be rounder given that it increases the field of view making the stars look smaller and therefore appear to be rounder (even though they actually aren't any rounder when you zoom in on them -it's just harder to tell when they are smaller). The same thing is true for having a larger chip, the larger chip means the individual stars appear to be smaller in the image (making them seem rounder -even though they are not).

#91 freestar8n

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Posted 25 November 2008 - 11:24 AM

I will spare you the details but the conclusion was that aperture rules (if your mount is up to the task).



Hmm - well I piped in at the end of that thread with the thing about pixels vs. arc-seconds - which is very different from aperture rules - and it seemed to be accepted. The thread does come up a lot and it is always a slippery topic - but if you focus on pixels vs. square arc-seconds it makes sense. There is weird stuff about film vs. ccd that confuses things and isn't relevant.

The adu and s/n of nebulosity for a pixel on a given camera just depends on f/ratio of the attached lens. There is a direct trade off of aperture and image scale at a fixed f/ratio and fixed camera, and no magic is involved. Stars behave differently and are much more dependent on aperture.

For the concrete example here - a nebular image with 6" at f/10 and 11" at f/10 will look just as bright (same adu's and per pixel s/n) but the 6" image will be smaller. If you then blow it up, it will have less detail and look noisier. But if you don't blow it up, you will have a nice, wider field and bright image with the 6" vs. the narrow field, but equally bright, 11" image.

So - aperture rules, and f/ratio rules - depending on what you mean.

Frank

#92 yg1968

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Posted 25 November 2008 - 12:33 PM

Thanks for the explanations. But I have some follow-up questions. If you use the same camera for both scopes, shouldn't the image size be the same? Also for a fair comparaison, shouldn't you compare both objects at the same size (by zooming on the object in the image with the smaller scope)?

#93 Strgazr27

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Posted 25 November 2008 - 01:48 PM

Thanks for the explanations. But I have some follow-up questions. If you use the same camera for both scopes, shouldn't the image size be the same? Also for a fair comparaison, shouldn't you compare both objects at the same size (by zooming on the object in the image with the smaller scope)?


The physical size of the image will be the same (Whatever size your chip is doesn't change regardless of what scope you mount it on). What does change is the image scale as well as FOV.

For those that don't know about it and want to see what changes can happen when you swap cameras and scopes go to Ron Wodaskis site and DL his CCDCalc. You can find it Here

It also needs to be understood that pixel size will also have an impact on FOV/Image scale. If you look at most imagers shooting at real long Fl's 15-1800mm and longer they are using cameras with larger pixels. Much larger than those found in todays DSLR's. This is the subject of being over/under sampled.

I will say I will put my 8" f/4 up against a C11 any day. My exposures will be shorter to reach the same SNR, my guiding errors will be less and my image just as bright. My FOV will be much larger but as I said before, with good guiding I can crop the image such that I can clome pretty close to the 11 as far as image scale goes. I may suffer a SLIGHT bit in resolution but it would be hard to tell for most.

#94 freestar8n

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Posted 25 November 2008 - 11:05 PM

If you use the same camera for both scopes, shouldn't the image size be the same? Also for a fair comparaison, shouldn't you compare both objects at the same size (by zooming on the object in the image with the smaller scope)?



Hi -

Well - you can do whatever you want with the images and things will change accordingly. But somehow the importance of f/ratio has been made into a myth, and oddly associated with a change from film to ccd. My point is - f/ratio is just as important and fundamental as it ever was, and it does have this nice property that, for the same camera and the same nebula, the s/n and adu levels on each pixel ONLY depend on f/ratio and aperture doesn't matter.

That isn't artificial, either. Fast optics really do let you capture more nice images quicker - because people don't blow things up to the same arc-second scale. They tend to use the wide shot regardless. If you do go ahead and zoom in, say on a planetary nebula, then aperture is all that matters for the same image scale.

Frank

#95 yg1968

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Posted 26 November 2008 - 10:23 AM

I see what you are saying and I agree. But I guess that it depends on your target. The larger scope you can use on smaller stuff like Stephen's quintet. The smaller 80ED scope is better for wider filed targets such as Andromeda or the Orion Nebula. In other words, there is something to be said for having a larger scope and a smaller one. You can also use a focal reducer with the larger scope to reduce the focal ratio.






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