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Equipment Discussions >> Mounts

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Kendahl
member


Reged: 02/02/13

Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Bluejay08]
      #5704609 - 02/28/13 12:46 AM

I'm aware of his retraction. It's encouraging but not a complete answer. There are still going to be tracking errors of a certain size. That size determines the longest focal length at which one can still get good images.

As a beginner, I am confused by the extreme range of recommendations I have found on various forums. There are people who are happy with a C8 on a CG-5. On the other hand, when I asked, elsewhere, about a Losmandy GM-8 versus a G11 to carry a C9.25, a couple of responses stated that both the mount and the telescope would be a disappointment and that I would be better off jumping straight to Astrophysics or Takahashi equipment. There seems to be consensus that mount capacity ratings should be discounted by 30% to 50% for astrophotography. Yet a guy in my local club is doing well with a C11 plus camera and piggy back guide scope on a CGEM. The CGEM's 8/3 issue and the C11's mirror shifts don't appear to cause him problems. It has been recommended that I begin at an image scale of no less than 2 arc-seconds/pixel. At that scale, the Ring Nebula occupies only 75 pixels. Rather than start with something small and cheap and upgrade repeatedly until I find a satisfying combination of mount and telescope, I would prefer to do enough research to jump straight to the final one.

Edited by Kendahl (02/28/13 12:53 AM)


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Midnight Dan
Postmaster
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Reged: 01/23/08

Loc: Hilton, NY, Yellow Zone (Bortl...
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Kendahl]
      #5704796 - 02/28/13 07:06 AM

The reason you find such a range of recommendations is that people have different tolerances for what's good enough. Fact is, there's no "hard" cutoff point where a mount has too much load on it. There's just a range of factors such as load and length of scope, that produce a range of results. It's up to each individual to decide where the cutoff point is. For some, a C8 on a CG-5 produces decent results. For others, the exact same setup won't be nearly good enough and not worth the frustration.

Generally speaking, the smaller the mount, the bigger the load, and the longer the focal length, the worse the results. And, as you've read, a good rule of thumb is that you should load a mount between 50% to 70% to get the best results. But again, some people are perfectly happy with the results even when exceeding those numbers, some times by a lot. And then of course there's the variation in the quality from example to example of the same mount.

So if you're looking for hard answers, you won't find them. Unless you're already in the game and have tried several mounts and know your own tolerance for quality level in your images, all you can do is go by the "rules of thumb".

A C8 will work on a 30 pound capacity mount like a CG-5, VX, Sirius, iEQ30, or HEQ5. But, when you add up the weight of all gear you'll have mounted along with it, it will be much happier on a 40-45 pound capacity mount like an Atlas, iEQ45, or CGEM. A C9.25 is starting to push that 40-45 pound mount class, but will certainly work on it. I think a C11 is too much and should be on the next class up with a 60 pound or more capacity, but again, there are those who will do it and be satisfied with the results.

-Dan


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freestar8n
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Reged: 10/12/07

Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Kendahl]
      #5704821 - 02/28/13 07:45 AM

Quote:

It has been recommended that I begin at an image scale of no less than 2 arc-seconds/pixel. At that scale, the Ring Nebula occupies only 75 pixels. Rather than start with something small and cheap and upgrade repeatedly until I find a satisfying combination of mount and telescope, I would prefer to do enough research to jump straight to the final one.




Hi and welcome to cloudy nights.

You have a refreshingly open perspective on the issues of autoguiding and PE, and I agree the information you find on the web can be confusing. For some reason there is a strong emphasis placed on the PE characteristics of a mount - when that is separate from how well the mount can create a small image of a star when autoguiding - which is the whole goal of good autoguiding in the first place.

In short, when you hear advice I would look at the actual results people get with the equipment they have. There are many plots showing a reduction of PE, but it is rare to see information on a corresponding reduction in star size, or fwhm in arc-seconds. There are many people who could not get good results with mid-range equipment and found a dramatic improvement spending for high end - but that doesn't mean they could not have achieved similar results with mid-range using the right techniques and software.

This leads to advice not to bother with mid-range equipment, and discouragement that small stars can ever be obtained - hence a push towards large pixels. There is no doubt that a small refractor and big pixels, in arc-seconds, is easier for imaging - but if you want high resolution in something like the ring nebula - then you will need long focal length and small pixels, in arc-seconds - but the result is achievable as demonstrated by many people, including my images on the MetaGuide site with cge and cge-pro (but not cgem).

There may be more effort and care required with mid-range equipment - but the existence of results at 2" fwhm and below is proof that such results are achievable without a high end mount - and such results benefit from small pixels.

More on topic - I think it's great the OP's PE was reduced, and that may well make the mount more forgiving when guiding - but the number I like to see is the achieved fwhm in arc-seconds in long exposures - which sounds like what you are interested in also, ultimately.

Frank


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Kendahl]
      #5704828 - 02/28/13 07:54 AM

Hi Kendhal

For me, my iEQ45 performs very, very well. I agree with Dan: one has to know the performance of several kinds of mounts. Getting down to 4.5 arcseconds is very good. Is it for anyone else? Only each one an answer that.

The weight, cargo capacity, PEC capability and GPS were features that attracted me when I bought mine, but the main feature which made my decision was the fact that it can get to 5 deg of latitude with no other accesories required because I'm located at 4.8 deg North. So, no matter how good or bad its competitors are, they didn't work for me.

Apart from that, I have been kindly surprised by the good performance of this mount, as many other owners of it have. I know at least two other cases in which the PE with PEC has been taken to 4 and 6 arcseconds, which are in the same range as I am. Tracking is good, alignment is easy wih the built-in routine and the go-tos are always into the FOV.

Having said that, it is not a Mach 1 or a Paramount, but is a very good mount. I'm in no way related to iOptron. I'm just a satisfied costumer.

Is it the right mount for you and within your price range? You have to evaluate that.

Let us know if you need more help.

All the best,

Alfredo


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: freestar8n]
      #5704841 - 02/28/13 08:08 AM

Hi Frank

How can the fwhm be measured? I'm very interested in measuring that.

Alfredo


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freestar8n
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Reged: 10/12/07

Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Alfredo Beltran]
      #5704868 - 02/28/13 08:25 AM

Hi Alfredo-

Most image processing tools provide a way to measure it. You just need to take a guided image of perhaps 5-10 minutes that is focused as well as you can. You need to know the plate scale, arc-seconds per pixel - which you can estimate using the focal length and the pixel size - or use plate solving software. It should be a raw or calibrated image that has not been stretched or manipulated at all.

As long as the stars aren't saturated, they should all have the same width when measured at half their height (fwhm). So the brighter ones look wider - but they are also "taller" - and when you measure them half way up to their peaks - the width will be about the same for all.

The fwhm in arc-seconds is an indication of how well you are imaging. Extreme professional results at excellent sites can be as small as 1", while a typical result with a guidescope is around 4" perhaps. I consider a good target to be 2".

So if you use Maxim or Images Plus or something - you can just enter the plate scale (arc-sec per pixel) and point at a star and read the fwhm.

If you get a big number it could be due to seeing or guiding, but I think a common limitation is just making sure the focus is excellent. Anyway - it serves as a quantitative measure of how good your imaging is - so you can measure it and try to improve it.

Frank


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: freestar8n]
      #5704968 - 02/28/13 09:29 AM

Hi Frank

It seems very simple! I will try it next time I'm outside and see what I can find.

Best regards

Alfredo


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freestar8n
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Reged: 10/12/07

Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Alfredo Beltran]
      #5704987 - 02/28/13 09:39 AM

Hi-

I just noticed you may be using a DSLR. In that case you can't measure with the raw images very well and you need to debayer them first. So I would calibrate them with darks/flats/bias - or however you do it - then debayer - then measure fwhm.

Frank


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: freestar8n]
      #5705001 - 02/28/13 09:46 AM

Quote:

Hi-

I just noticed you may be using a DSLR. In that case you can't measure with the raw images very well and you need to debayer them first. So I would calibrate them with darks/flats/bias - or however you do it - then debayer - then measure fwhm.

Frank




Very good suggestion Frank, thank you!

Alfredo


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Phillip Easton
sage


Reged: 12/24/10

Loc: DFW
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Alfredo Beltran]
      #5705164 - 02/28/13 11:40 AM

Alfredo,

You could also get a trial of CCD Inspector. You can easily measure FWHM of several images. It is a good tool for analyzing your images to help troubleshoot issues with your imaging system.

http://www.ccdware.com/products/ccdinspector/

I would take images with short exposure 15-30 sec and then at times you normally image at. This way you can get a better indication of performance of the mount. If the two sets are close then I would think the mount is doing what you want. If the short subs have large FWHM then of course there may be other issuse like colimation, focus, etc. affecting your performance.

Here are the results from my last imaging session. I am imaging at 0.89 arc-sec/pix with my Canon 60D and have adapted a Hotech T-adapter to keep the camera centered. From an analysis of 10 files while guiding for 5 mins each, I neglected to get shorter exposures for analysis last time to compare.

Min FWHM: 2.08"
Max FWHM: 3.50"
Curvature: 31.6%
Tilt in X: -0.1"
Tilt in Y: -0.3"
Total Tilt: 9% @107 deg
Collimation 3.7"
Stars Used: 2483

The numbers don't tell the whole story. When you look at a plot you see how the FWHM changes across your image. Mine is fairly flat but I get curvature in the extreme corners so that is were the stars start getting fat and the max goes to 3.5. The average value was around 2.5.

The rest of the information tells you about colimation and whether your camera is tilted.

Cheers!

Edited by Phillip Easton (02/28/13 11:41 AM)


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Alfredo Beltran
sage


Reged: 08/01/10

Loc: Bogota, Colombia
Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Phillip Easton]
      #5705259 - 02/28/13 12:31 PM

Quote:

Alfredo,

You could also get a trial of CCD Inspector. You can easily measure FWHM of several images. It is a good tool for analyzing your images to help troubleshoot issues with your imaging system.

http://www.ccdware.com/products/ccdinspector/





Hi Phillip

Thanks for the suggestion. I will take a look. Seems very promising.

Best regards

Alfredo


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freestar8n
Post Laureate
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Reged: 10/12/07

Re: iEQ45 PE reduced to 4.5 arcsec peak to peak!! new [Re: Phillip Easton]
      #5705401 - 02/28/13 01:52 PM

Short exposures 1-15 seconds can tell you about the optics - but for assessing the autoguiding of the mount it would be better to go for many minutes - preferably at least one worm period. That's what you do when you do deep sky imaging - so it makes sense to use the mount to do what you need to do - and then measure it to see how well you did.

If someone gets small fwhm with a mount - that is a direct indication the mount is capable of achieving that fwhm in a deep sky image - because it did.

Frank


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