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Ioptron Cem25p astrophotography question

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#1 SunBlack

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Posted 11 January 2019 - 04:20 PM

hello, would i have pinpoint stars with 3 minutes at 0° DEC with a 300mm lens on full frame dslr, on a cem25p?



#2 photoracer18

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Posted 11 January 2019 - 04:59 PM

How are you focusing it? How are you polar aligning it? What is meant by 0 degrees DEC?

#3 OldManSky

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Posted 11 January 2019 - 05:03 PM

Depends on YOUR cem25p.  And on the imager you're using.  And the wind.  And the seeing.  And...well, you get the idea.

 

The cem25p "guarantees" +- 10 arc-secs PE.  That's a potential swing of 20 arc-secs.  It doesn't guarantee when it will happen.

 

If your image scale is 20 arc-secs per pixel, you'll never see that.  If your image scale is 10 arc-secs per pixel, it'll be minor.  If your image scale is 4 arc-secs per pixel, you'll see it in the images.

 

You can use the PEC in the mount to get much of that out, and then you'll be fine at most image scales.  But unfortunately the PEC isn't permanent -- so you'd need to do that at the start of every night.  

 

And we haven't even talked about a good polar alignment so you don't get DEC drift yet...

 

Have you measured your mount's PE yet?  That'll tell you whether you will see it or not, assuming really good polar alignment.

 

You can calculate your image scale here: http://celestialwond...eScaleCalc.html (free calculator)

 

I just managed to measure my cem25p's PE last night (finally had enough clear sky and the guider working to do it) -- the mount is just over 3 weeks new.

It shows about +- 6 arc-secs.  Better than spec.  But I'll still guide it out, since I'll be imaging at about 2.4 arc-secs / pixel (usually!).



#4 scadvice

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Posted 11 January 2019 - 05:27 PM

Not sure where your going with this question but...

 

If your polar alignment is good you should have as pinpoint as the "seeing" will allow.

 

"Seeing is dependent upon the degree of turbulence in the Earth's atmosphere for a given telescope. Scintillation, the “twinkling” of stars to the unaided eye, is a commonly known result of turbulence in the higher reaches of the atmosphere."

 

The mount can do what you want be with certain limits as above and I think you can also look at this site for resolutions possible with that camera and lens. If it's not here I'm sure someone else will post the right site.

 

http://www.blackwate...maging-toolbox/



#5 SunBlack

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Posted 11 January 2019 - 05:48 PM

Dslr lens manual focus. Live view. 0 degrees Dec I mean imaging at equatorial .

Dslr would be a 6d or a 5d mkiii .

Polemaster. 

No autoguide. 

3m is possible with pinpoint stars?



#6 gotak

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Posted 11 January 2019 - 05:56 PM

Likely no as your image scale is 4.5 arc second per pixel. But you'll get some subs that are good. Likely 1 minute would work better.


Guiding will fix that easily. You'll need a cem25ec for longer exposures unguided which is a lot more expensive.

#7 SunBlack

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Posted 12 January 2019 - 01:23 AM

1m???? Gosh ..... I need 3 minutes at 300mm!

#8 Tapio

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Posted 12 January 2019 - 02:16 AM

You can stack those 1min exposures.

#9 SunBlack

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Posted 12 January 2019 - 03:01 AM

1m will produce heavy underexposed frames

#10 Tapio

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Posted 12 January 2019 - 03:18 AM

What target ?
And you can stretch the stacked image to reveal more.

#11 SunBlack

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Posted 12 January 2019 - 03:46 AM

Hard targets: dark nebulae, faint emission nebulae, etc. Underexposed frames are useless

#12 sg6

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Posted 12 January 2019 - 04:24 AM

I think you are tinking of photography not astrophotography.

In AP you get several images and stack one on the other to build up the final image. In effect a 3 minute exposure is made up of say 3 one minute exposures  all stacked on one another. Better still is you take 5 or 10 one minute exposures and stack the whole lot on top of one another.

 

An unguided CEM25 will begin to fail at about 60 seconds, it could begin to drop performance after 40 seconds. And all depends on the set up, balance, polar alignment.

 

Pin point stars are a feature of the lens, the mount just tracks. Bad tracking would result in trailing of the stars but that is with a good well focused lens give sharp tracks.

 

If not already done then set up the equipment and get yourself say ten exposures each of 60 seconds, then download Deep Sky Stacker and stack the gathered images.

 

It is a little more involved, but not too much, at least at the simplest level. At this stage stay with standard jpegs, DSS will stack jpegs. For extraction of any color you will need to post process and that is more involved and the software used determines the steps required.



#13 gotak

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Posted 12 January 2019 - 08:11 AM

Guiding isn't hard and in the grand scheme of things, not that expensive to gear up for.

#14 SunBlack

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Posted 12 January 2019 - 08:21 AM

Hello folks. I am an experienced imager. I just want opinions on tracking accuracy of the cem25p. I have not it yet. I thought to be in the market for it soon.
I do exclusively unguided astrophotography. Never in my plans to buy an autoguider

#15 SunBlack

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Posted 12 January 2019 - 08:23 AM

It is a no sense for me to made a 3x1m stack instead of 3m single exposure. To use 1m I should raise up ISO at too high limits. Remember I want to image dark nebulae and faint emission nebulae with uhc filter which absorbes a lot of light

#16 gotak

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Posted 12 January 2019 - 09:16 AM

Cem25ec then. Since it's spec at 0.3" RMS, you can shoot likely any length of exposures with DEC and diffraction being the only limiting factors.

#17 SunBlack

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Posted 12 January 2019 - 10:11 AM

Ahahah 2520 euro ahahah are u seriously joking? Ahahah i am sub 1000 euro budget

#18 ThatsMyCoffee

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Posted 12 January 2019 - 10:29 AM

It is a no sense for me to made a 3x1m stack instead of 3m single exposure. To use 1m I should raise up ISO at too high limits. Remember I want to image dark nebulae and faint emission nebulae with uhc filter which absorbes a lot of light

 

If you want good results, and the mount can't do better than 1 minute exposures, you're going to be taking a lot of subs.  Not 3 x 1.  More like 50 or 100 or 300...  Stacking so many photos is what helps the signal to noise ratio.  You may need a higher ISO (or not...), but in the end, the result will be less noisy than if you did a single photo at 3m.



#19 SunBlack

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Posted 12 January 2019 - 10:46 AM

Hey if i do 3m 6400 iso + uhc, how much iso for 1m? More than 12800 for sure. Losing DR and details all washed out by noise, even stacking 100 150 200 subs

#20 Tapio

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Posted 13 January 2019 - 05:02 AM

Using 12800 instead of 6400 you are just using multiply operation in camera - you could do that in computer too.
If you are limited to say 1min subs you really should understand the concept of stacking.

#21 ChrisWhite

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Posted 13 January 2019 - 09:31 AM

The OP understands stacking. He wants a mount to do unguided imaging at 3 min with perfect stars.

I don't think you can expect that from the cem25p.

Are you using a 300mm f4 lens? Something light? You might look at astrotrac 320. It does not have goto, but might have low enough PE. There are other limitations, but might be worth a look. I think it has PE of 5 arcseconds peak to peak over 5 min.

You could also look at stand alone guiders.

The cem25p is a great mount but you do need to guide.

Edited by ChrisWhite, 13 January 2019 - 09:35 AM.


#22 SunBlack

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Posted 13 January 2019 - 12:37 PM

300F4 yeah



#23 StarMike8SE

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Posted 14 January 2019 - 09:46 AM

You either need to look at guiding, or buy a higher end mount.  The CEM25P needs to be guided




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