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130 PDS v 200 PDS for viewing and imaging?


Welrod50
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All,

Been a while since I posted on here - working lots of hours and also managing to actually get outdoors at night lately and do a bit!! So hello again :)

Anyway, I searched for this and drew a blank so am generating a new post.

I currently have a SW 130 PDS on an EQ3 pro synscan which at best gives me 1 minute subs before I get trailing (albeit slight, but it's there). I have decided I have two choices and would like some help deciding:

1. Auto guide. I am thinking SW syn guider or Neximage guider. I don't want to have to mess about with computers etc, so this idea seems ideal. If I can shoot 3 minutes or less I will be a happy man. I currently shoot 45 second subs which give me 50% more data than a 30 second sub. Usually 1600 iso with the excellent Pentax Kr, or iso 4000 if it's really faint. This iso seems very high, but it does give me a much brighter image with not too much more noise!

2. A larger aperture scope without autoguider. Now, i am thinking 200 PDS on a HEQ5 so it will take the weight of the OTA with a 2lb camera stuck to the side of it.

My query really is this...will a 200 PDS gather a lot more light than the current 130? Would I be able to shoot 45 second subs at 1600 iso and gather the same/more data than say, the 130 PDS at 2 or 3 minutes??

Does anybody have any direct experience of this and are able to assist?

Cheers all and clear skies!!! :icon_salut:

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The 150 and 200 are really different scopes! I had a 150PDS but now use a 200PDS, almost twice as big !!! I use a HEQ5 pier mounted and an unguided Canon1000d and have great results.

I would not even consider the 150, go for the 200PDS, totally different beast!!

Ron

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Cheers guys,

Ron may I ask, is there a noticeable difference between the 150 and 200 in light capture? I know for visual it has, on paper, a big advantage and an even bigger advantage over my current 130 PDS. How might this translate in real world terms though from a mildly light polluted back garden in Staffordshire?? Would it make a big difference with galaxies? I currently get slightly coloured blobs with a bit of spiral arm structure if I'm lucky. Would a 200 PDS allow that much more detail to be captured?

Thanks :)

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If you are thinking with imaging in mind, there is absolutely no difference in image brightness between a 45sec sub with a 130PDS or a 200PDS as both are f/5. The 130 will give a wider view and be easier to guide,the 200 will give a more 'zoomed in' view but will be much more difficult to guide due to the increased focal length. The one thing you don't need for deep sky imaging is big telescope, in fact it is often a hindrance.

I would go for that HEQ5 with an auto guider but keep the 130PDS. That will thrash a 200PDS without an auto guider.

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OK, I get that - thanks. I wonder, would not an 8" aperture scope at f5 be better than a 5" aperture scope at f5? Or am I barking up the wrong tree??

Thanks again :)

ps I am leaning towards autoguiding, albeit with the current EQ3 Pro as I don't think a small guidescope will add enough weight to be detrimental to it.

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At first look it would seem that an 8" scope should beat a 5" scope, and for visual that is surely the case however, for prime focus imaging f/5 is f/5. You get more light with the 200 but due to the increased focal length it (you could imaging this as increased magnification) the extra light is spread out over a larger area so ends up the same brightness for extended objects.

Guiding is the key to long exposures and long exposures are the key to good DSO images. I don't know how well the guiding works on the EQ3Pro. I would hope it would be good, but the motors have only half the resolution of the motors in the HEQ5 (EQ5Pro is the same as the EQ3). The mount is by far the most important part of a DSO imaging set-up. An autoguider is part of the mount! Optics come last in priority. As well as precision, stability / wind resistance comes into it. This will determine how many subs you get to keep. You will get to keep more of your subs with the HEQ5 than with the EQ3. It is not just about weight but about stability and the focal length at which you are imaging, longer focal length is more difficult.

That said, there is no reason why you shouldn't get to keep the majority of say 3-5 min subs with a 130PDS on an EQ3 with guiding. The difference is that you ought to be able to keep almost all of your 5-10min subs with the same scope on an HEQ5 with a guider.

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Brill, thanks for that. My EQ3 Pro has autoguider interface built into the handset. Does the HEQ5 have this feature also do you know? I think I will be investing in a HEQ5 if it does.
Thank you again !
:)
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Both the HEQ5 Pro Syntrek and Pro Synscan models have the ST4 autoguider port. It's on the mount body rather than the handset. Not sure about the base model HEQ5 though? Since you wanted to avoid computer hook-up, you want the Synscan version.

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  • 1 year later...

I must choose to express a differing opinion. F ratio has only to do with exposure values. Number of photons captured in a given time is strictly a function of aperture. A 200mm will capture 1.77x the photons in the same amount of time as the 150. Both of these scopes are relatively wide field at prime focus, so unless one is trying to get the very largest objects in FOV without a mosaic, more photons concentrated on the target means more of the fine detail will be above the read noise in the same length exposure. If you must crop to the target anyway, the "equal brightness" of the smaller aperture rapidly goes away.--Jack

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I must choose to express a differing opinion. F ratio has only to do with exposure values. Number of photons captured in a given time is strictly a function of aperture. A 200mm will capture 1.77x the photons in the same amount of time as the 150. Both of these scopes are relatively wide field at prime focus, so unless one is trying to get the very largest objects in FOV without a mosaic, more photons concentrated on the target means more of the fine detail will be above the read noise in the same length exposure. If you must crop to the target anyway, the "equal brightness" of the smaller aperture rapidly goes away.--Jack

To look at aperture alone is almost meaningless with regards to AP. You are comparing apples with oranges here.

With my camera the 200 yields a resolution of 1.33 arcseconds/pixel, the 150 yields 1.77 arcseconds/pixel. The 150 will illuminate the chip faster than the 200 and the 130 will be faster still. Think of it as the amount of sky focused on the chip rather than the size of the hole in the end of the telescope. More sky = more photons.

But that's beside the point as all three telescopes provide a completely different field of view, bigger aperture doesn't make it better, just different.

Choose the telescope for the field of view and the resolution for your chosen camera, not because it's got a bigger aperture.

Edited by wuthton
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As far as you go, I agree. The objective , however is not usually just to illuminate the chip, but to capture the object with an acceptable exposure, and as much detail as possible. To use analogy, carrying the logic to an extreme a 200" Palomar would be inferior to a 150PDS, or an 80mm with a focal reducer would beat a 200mm Most DSO's clusters, etc. are quite small, and would fit well in the FOV of either scope. Less cropping is better.  Olly Penrice has written some nice pieces supporting the aperture rules POV.

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To use analogy, carrying the logic to an extreme a 200" Palomar would be inferior to a 150PDS, or an 80mm with a focal reducer would beat a 200mm Most DSO's clusters, etc.

It depends on the mount, camera and target. All three can be the "best" depending on the circumstances.

With my mount and camera I'd choose the 130pds (actually I'd choose a fast frac) and not the 200. I wouldn't touch the 250 with a ten foot pole.

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How about the 200pds on HEQ5 as you suggested but retain the 130pds OTA for use when you want a more stable imaging platform. You would then have a 200pds on HEQ5 for visual / high magnification work and AP under good conditions but the 130pds on the HEQ5 for AP in poorer conditions (or just when you don't want to lug a 200pds around)

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I had forgotten about this thread :)

In the end as much as I loved the 130PDS and the results it gave me, I opted to go smaller to lessen the weight on the mount and give a wider FOV for DSO work and purchased a TS 65mm quadruplet refractor.   The mount doesn't appear to notice the difference in autoguide accuracy and stress and still goes for ages (I once tried a 15 minute sub, which had zero trailing!) But what I have gained is an easier scope to move about which provides superb subs now with my full spectrum EOS 500D and CLS CCD filter fitted. 

Still using the EQ3 Pro which guides absolutely fine!  Problem now is, I cannot seem to get a clear night anymore.  I seem to be at work on either noons or nights whenever the stars are out.  When I'm off, it is cloudy.  Go figure :rolleyes:

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