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to view peleades?

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#1 JamesL

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Posted 30 December 2012 - 12:48 AM

Hi folks, I am trying to view Peleades in my c8 f/10 and cant seem to get the whole cluster in view. I purchased a meade 4000 series 56mm super plossl for this purpose but it still doesnt all fit in the field of view.Do I need to get a focal reducer? Would one be handy to have for wider views or would it be something that wouldnt be used much? I just look visually, no photograghy, thanks..James

#2 Jon Isaacs

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Posted 30 December 2012 - 01:14 AM

James:

No matter what you do with a C-8, fitting in the entirety of Pleiades is beyond it's capabilities. A focal reducer is a help but the focal reducer cannot change the fundamental limitations of the telescope.

The C-8 has a focal length of 2032 mm with a rear port of 38mm. This means that anything greater than a 1.07 degtree field will vignette. You can push this but the Pleiades are close to 2 degrees.

The 55 mm Plossl provides a 1.3 degree TFoV you might get some more with a focal reducer but the exit pupil will be quite large and you may begin to see the shadow of the secondary. There are 40mm eyepieces that provide the same TFoV as the 55 mm Plossl, they would be a better bet.

In general, smaller, shorter focal length scopes and binoculars are the best choices for viewing the Pleiades, the strengths of your C-8 lie elsewhere.

Jon

#3 JamesL

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Posted 30 December 2012 - 02:10 AM

ok, thanks Jon for that explaination. I have an old dob 8"f/4 maybe ill clean it up and take a look through that one but it doesnt track, but thats ok.

#4 Tony Flanders

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Posted 30 December 2012 - 06:50 AM

Binoculars do a great job on the Pleiades. Life without binoculars is barely worth living, and astronomy without binoculars is barren indeed. If you don't already own a pair, they're the best astro investment you will ever make.

A high-quality finderscope also does a fine job.

#5 Jon Isaacs

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Posted 30 December 2012 - 07:14 AM

ok, thanks Jon for that explaination. I have an old dob 8"f/4 maybe ill clean it up and take a look through that one but it doesnt track, but thats ok.


With the right eyepiece, your 8 inch f/4 would take in the entire Pleiades. Normally I would not suggest a 55 mm Plossl for an f/4 telescope because of the whopping 14 mm exit pupil. With your eye only open to 7mm or less, most of the light does not enter the eye.

But the Pleiades are plenty bright and the large exit pupil transforms your 8 inch f/4 into a 4 inch f/8 which should provide cleaner views away from the center of the field. The CO would be doubled.. that could be a problem.

A 3 inch off-axis aperture mask would do the trick. It's just a piece of cardboard with a hole.

Jon

#6 edwincjones

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Posted 30 December 2012 - 07:35 AM

Binoculars do a great job on the Pleiades. Life without binoculars is barely worth living, and astronomy without binoculars is barren indeed. If you don't already own a pair, they're the best astro investment you will ever make.

A high-quality finderscope also does a fine job.



:waytogo: :waytogo:

edj

#7 newtoskies

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Posted 30 December 2012 - 08:05 AM

I found binos great for M45.

#8 Ed D

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Posted 30 December 2012 - 08:37 AM

Jon and Tony nailed it! The only thing I can add is that with the 8" f/4 and a good wide FOV eyepiece the lack of tracking is almost a non-issue. Make sure the Dob bearing surfaces are clean and you have smooth motions.

Ed D

#9 lamplight

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Posted 30 December 2012 - 10:55 AM

See a used pair of 7x50 bushnell fog proof for $60, good deal? Need to get a pair , think wife will like it if she ever decides to observe with me again. Pleiades is gorgeous through my stellarvue finder!

#10 Achernar

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Posted 30 December 2012 - 11:22 AM

Your telescope as it is right now can't take in the Pleiades into a single field of view not only due to the focal length, but also due to the diameter of the rear port where the diagonal is attached. Using a reducer with that eyepiece will result in a drastic loss of aperture and increased sky glow, and there could also be vignetting due to the size of the rear port. You may be able to use that reducer with a 30mm or so 82 degree eyepiece, or even a 24mm 82 degree eyepiece, which will accomadate an object this large. Using a 30mm eyepiece with a F/6.3 focal reducer will give you an exit pupil just under 5mm, a good range for both light pollution and aging eyes considerations. And you will get a true field of view nearly two degrees across, more than enough for the Pleiades, or Beehive clusters. A focal length no greater than about 1200mm is a must if you want to fit objects as large as the Pleiades into one view. A smaller telescope with shorter focal length and a wider field of view would be an invaluable partner for your C-8. I can fit the Pleiades into the field of view of my 6 and 10-inch Dobs with an Explore Scientific 24mm 82 degree eyepiece. Both telescopes have a focal length of 1,200 and 1,315mm respectively, similar to what a C-8 would have with a F/6.3 focal reducer in place. The reducer will also help you a lot with other deep sky objects like the Andromeda Galaxy and M-42, which benefit from the larger field of view. The reducer also improves star images at the edges of the field, SCT's do have coma but not as much coma as my F/4.5 Dobs do without a Paracorr in place. If you decide to try prime focus photography, it will dramatically reduce exposure times.

Taras

#11 WAVT

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Posted 30 December 2012 - 01:36 PM

a 32mm plossl eyepiece will take in the Pleaides quite nicely in your 8" f/4 scope. The exit pupil is still a bit oversized but I suspect the view will be quite good. If your 8" f/4 scope takes 2" eyepieces you would have even more options for taking in a big wide field.

#12 kenrenard

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Posted 30 December 2012 - 03:56 PM

Binoculars do a great job on the Pleiades. Life without binoculars is barely worth living, and astronomy without binoculars is barren indeed. If you don't already own a pair, they're the best astro investment you will ever make.

A high-quality finderscope also does a fine job.



Nothing works as easy as binoculars. As a beginner not that long ago I found binoculars to be the best tool you can use. I use my 10 X 50 binoculars all the time for astronomy and birds.

#13 JamesL

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Posted 30 December 2012 - 11:54 PM

ok, I ordered a focal reducer and I have a 32mm 2"Rini eyepiece and 11mm es 82 eyepiece. Its cloudy tonight(snow coming again)so I cant use either scope tonight. The 8" dob is one that I built a few years ago and its ra movement is kind of stiff but the views were nice.Also it is too heavy and bulky to set up. Think ill concentrate on the c8.Last night I had great views of jupiter with 17mm tv plossl and 10mm orion explorer ii eyepieces. Then looked at orion nebula with es 11mm 82 and it was awesome!Having lots of fun with the c8.Also have the 20 and 15mm superviews, 25mm silver top. A new eyepiece is always coming in the mail, I gotta stop spending for a while. Anyway thanks for all of your input...James

#14 Mark Costello

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Posted 02 January 2013 - 02:45 PM

a 32mm plossl eyepiece will take in the Pleaides quite nicely in your 8" f/4 scope. The exit pupil is still a bit oversized but I suspect the view will be quite good. If your 8" f/4 scope takes 2" eyepieces you would have even more options for taking in a big wide field.


I agree. I have a 5" refractor with a focal length of 825 mm and view the Pleiades often with a 31mm Celestron Ultima eyepiece (gives about 27X and a 2.6 deg RFOV). It does the job of framing the entire cluster. A couple of weeks ago, I observed and "mapped" M45 with a 17mm Celestron (49X and about 1.4-1.5 deg RFOV). The cluster was a bit big for the field of view but I'm glad I did this anyway because it probably helped to catch and map a few extra of the dimmer stars.

#15 Widespread

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Posted 03 January 2013 - 11:17 AM

I have an 8SE (C8), and had the same problem with insufficient TFOV for large objects.

I got an Orion ST80 for $179 and sold off the Expanse EPs on Astromart for a net cost of about $115.

Since you have a fast Dob, this may be superfluous information, but I think a short FL refractor is a great complement for powerful but FOV-challenged SCTs.

If I'd known how much I would love this little refractor, I would have gotten small ED scope. (I still might ;p)

Best,
David

#16 kenrenard

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Posted 03 January 2013 - 11:52 AM

If I'd known how much I would love this little refractor, I would have gotten small ED scope. (I still might ;p)


I know what you mean. I have an astro Tech 72ED and its a great wide field scope.

#17 soliari

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Posted 03 January 2013 - 12:00 PM

I agree. I have a 5" refractor with a focal length of 825 mm and view the Pleiades often with a 31mm Celestron Ultima eyepiece (gives about 27X and a 2.6 deg RFOV). It does the job of framing the entire nebula. A couple of weeks ago, I observed and "mapped" M45 with a 17mm Celestron (49X and about 1.4-1.5 deg RFOV). The cluster was a bit big for the field of view but I'm glad I did this anyway because it probably helped to catch and map a few extra of the dimmer stars.


I hope you don't mind me jumping in on this thread, but what I'm gathering from what everyone is saying that you need right around 2 degrees for TFOV to view the Pleiades with any success? If so, it seems my 1000mm refractor will have a hard time viewing it ... I was looking at purchasing a 44mm eye piece with 40 degrees FOV. If I calculate correctly that should yeild 1.7 degrees of TFOV ... too narrow for gathering the entirety of the Pleiades?

#18 ensign

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Posted 03 January 2013 - 01:26 PM

To the OP, I solved the problem of narrow fields in an SCT by dual mounting a small (80mm) refractor with the SCT. The little scope does double duty as a finder and a wide-field viewer.

With a 40mm eyepiece in the 80mm refractor, I get a whopping 5.2 degrees of true field - way more than enough for the Pleiades and other extended objects. The same EP in the SCT gives a mere 1.1 degree true field, but the views are magnificent.

#19 Tony Flanders

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Posted 03 January 2013 - 01:42 PM

I hope you don't mind me jumping in on this thread, but what I'm gathering from what everyone is saying that you need right around 2 degrees for TFOV to view the Pleiades with any success?


I'm not sure about "success." The Pleiades are still pretty nice viewed piecewise. But to my taste, you need a 2-degree field of view to frame them to best aesthetic effect. Maybe even a little more than 2 degrees, if anything.

If so, it seems my 1000mm refractor will have a hard time viewing it.


That's correct, unless your refractor has a 2-inch focuser. The maximum FOV achievable with a 1000-mm focal length and a 1.25-inch eyepiece is about 1.6 degrees. Personally, I would use a 32-mm Plossl rather than a longer eyepiece with a narrower apparent field of view to achieve that.

#20 soliari

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Posted 03 January 2013 - 02:55 PM

Tony,

What I meant by success was to frame them in all together.

Hmm ... i didn't think about the 2" eye piece route. Right now i'm setup for 1.25, but i think it can easily host a 2".

#21 Tony Flanders

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Posted 03 January 2013 - 09:46 PM

Right now i'm setup for 1.25, but i think it can easily host a 2".


On a scope with a slow focal ratio, using 2-inch eyepieces will open up whole new worlds. Expensive but well worth it.

#22 Dennis_S253

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Posted 03 January 2013 - 11:36 PM

I guess I don't understand what you are all looking at. Sure there maybe a couple other stars that you could include but...

Attached Files



#23 soliari

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Posted 03 January 2013 - 11:59 PM

Dennis, I am not well versed at what you're displaying here. Can you tell the novice blatantly?

#24 Jeff2011

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Posted 04 January 2013 - 12:28 AM

With my 8in Dob and my 38mm ep I can view the whole Pleiades. Tracking is not that big of an issue with a low power wide angle ep.

Happy viewing!

Jeff

#25 Allan...

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Posted 04 January 2013 - 12:34 AM

Hey Jeff.....I was curious about the Q70 38mm for an XT8. I am almost 60 (so eyes not the greatest exit pupil) and I was told by some, that a 38mm is too low; it would have an exit pupil of 6.44 and Im sure that my eyes are about a 5 or less now. Any comments? thanks, Clare The lowest that I have now is a 30mm Owl 2" and its OK but not as bright as a 24 or 25mm (for MY eyes, anyways).






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