#1  
Old 17-06-2011, 10:00 AM
binofied
Registered User

binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Question Why go bigger?

Obviously my last question was to vague. What I want to know is what is the overriding reason to use a bigger diameter mirror in imaging?

Is it purely to get the focal length to frame the object better with your specific CCD size or does it give you resolution or contrast gains? Other than focal length are there any reasons to choose say a 12" GSO RC over a 6" GSO RC?

I am in the process of getting an STL-11000, I have a Losmandy G11 with an O-Vision worm and I'm thinking of buying a 10" GSO RC. Is this a good match?

Dave
Auckland
Reply With Quote
  #2  
Old 17-06-2011, 11:17 AM
Logieberra's Avatar
Logieberra (Logan)
Registered User

Logieberra is offline
 
Join Date: Mar 2010
Location: Canberra
Posts: 1,546
A bigger mirror means more light gathering ability. Stars and deep sky sky objects are fait points of light a long way away... with a bigger mirror you'll see much more detail, or in the case of imaging, your images will go 'deeper' with a bigger mirror...

Check out this link, head 1/3 down the page on the right hand side, and click the M13 Comparison image... I hope this helps.
http://www.obsessiontelescopes.com/t...22UC/index.php

I also have a G11, and have been watching Paul's rolling review of the GSO 12" RC (non-CF version) with great interest. I think for the time being the 10" GSO RC (CF version) would be a great combo with the G11.

Good luck
Reply With Quote
  #3  
Old 17-06-2011, 01:31 PM
binofied
Registered User

binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Yes I have seen those visual representative type images before, has anyone ever done them for real, from an imaging point of view, with different size telescopes to show what the difference actually is?

So say I had a 6" F10 and a 12" F5 both would have the same focal length and image scale. It would take heaps longer to gather the same number of photons on the 6" due to the low F number. Would they both produce exactly the same image if you gave them different length exposures to account for the smaller aperture or would the bigger telescope have more detail and win the day?

This review of the 12" RC is definitely part of why I am asking this question. The current wisdom says a G11 won't carry the 12" but I have heard of C14 tubes on the G11. So is there any advanage going for a 12" rather than a 10"?
Reply With Quote
  #4  
Old 17-06-2011, 01:42 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
Resolution is the other big one. A well set up big scope should always beat a smaller aperture instrument. You won't always be able to see the difference if seeing is poor, but when its great that's when the extra resolution (and light gathering) of the big instrument pulls away.

BUT, but instruments are big heavier, a PITA to set up (better in an observatory), very expensive, require very high quality (EXPENSIVE) mounts, usually require exacting collimation. They also tend towards longer focal lenghts with tighter fields of view that won't suit larger objects. You can get around this to varying degrees with EXPENSIVE correctors and focal reducers.

Astrophotography requires intimate experience with a number of key variables (tracking, balance, software, FOV, image processing, focus, etc) for success. All these things are very exacting once you get past a focal length of 1m. The 2m FL of the 10" will be challenging unless you have lots of experience and high quality gear.

Most people have a lot more fun imaging at under 1m until they've really been pulled over the imaging event horizon....

CCDcalc (freeware) is worth a play to get a feel for the effect of FOV with different scopes and cameras.
Judging by your website pics you're no newbie though....

Last edited by RobF; 17-06-2011 at 06:42 PM. Reason: Spelling
Reply With Quote
  #5  
Old 17-06-2011, 01:52 PM
binofied
Registered User

binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Thanks Rob, I have been working at 1600mm for several years now with my G11 with an O-Vision worm, Meade 10" OTA with 0.63x reducer and an ST2000. I am about to buy an STL-11000 which I think will be to much for the Meade and I'm looking to get some better optics hence the interest in the GSO 10" or 12". Find attached one of my better images, so you can see where I am at. To get a bigger OTA is the real question. Yes I do have CCDcalc a great program.
Attached Thumbnails
Click for full-size image (cent a.jpg)
164.6 KB99 views
Reply With Quote
  #6  
Old 17-06-2011, 06:44 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
Great Centaurus A

Be interested to hear how you go in the quest. Hagar (Doug) has experience with 10" GSOs and just may have had his 11000 based camera in too if I recall correctly. Might be worth a PM.....
Reply With Quote
  #7  
Old 17-06-2011, 07:24 PM
DavidTrap's Avatar
DavidTrap (David)
Really just a beginner

DavidTrap is offline
 
Join Date: Jun 2009
Location: Brisbane
Posts: 2,828
Very impressive Centaurus A - some might ask why try and improve on that???

Is a GSO RC going to be better than a Meade OTA? I would have thought they were in the same "class".

DT
Reply With Quote
  #8  
Old 17-06-2011, 10:11 PM
ericwbenson (Eric)
Registered User

ericwbenson is offline
 
Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 200
Main answer: a bigger bucket gives you more photons per unit time. As a bonus, if the seeing conditions support it, the resolution increases with aperture too. See this page for a comparison between a 4" refractor and a C11:
http://www.faintgalaxy.com/ngc6720.htm

However in your case, since you are looking at a big chipped camera you really should be worried about the fully corrected field diameter i.e. the aberration free image circle, or don't bother with a big chip. On a Schmidt-Cass the corrected circle is very small, perhaps <5 mm in diameter, see this pic:
http://www.faintgalaxy.com/ngc6341.htm
the seeing was very good and the increase in coma from the center is evident (magnify the image in your favorite software).

The RC is better than the SC no doubt, but without a field flattener the STL on the RC may still be disappointing, so factor that into the image train. Actually very few telescopes can present a large enough image circle and fully illuminate the STL-11K, and their cost is usually shocking, but they do exist. See here for a comparison:
http://www.faintgalaxy.com/ic0434.htm
same camera as above, close to the same focal length but different optical design.

EB
Reply With Quote
  #9  
Old 17-06-2011, 10:35 PM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
Yes, and that's another reason people love Tak FSQ106s or ASA Newts - its tough to get such big flat circle of light as Eric has said.
Reply With Quote
  #10  
Old 18-06-2011, 11:43 AM
binofied
Registered User

binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Thanks for all you replies guys. Yeah I was aware that I would have to use a flattener with the GSO, I read somewhere that over about 24mm image circle it was really needed. My current thinking is that on day one I have my Meade with a 0.63 reducer/ flattener. At least I will be able to use the same area as my ST2000 and probably a bit more before I have to start throwing away pixels that are to bad either from vignetting or coma etc. I also have a 5yo WO Megrez 80mm F6 APO triplet and a Televue 0.8 flattener, that I think probably will have a biggish corrected FOV. So I am then looking at my next move.

Looks like I may be a bit constrained by my G11 as to the max size scope I can carry and still expect very good guiding etc. I have tried out an 8" GSO RC on this mount and it is so light it handles it with ease. It gives EXACTLY the same results in terms of resolution and image scale as my Meade 10" OTA at F6.3 with reducer. The only measurable or visible difference was a 20% drop in peak brightness of the stars on the 8". The 8" should have been worse but it had several things going for it. No corrector plate or focal reducer and higher quality in the reflectivity of the new coatings. Both ran at 1600mm and I could not tell the images apart visually by flicking between them and only just using measurements. The FWHM was identical for example. In a blind test my friend could not tell me which one was which.

I wanted to get a sense of the improvement I may get from going from an 8" RC (the benchmark being so much like my current setup) to either the 10" or 12" GSO RC.

Adding the STL-11000 adds extra complication due to it's large field. I don't know if combo focal reducer & flattener for these scopes exist to mitigate the extra focal length making tracking etc harder. However I don't have any tracking error issues now so an extra 400mm or so shouldn't be to much worse.
Reply With Quote
  #11  
Old 18-06-2011, 03:07 PM
mithrandir's Avatar
mithrandir (Andrew)
Registered User

mithrandir is offline
 
Join Date: Jan 2009
Location: Glenhaven
Posts: 4,161
Losmandy say a G11 can carry 60lb of instruments as long as the scope is not too long and acting like a sail, like a big Newt out in the middle of a field. Rainer has 2 G11s with 65lb payloads. A picture of one of them is linked from here.
Reply With Quote
  #12  
Old 18-06-2011, 05:53 PM
binofied
Registered User

binofied is offline
 
Join Date: May 2005
Location: Auckland New Zealand
Posts: 44
Well I was thinking the 12" is 22kg Camera is 2kg finder and SBIG remote guider about 1kg = 25kg or 55lbs. On the limit I know.

The mount is totally rock solid you can bash it on the side and not feel it move with the other hand. It's mounted on a massive concrete pier that is at least 1 meter square going down several meters! I used to hold a 22" cassagrain. So there is zero chance of any vibration type issues.

We are surrounded by trees and in a bit of a dip. I can't be bothered imaging anything low down so this doesn't ever cause an issue. Even in howling winds we hardly notice it blowing in the observatory. I currently have a large yellow dew sheild on the front of the 10" so I would have had wind issues by now if I was going to.

So it's just a question of if the mount can track and autoguide well enough with the extra load of a 12". It sure would be nice to have an instrument of this size.
Attached Thumbnails
Click for full-size image (IMG_3889.JPG)
191.8 KB31 views
Click for full-size image (IMG_3895.JPG)
144.9 KB36 views
Reply With Quote
  #13  
Old 18-06-2011, 05:57 PM
gbeal
Registered User

gbeal is offline
 
Join Date: Oct 2004
Location: New Zealand
Posts: 4,301
Really, this is leading to, "why not just try it", sorry mate.
Gary
Reply With Quote
  #14  
Old 19-06-2011, 04:56 AM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Bribie Is.
Posts: 3,124
Quote:
Originally Posted by binofied View Post

Adding the STL-11000 adds extra complication due to it's large field. I don't know if combo focal reducer & flattener for these scopes exist to mitigate the extra focal length making tracking etc harder. However I don't have any tracking error issues now so an extra 400mm or so shouldn't be to much worse.
The 11000 series sensors are 24x36mm (i.e. 43mm corner-to-corner). I have an ATIK 11002M and I think the SBIG STL series will be the same. To get best use out of such a chip, without vignetting, you have to give thought to just how much of the chip your scope can cover with a flat field. My main scopes are in the C14 size-range and their rear cell is 48-50mm I think (the EdgeHD1400 is a bit bigger than the C14). But then, a reducer or reducer/flattener is likely to bring that down to 40mm or so. The standard Celestron .63 reducer has a clear glass diameter of no more than 40mm. An AstroPhysics .67 is a little bigger but not much. That means that you have to seriously think about using 3" gear to get the coverage.
Peter
Reply With Quote
  #15  
Old 25-06-2011, 09:10 AM
garymck (Gary)
Registered User

garymck is offline
 
Join Date: Aug 2005
Location: Geelong
Posts: 685
I've never actually understood the pursuit of bigger scopes for imaging. Though I can understand going for a bigger scope, but keeping the focal length the same. Once our image scale is at our average level of seeing (fl dependent on pixel size and average local seeing) what is the purpose of going for a longer focal length? There are no gains to be had in extra detail, because detail will be seeing limited ..... AO systems might stretch things a bit, however I think the pursuit of bigger stuff is more about the pleasure of ownership than in getting more information....There are of course lots of fabulous images taken with 16" f8/f6 RC's etc, but I honestly doubt if they would be any better than if they had the same exposure with an 8" RC and were processed with the same level of skill.....

my 2 cents
cheers
Gary
Reply With Quote
  #16  
Old 25-06-2011, 10:24 AM
RobF's Avatar
RobF (Rob)
Mostly harmless...

RobF is offline
 
Join Date: Jul 2008
Location: Brisbane, Australia
Posts: 5,509
I don't think you can ignore the benefits of pulling in more photons per unit time and the likelihood that you will end up with more resolution. There's more than one thread here about big scope versus small through poor seeing and what the final result will be.

I suspect in this case the question is more about CCD real estate size than scope size. It really is genuinely hard to evenly illuminate the larger CCDs becominga available to amateurs, but its hard to ignore how nice it is to swim around in the big fields they can realise rather than having to resort to mosaics.

Personally I'm constantly amazing and pleased by what my relatively straightforward 8" rig can do, so thank goodness we don't all need 16803 chips and 8" Taks (even if they did make them... )
Reply With Quote
  #17  
Old 25-06-2011, 10:41 AM
Hagar (Doug)
Registered User

Hagar is offline
 
Join Date: Oct 2006
Location: Australia
Posts: 4,646
Hi Dave, Been watching and reading a lot of the discussions lately on various scopes for imaging and gather from what you are saying that your main interest is in photographing nebulas. If this is the case then my advice would be one of the wider field scopes such as the FSQ or maybe an 8 or 10" Orion optics such as Mike Sidonio's new beast. May be these are quite expensive scopes but well worth the expense. I can relate to an FSQ as I have one and find the only drawback is the need to maintain focus with changing temporatures. At 7KG it is a beautiful flat field and with the 11000 is a huge field for even the big nebulas. The one thing with the Tak and the 11000 is often very small stars only fill just 1 pixel hence my move back to a smaller pixel size camera. I have plenty of images on my site from the 11000/FSQ combo under Atik 11000 images and recent images and some Hi res images.
Star size with the refractor is much smaller and more pinpoint than you will get with the RC's and Cassagrain scopes. Certainly a fast scope eleviates the need for appature to some degree.

Good luck
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 07:48 AM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
SkyWatcher Australia
Advertisement
OzScopes Authorised Dealer
Advertisement
NexDome Observatories
Advertisement
Bintel
Advertisement
Celestron Australia
Advertisement
Meade Australia
Advertisement
Lunatico Astronomical
Advertisement
Astronomy and Electronics Centre
Advertisement