Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Stopped down Celestron 9.25 vs Orion ED80

This topic has been archived. This means that you cannot reply to this topic.
27 replies to this topic

#1 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 16 October 2005 - 11:06 PM

This post is to help answer the question "does a stopped down SCT equal a similar size APO" asked in another thread. One of the mentioned APO strong points is the ability to show more detail under poor seeing conditions. This was certainly tested tonight.

The scopes are the Orion ED80 about a year old. Good optics and average sample of the ED80 line. The Celestron is a 9.25 XLT about a year old. Collimation was carefully checked last night.

A mask for the Celestron was fabricated with an 80mm aperture off center. Three objects were observed. M27, M31 and Mars. The moon was very full and overpowering. The OTAs were allowed to cool 4 hours in an open observatory dome. Both OTAs were mounted side by side on a Celestron CGE with a Losmandy dual dovetail plate.

First, visual observations on Mars.

Several EPs were tried in both scopes including Nagler 3-6 zoom, Meade 8.8 UWA, Nagler 11 & 13mm T6 and Orion 3.8 Lanthanum.

Best EP for the Orion ED80 was the 3.8mm Orion Lanthanum. Best EP for the Celestron was a Pentax 20mm XWA.

I did a test the previous night on these and two other scopes with Mars closing on the 12 o’clock position. The results were different with the 9.25 the best performer. During this test a day later Mars was a little below the 10 o’clock position with lots of distortion. The Orion ED80 showed ample detail and cloud bands. Not fantastic mind you but very visible. The Celestron 9.25, when stopped down to 80mm, showed little to no detail. Just a big orange disk with some occasional markings. When the mask was removed and the full 9.25 aperture was brought in the detail improved greatly. Last night when seeing was much better the 9.25 beat the ED80 and two other scopes handily. Tonight, with poor conditions, the results were about equal. So the 9.25 in poor conditions and stopped down to 80mm did not do nearly as well as the Orion ED80. This with lots of moon light and atmospheric distortion.

Then I got a little overly enthusiastic and decided to shoot some images in these dismal conditions.

#2 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 06:54 AM

These images are JPEG images from a Hutech modified Canon 300D. All exposures are 2 minutes at ASA 800. There are no darks, flats are heavy processing on any image. Also, each image has only had a mild color balance and small contrast/brightness enhancement. The idea is to post "Raw" images as acquired by each OTA. You can extrapolate what a highly processed image would look like from the single frame raw images.


Here is M27 from the Celestron 9.25 without the stopped down mask applied:

Attached Thumbnails

  • 645581-M27 Celes07_JFR.JPG


#3 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 06:57 AM

The Orion ED80 raw image (raw as exposed with JPEG setting on Canon, not raw image mode on the Canon). 2 minutes, ASA 800, minimal processing.

Attached Thumbnails

  • 645584-M27 ed310_jfr.jpg


#4 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 06:59 AM

And now the Celestron 9.25 on M27 stopped down to the same aperture (80mm) as the Orion ED80. Exposure 2 minutes at ASA 800, minimal processing. I can't make M27 out on my monitor.

Attached Thumbnails

  • 645586-M27 Celes208_JFR.JPG


#5 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 07:04 AM

The OTAs were then moved to M31. This is the Celestron 9.25 without the mask. Yes, there is trailing but I did not want to mount the STV for autoguiding in this test. I think it shows the smaller aperture is much more forgiving of mount misalignment.

Attached Thumbnails

  • 645587-M31 Celes513_JFR.JPG


#6 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 07:08 AM


Then the aperture mask was added moving the Celestron 9.25 aperture to 80mm. Andromeda with mild color correction and some brightness/contrast adjustment.

Attached Thumbnails

  • 645590-M31 Celes412_JFR.JPG


#7 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 07:10 AM

The camera was then moved over to the Orion ED80. Andromeda 2 minute exposure at ASA 800. Note the dust lanes beginning to show. Also companion galaxies. With this detail from a single unprocessed exposure it shows real potential for multiple images stacked and darks subtracted. I think Focal Length plays a very important part in the results achieved in this test.

Attached Thumbnails

  • 645591-M31 ed311_jfr.jpg


#8 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 07:22 AM


If the focal lengths had been similar the results would have been closer. However, the visual comparison with similar magnifications on Mars showed the stopped down SCT does not equal the image quality of the Orion ED80 (in this test, on this equipment, under these conditions).

In taking the above images two T-Ring adapters were used. One on each OTA. A Stiletto Pro focuser with a 300LPI insert was used to focus both OTAs. The same Hutech modified Canon 300D was then moved between the OTAs for each photo.

On the previous night the Celestron 9.25 beat the Orion ED80 in similar observations. However, in this test Mars was much lower in the sky and conditions were marginal. The refractor came through in spades while the fine Celestron 9.25 grappled with seeing problems. My conclusion (IMHO) is different OTAs have different performance characteristics that different circumstances can accentuate or aggravate. While simulated performance curves can provide a guide to the expected potential of a given instrument, real world observations have too many variables to make their use absolute.

#9 sixela

sixela

    Hubble

  • *****
  • Posts: 15260
  • Joined: 23 Dec 2004

Posted 17 October 2005 - 06:21 PM

And now the Celestron 9.25 on M27 stopped down to the same aperture (80mm) as the Orion ED80.


Effective f/29 against effective f/7.5...it's hardly a surprise to any photographer to see that the exposure times for the former are longer, is it?

#10 sixela

sixela

    Hubble

  • *****
  • Posts: 15260
  • Joined: 23 Dec 2004

Posted 17 October 2005 - 06:23 PM

I think Focal Length plays a very important part in the results achieved in this test.


Actually, if you don't care about the image scale, only the f/ratio is what determines this.

Given M31's size, the ED80 or e.g. one of the Meade SN are the natural born winners.

#11 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5746
  • Joined: 30 Sep 2003

Posted 17 October 2005 - 07:39 PM

Jerry, thanks for posting the pics, but as sixela points out, equal exposure times are not going to give equal exposures unless the focal ratios are the same - which they are not. You need to compensate exposure time for the difference in effective focal ratio to get equal exposures.

#12 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 09:00 PM


Hello guys. The exposure times and focal ratio have no bearing on what this is to show. The loss of light and the effect loss of light have on the image at longer focal rations is precisely what it is to show. This test has only one purpose. To address the question "Does a stopped down SCT equal the performance of a similar sized APO on Mars". This test took the question to heart and showed what a stopped down APO does to the light gathering. No more than that.

#13 NorthCoast

NorthCoast

    Vanguard

  • *****
  • Posts: 2308
  • Joined: 05 Dec 2004

Posted 17 October 2005 - 10:04 PM

Question One: Does image scale change when stopping down the SCT for visual use? I would think that the image would be the same size at the EP both with and without the mask...

Question two: I am not an astrophotographer, but don't the rules for additive light and f ratio apply similar to still photography e.g. faster f ratio = more light for any given aperture (in the case both at 80mm)?

Third question: Given #1 and #2 wouldn't size have to consider both aperture and focal length of the system to be really apples to apples.

Still learning,
Mark

#14 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 17 October 2005 - 10:26 PM

Question One: Does image scale change when stopping down the SCT for visual use? I would think that the image would be the same size at the EP both with and without the mask...

Question two: I am not an astrophotographer, but don't the rules for additive light and f ratio apply similar to still photography e.g. faster f ratio = more light for any given aperture (in the case both at 80mm)?

Third question: Given #1 and #2 wouldn't size have to consider both aperture and focal length of the system to be really apples to apples.

Still learning,
Mark


Hello Mark:

Question 1: No

Question 2: Yes

Question 3: Not trying to compare apples to apples. Trying to show you can't make an apple into an orange by stopping it down. You therefore show an apple, then an apple stopped down which looks like a prune. You then show the orange and they don't look the same. :grin:

#15 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5746
  • Joined: 30 Sep 2003

Posted 17 October 2005 - 11:10 PM

Question 2: Yes. Relative image brightness is related to the effective focal ratio. Not only more light at a given aperture, but equal focal ratios give the same amount of light regardless of aperture.

#16 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5746
  • Joined: 30 Sep 2003

Posted 17 October 2005 - 11:19 PM

Jerry, you may already now this, but the brightness difference between two focal ratios can be calculated:

(Effective focal ratio A / Effective focal ratio B)squared

The factor can also be used to adjust exposure time to gain equivalent exposures. JFYI.

#17 wilash

wilash

    Fairy Godmother

  • *****
  • Posts: 5746
  • Joined: 30 Sep 2003

Posted 17 October 2005 - 11:41 PM

Question 1: No. Focal length controls image scale, not focal ratio. This is confusing because astronomers sometimes use focal ratio to describe field of view, but there is an assumption that the aperture is constant.

#18 NorthCoast

NorthCoast

    Vanguard

  • *****
  • Posts: 2308
  • Joined: 05 Dec 2004

Posted 18 October 2005 - 10:59 AM

O.k. so simple question: if I mask my SCT the "size" of the image in the EP does not change but the scale in ralation to the FOV does?

I'm at work, I guess I'll have to try this at home with my SCT and just experience the results.

Thanks,
Mark

#19 NorthCoast

NorthCoast

    Vanguard

  • *****
  • Posts: 2308
  • Joined: 05 Dec 2004

Posted 18 October 2005 - 11:05 AM

Question 3: Not trying to compare apples to apples. Trying to show you can't make an apple into an orange by stopping it down. You therefore show an apple, then an apple stopped down which looks like a prune. You then show the orange and they don't look the same. :grin:


Fruit salad... they are different I get it now. ;)

Although I don't have an APO I do own a 80mm Acro I could compare to my stopped down SCT. Not sure why I would want to, but now I'm curious...

Later,
Mark

#20 sixela

sixela

    Hubble

  • *****
  • Posts: 15260
  • Joined: 23 Dec 2004

Posted 18 October 2005 - 11:45 AM

Question One: Does image scale change when stopping down the SCT for visual use?


No. The focal length is unchanged. But as the aperture is diminished, the f/ratio changes drastically.

faster f ratio = more light for any given aperture (in the case both at 80mm)?

Not more light - shorter exposure times. There are two ways to skin that f/ratio cat: more aperture, or smaller image scale (faster f/ratio, or for a given aperture less focal length).


Third question: Given #1 and #2 wouldn't size have to consider both aperture and focal length of the system to be really apples to apples.

Yes. Which is why, for determining exposure times necessary to expose a given film with an extended object, f/ratio is important (and it's focal length divided by aperture).

#21 sixela

sixela

    Hubble

  • *****
  • Posts: 15260
  • Joined: 23 Dec 2004

Posted 18 October 2005 - 01:15 PM

O.k. so simple question: if I mask my SCT the "size" of the image in the EP does not change but the scale in ralation to the FOV does?


No. The field of view is just as large, it just gets illuminated less. What you have to grasp is that each point in the field gets light contributions from the *entire* aperture...

#22 NorthCoast

NorthCoast

    Vanguard

  • *****
  • Posts: 2308
  • Joined: 05 Dec 2004

Posted 19 October 2005 - 08:10 AM

So does "shorter exposure time" and "gets illuminated less" indicate that there is more or less light at any given point?

Example: 80mm f/10 vs 80mm f/5; the f/5 has shorter exposure time because the f/10 gets illuminated less.

Thanks,
Mark

#23 jrcrilly

jrcrilly

    Refractor wienie no more

  • *****
  • Administrators
  • Posts: 35468
  • Joined: 30 Apr 2003

Posted 19 October 2005 - 08:15 AM

So does "shorter exposure time" and "gets illuminated less" indicate that there is more or less light at any given point?

Example: 80mm f/10 vs 80mm f/5; the f/5 has shorter exposure time because the f/10 gets illuminated less.

Thanks,
Mark


It's more complex than that. For example, point sources like stars will require the same exposure time for a given aperture regardless of F ratio. For extended objects, the "faster" telescope will require less exposure time because the light from an object is spread out over a smaller area, and is thus more intense at each pixel. It's still the same amount of light, just distributed differently.

#24 NorthCoast

NorthCoast

    Vanguard

  • *****
  • Posts: 2308
  • Joined: 05 Dec 2004

Posted 19 October 2005 - 10:50 AM

I think I see...

Is there a good text book on telescope physics/optics?

Thanks again,
Mark

#25 JerryWise

JerryWise

    Cosmos

  • *****
  • topic starter
  • Posts: 9684
  • Joined: 26 Dec 2003

Posted 19 October 2005 - 01:26 PM

.....It's more complex than that. For example, point sources like stars will require the same exposure time for a given aperture regardless of F ratio. .......


Uh-Oh. I didn't know that!!!.

Now John, I would never question anything you say the way you have tutored so many of us from knocker head rookies to budding Carl Sagens. But I don't understand something on the stars. I see Tonk, Ralph, Corn, you and the others image some beautiful stuff in the competition. It seems I remember seeing exposures (say M45) with F4 Newts. that look like the stars have bloomed and over exposed. Would an F10 SCT exposed for the same time have equivalent blooming?


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics