Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Video and Electronically Assisted Astronomy

Pages: 1 | 2 | (show all)
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Coma with GSO x0.5 focal reducer on a Newt?
      #5854428 - 05/11/13 09:31 PM

I post another thread in the Beginner Imaging forum, but I guess the gear and image were too much different from those beautiful photo-like images there. People there may be shocked by my low-resolution image. So I repost it here, which may be more appropriate.

---------------------
I had GSO x0.5 FR for my Orion XT8 F/6. Doing some short-time one-frame quick viewing with SX Lodestar-C (1/2" sensor). I notice significant coma at the edge of scene


m51_f3_30s

Compare to F/6 with no FR:

m51_f6_60s

So my question is should I get a high quality FR? Any suggestion for my scope?

And I do plan to get some 10" F/4 Newt and many told that Coma corrector is must. So may be I just need a Coma corrector to fix the GSO x0.5 issue?

As a side note, the transparency yesterday was quite poor, which worsened the LP so much less contrast shown in these pictures.

Thanks for the help to a newbie.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dwight J
scholastic sledgehammer
*****

Reged: 05/14/09

Loc: Lethbridge, Alberta, Canada
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5854514 - 05/11/13 10:36 PM

Others have had better results with the Antares 0.5X focal reducer. I have a GSO FR too and I get similar results to yours - lots of distortion around the outer third of the field.....

Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Dwight J]
      #5854524 - 05/11/13 10:41 PM

Really!? That is good (and oops!) to know. So you think the Antare 0.5x will be better?

Thank!


Post Extras: Print Post   Remind Me!   Notify Moderator  
geminijk
scholastic sledgehammer


Reged: 04/03/08

Loc: TN
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5854644 - 05/11/13 11:55 PM

I too use a GSO, and see coma as well. Looks like we have found a theme with them. I have heard from others that we could play with spacing, but haven't tried, bought a f3.3.

John


Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: geminijk]
      #5854687 - 05/12/13 12:34 AM

It will be great if someone use Antare's can confirm its improvement. Otherwise, one has to buy several hundred bucks' one?

@Geminijk
Which f/3.3 FR you purchase? And does it work for your AD12 (a newt)?

Thanks!


Post Extras: Print Post   Remind Me!   Notify Moderator  
jchaller
Pooh-Bah


Reged: 05/29/08

Loc: Tenino, WA.
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5854701 - 05/12/13 12:58 AM

I seem to remember a discussion involving the GSO and Antare focal reducers several years back. There were negative comments on the GSO, so I went with the Antares. I could not find that particular thread, but I did find this one:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4626393/page...


Post Extras: Print Post   Remind Me!   Notify Moderator  
nytecam
Postmaster


Reged: 08/20/05

Loc: London UK
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5855469 - 05/12/13 01:08 PM Attachment (6 downloads)

Puck - impressive M51 for 30s When I do a gamma contrast stretch on your f/3 M51 image via your x0.5 FR, the coma seems confined to the left-hand side as if out-of collimation or perhaps the FR is tilted

Post Extras: Print Post   Remind Me!   Notify Moderator  
mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: nytecam]
      #5855490 - 05/12/13 01:27 PM

definitely out of collimation on top lefthand corner but methink that even after collimation there would be enough coma at the edges. A reducer+coma corrector might work better.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: mattflastro]
      #5855840 - 05/12/13 04:01 PM

Yes, I was also suspect the asymmetric coma. Come to think of it, I should do collimation more religiously since all the moving-around involved with mounting XT8 every time.

However, I do have another shot at the same session which show symmetric coma:

m81_60s

The forcus was off probably due to the dew on primary and I cannot make it better. But the coma was around.

People at Beginner Imaging forum thinks FR and Newt (even f/6) will never work. I do like to hear people's opinion on this since I try to find what to do next.

Should I buy a better FR (if that ever possible to work with XT8) or just get a F/4 Newt. The power of large F/# demonstrate itself very well in my video session.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5855952 - 05/12/13 05:25 PM

Quote:

Yes, I was also suspect the asymmetric coma. Come to think of it, I should do collimation more religiously since all the moving-around involved with mounting XT8 every time.

However, I do have another shot at the same session which show symmetric coma:

m81_60s

The forcus was off probably due to the dew on primary and I cannot make it better. But the coma was around.

People at Beginner Imaging forum thinks FR and Newt (even f/6) will never work. I do like to hear people's opinion on this since I try to find what to do next.

Should I buy a better FR (if that ever possible to work with XT8) or just get a F/4 Newt. The power of large F/# demonstrate itself very well in my video session.




if you look at the stars in your first image (the one with loads of coma at the top lefthand corner) , you can determine how much coma you would get with a collimated and focused scope . All you have to do is look at the bottom righthand of the image and measure a circle equal to half the image diagonal, but with the center at the bottom righthand in the spot with tightest stars . You will notice that the stars are about 3 times smaller in the first image than the second, and the coma a half field diagonal away is not as bad as in your second image (because the second is really badly out of focus). Get a Bahtinov mask, or make a cardboard Hartman mask, or use digital zoom to magnify when you focus .


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: mattflastro]
      #5856172 - 05/12/13 07:27 PM

For fast Newt (or slow Newt with FR added), coma will always be there unless you have a matched Coma Corrector.
For an even faster Newt with its entire image circle shines (due to the use of a FR) on a CCD imager, the edge distortion will be there.

I.e., there is no free lunch anywhere in the optical world. High quality fast lens or OTA costs quite a bit. Newt being an entry level product should be a hint.

Clear Skies!

ccs_hello


Post Extras: Print Post   Remind Me!   Notify Moderator  
ccs_hello
Postmaster
*****

Reged: 07/03/04

Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: ccs_hello]
      #5856198 - 05/12/13 07:43 PM Attachment (6 downloads)

On the other hand, combining a CC and FR altogether is difficult and I've never tried it.
Usually CC needs to be fairly closed to focal plane (about 55mm inside) while FR sits much deeper inside. I *think* CC should take place first then followed by a FR. However per my earlier described optical sequence, it is not in that order.

Clear Skies!

ccs_hello


Post Extras: Print Post   Remind Me!   Notify Moderator  
mattflastro
Vendor - Astrovideo Systems


Reged: 07/31/09

Loc: Brevard County , FL
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: ccs_hello]
      #5856236 - 05/12/13 08:03 PM

Quote:

On the other hand, combining a CC and FR altogether is difficult and I've never tried it.
Usually CC needs to be fairly closed to focal plane (about 55mm inside) while FR sits much deeper inside. I *think* CC should take place first then followed by a FR. However per my earlier described optical sequence, it is not in that order.

Clear Skies!

ccs_hello



Lambda Research still offers a free download for their optics design software called Oslo .
There is a comprehensive user manual, tutorials , and a truckload of examples both in the user support area of their website and in the actual software download .
I recall several years ago they had a few working examples of newtonian reducers and correctors including Ross, Wynne and a combination of focal reducer plus coma corrector in one .
Their demo software was at that time limited in the number of surfaces but the limit varied depending on version (never less than 10 surfaces) so any version allows analysing a combined netwonian reducer plus coma corrector .
I don't know how it works now because I have the paid version which I haven't upgraded in a few years but I see they still have the free limited edition.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: ccs_hello]
      #5856246 - 05/12/13 08:12 PM

Quote:

For fast Newt (or slow Newt with FR added), coma will always be there unless you have a matched Coma Corrector.
For an even faster Newt with its entire image circle shines (due to the use of a FR) on a CCD imager, the edge distortion will be there.

I.e., there is no free lunch anywhere in the optical world. High quality fast lens or OTA costs quite a bit. Newt being an entry level product should be a hint.





For me, my goal was to get DSO details at shortest time and most economic route possible. So a fast, large aperture, newt seems to be the solution. And I don't desire artistically perfect round stars all the way to the edge. As long as the coma or distortion wasn't so bad that I could mis-recognize a star as tiny galaxy, I will be happy.

I think that goal is achievable with a Newt or maybe Schmidt-Newtonian. Let me spend more time on this system: making sure my collimation is good next time. (I used to do it religiously with a Dob mount, but now polar align & GOTO align have cost so much time so I skipped collimation. )

I'll report back later to see if I will need a new FR or new f/4 scope.

Edited by Puck Ja (05/12/13 08:13 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: mattflastro]
      #5856254 - 05/12/13 08:15 PM

Quote:

Lambda Research still offers a free download for their optics design software called Oslo .
There is a comprehensive user manual, tutorials , and a truckload of examples both in the user support area of their website and in the actual software download .
I recall several years ago they had a few working examples of newtonian reducers and correctors including Ross, Wynne and a combination of focal reducer plus coma corrector in one .
Their demo software was at that time limited in the number of surfaces but the limit varied depending on version (never less than 10 surfaces) so any version allows analysing a combined netwonian reducer plus coma corrector .
I don't know how it works now because I have the paid version which I haven't upgraded in a few years but I see they still have the free limited edition.




I use Zemax EE daily in my day job. If I had time to chase down the prescriptions of these FR or CC. It will so easy to determine for me.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Puck Ja
professor emeritus
*****

Reged: 11/21/12

Loc: OK, USA
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: ccs_hello]
      #5856344 - 05/12/13 09:06 PM

Oh, and now I notice the attachment. Thanks a lot for the Excel sheet, I'll try it out later.

Quote:

On the other hand, combining a CC and FR altogether is difficult and I've never tried it.
Usually CC needs to be fairly closed to focal plane (about 55mm inside) while FR sits much deeper inside. I *think* CC should take place first then followed by a FR. However per my earlier described optical sequence, it is not in that order.

Clear Skies!

ccs_hello




Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: Puck Ja]
      #5856648 - 05/13/13 12:14 AM

The aberration we see here is not coma in the normal or classic sense. It might be more properly called astigmatism combined with defocus due to field curvature. The already present field curvature is only exacerbated by a simple doublet used as a reducer.

It would be informative to try a series of images while slightly varying the focus between images. If the aberration changes from a radial orientation as seen here to a tangential orientation (flipped by 90 degrees), then it's astigmatism.

It's almost certain that a native f/4 Newt will deliver a better image. There will definitely be less field curvature, and the comatic blur will be of a smaller magnitude (and with the fans pointing outward, not inward.)


Post Extras: Print Post   Remind Me!   Notify Moderator  
mclewis1
Thread Killer
*****

Reged: 02/25/06

Loc: New Brunswick, Canada
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: GlennLeDrew]
      #5857116 - 05/13/13 10:03 AM

So some ideas and comments for Puck Ja ...

More accurate focusing (using something like a Bahtinov mask) will make the existing setup work better (reduce the aberrations but likely not remove them entirely). Accurate focus is just a good thing to be able to do, it will help in every type of imaging you do now and in the future.
Collimation should be checked regularly. No matter what direction you go in the faster f ratios will put more pressure on the accuracy of your collimation. Getting better at it (ability to quickly check and tweak if necessary) will be like the more accurate focusing a capability in that will help you in everything you do going forward.
Varying the focus to investigate the type of aberration (not going to fix anything but will answer a few questions).
A coma corrector is likely not going to make things appreciably better.
A faster Newt running at it's native f ratio will produce better images ... but at big $.
A better quality focal reducer will also produce better images.

Given that I suspect the budget for changes is relatively limited I would personally purchase either an Antares 1.25" or 2" focal reducer from a source that would also allow me to return it if it didn't meet my expectations. Perhaps starting with the 1.25" model to see if it works out well.

I've personally not used the Antares 1.25" .5x reducer but I do have the 2" .5x model and found it produces much better quality images than my OPT branded (likely GSO sourced) 1.25" .5x reducer. In fact the 2" produces better quality images than my late model Meade 3.3 reducer and the only reason I don't use it more often is I want the additional focal reduction from the 3.3 unit. For an F6 Newt I think .5 to .6x reduction would be great.

The downside to the 2" reducer is that it has a longer focal length (and requires longer spacing) and this means more in focus travel is required. My rough calculations indicate it will need something in the area of 80mm of in focus travel where the 1.25" version only requires about 25-30mm. This may require that you relocate the primary mirror upwards a bit.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: mclewis1]
      #5857301 - 05/13/13 11:47 AM

I have an 8" 800m F4.0 and the AT Coma Corrector. Coma is not an issue unless I use a focal reducer. I have tried all of the focal reducers with the recommended spacing and I always get coma with them even when I use the coma corrector as well.

If anyone has a better experience with similar equipment I would love to hear how you did it.


Post Extras: Print Post   Remind Me!   Notify Moderator  
GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Coma with GSO x0.5 focal reducer on a Newt? new [Re: mpgxsvcd]
      #5857433 - 05/13/13 12:49 PM

The degree of coma depends also on sensor size. The little video chips having widths of ~6mm--in addition to their small pixel counts--are rather less afflicted by coma (and field curvature) than larger CCDs. An additional advantage of the wee chips is that you can apply more aggressive focal reduction before aberrations and vignetting become offensive.

An f/4 Newt with reduction via inexpensive 1.25", so-called 0.5X reducer spaced to give 0.7X (and *perhaps* as low as 0.6X) on a video cam should deliver acceptable images. At f/2.8 (0.7X), that's an image whose surface brightness is 2X (a full f/stop) brighter than at f/4.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | (show all)


Extra information
2 registered and 5 anonymous users are browsing this forum.

Moderator:  csa/montana, JayinUT 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1113

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics