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Coma with GSO x0.5 focal reducer on a Newt?

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#1 Puck Ja

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Posted 11 May 2013 - 08:31 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. :lol: So I repost it here, which may be more appropriate.

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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

Posted Image
m51_f3_30s

Compare to F/6 with no FR:
Posted Image
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. :)

#2 Dwight J

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Posted 11 May 2013 - 09:36 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.....

#3 Puck Ja

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Posted 11 May 2013 - 09:41 PM

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

Thank!

#4 geminijk

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Posted 11 May 2013 - 10:55 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

#5 Puck Ja

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Posted 11 May 2013 - 11:34 PM

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!

#6 jchaller

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Posted 11 May 2013 - 11:58 PM

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.cloudynig...4626393/page...

#7 nytecam

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Posted 12 May 2013 - 12:08 PM

Puck - impressive M51 for 30s :grin: 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 :o

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#8 mattflastro

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Posted 12 May 2013 - 12:27 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.

#9 Puck Ja

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Posted 12 May 2013 - 03:01 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:
Posted Image
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. :)

#10 mattflastro

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Posted 12 May 2013 - 04:25 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:
Posted Image
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 .

#11 ccs_hello

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Posted 12 May 2013 - 06:27 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

#12 ccs_hello

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Posted 12 May 2013 - 06:43 PM

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

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#13 mattflastro

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Posted 12 May 2013 - 07:03 PM

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.

#14 Puck Ja

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Posted 12 May 2013 - 07:12 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.


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. :o)

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

#15 Puck Ja

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Posted 12 May 2013 - 07:15 PM

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.


:grin: 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. :)

#16 Puck Ja

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Posted 12 May 2013 - 08:06 PM

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

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



#17 GlennLeDrew

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Posted 12 May 2013 - 11:14 PM

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.)

#18 mclewis1

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Posted 13 May 2013 - 09:03 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.

#19 mpgxsvcd

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Posted 13 May 2013 - 10:47 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.

#20 GlennLeDrew

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Posted 13 May 2013 - 11:49 AM

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.

#21 mpgxsvcd

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Posted 13 May 2013 - 12:11 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.


I tried all of the different spacings. It always had more coma than I was willing to accept when I looked at the full field of view. However, if I just use the in camera 2x lossless crop mode it gives me almost the same field of view that the small chip cameras have and coma is not an issue. It still has at least twice the resolution as the smaller cameras in that configuration. However, I would trade that extra resolution for larger pixels anytime.

#22 Puck Ja

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Posted 14 May 2013 - 03:41 PM

Thanks to all the help and ideas.

Last night I collimated my XT8 before mounting. After mounting, it was off a little bit, so I re-collimated with Glatter Laser kit. And I triple checked the FR was threaded correctly. I have also tried different 2"-to-1.25" adapter, including Howie's parallizer. And there is no dew last night.

Unfortunately, I still see the same distortion at edges.

Posted Image
m51_f3_prog60s_p1

So I will try to get Antare's 0.5x FR just as a double check since it is not too expensive.

And I will be go overseas for several months so I will not have chance to work on this issue until July. Maybe I'll get a F/4 scope at that time. :) Thanks to all the help and I will keep you updated later.

#23 mattflastro

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Posted 14 May 2013 - 08:54 PM

Thanks to all the help and ideas.

Last night I collimated my XT8 before mounting. After mounting, it was off a little bit, so I re-collimated with Glatter Laser kit. And I triple checked the FR was threaded correctly. I have also tried different 2"-to-1.25" adapter, including Howie's parallizer. And there is no dew last night.

Unfortunately, I still see the same distortion at edges.

So I will try to get Antare's 0.5x FR just as a double check since it is not too expensive.

And I will be go overseas for several months so I will not have chance to work on this issue until July. Maybe I'll get a F/4 scope at that time. :) Thanks to all the help and I will keep you updated later.


Just to gather more information, if I were you I would ask Al Nagler from Televue about using a Paracorr with a focal reducer on a F/4 newtonian . Give him a call or email or whatever works nowadays . Even if a Paracorr seems expensive, it's less than changng scopes, getting a new mount for the new scope etc.

#24 Puck Ja

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Posted 27 July 2013 - 10:32 PM

I was out of home for almost 2 mo so this GSO FR issue was not pursed until recently. Jim at ScopeStuff was very nice to allow me to exchange it with a Antares 0.5X FR.

It arrived yesterday and I had a chance to tried it out. I was so happy to see NONE of those distortion with this new Antares FR. :jump:

So my XT8 is great and so was my alignment. The poor quality of the GSO FR was to be blamed. :flame: I didn't even bother to make sure the alignment of my XT8. The 60-sec M51 showed nice star shape to the edge:
Posted Image
m51_60s_dk

So here is the closure of this issue. For the benefit of any one who might get such problematic RF: try to get a different one. :)






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