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EAA with newt and Paracorr?

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13 replies to this topic

#1 geminijk

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Posted 24 January 2020 - 04:07 PM

So I'm getting increasing annoyed with the coma and possible tilt i'm getting with my 8"f3.9 and 294 combo, and was wondering if the possibility of being able to use a Paracorr has ever been tried with imaging? 

 

The one thing I like the idea of is to use the paracorr too visually with my 12". I'm aware of the $500 steep price tag, andI do have a baader MCPII, and although i'm at the 55mm mark, still coma, and seems more on one side than the other. IDK...even considering going to the 190 MakNewt to remove this coma once and for all. 

 

Thoughts ? 

 

John

 



#2 photoracer18

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Posted 24 January 2020 - 04:28 PM

Have you rotated the coma corrector to eliminate it as being part of the issue? Or it could be a collimation issue. Paracorr won't fix that.



#3 geminijk

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Posted 24 January 2020 - 04:49 PM

I have removed the Baader a number of times for other reasons, but each time the issue is oriented the same, the right of my images are worse than the left. As far as collimation, I recently collimated via a new HoTech SCA laser. This weekend i was going to unmount the scope and really fine tune if I can, so confirmation of any collimation issue will have to wait until next clear night, which is not going to happen for the next week at least. 

 

Here is a sample of the upper-right corner. 

 

 

Annotation 2020-01-24 154746.jpg

Regards

 

John



#4 Todd N

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Posted 24 January 2020 - 05:12 PM

I've notice this issue with others and myself with a 6" f/5 newt and a Lumicon CC. Despite the statement that any such CC can be used between these f-ratios(usally f/4-f/6) they seemed to be optimal at the slower end of f/6. I would have doubts about the Paracorr correcting what the Baader largely can't. Maybe it's better but I don't know. I've come across several references over the years that CCs like mine which I think is a Ross design are more ideally suited to hyperbolic mirrors which or coarse are not common in newts. One cheap and short term solution is to stop the newtonian down to a slower f-ratio which will reduce coma; keep coma correcter in place. This is ideally to be done on the mirror surface but putting the stop it in front of the scope seems to work fine. Use a cutting compass to cut a smaller circular aperture on heavy construction paper and mount it centered in front of the scope. Try different apertures from f/4.5 to f/6 to optimize coma correction and photographic speed.


Edited by Todd N, 25 January 2020 - 09:28 AM.


#5 Alien Observatory

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Posted 24 January 2020 - 09:13 PM

The Skywatchers F4 Newt has a Coma Corrected available, but nor sure if it would work on your telescope... Pat Utah :)

https://www.skywatch...tonian-8-205-mm


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#6 Rickster

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Posted 26 January 2020 - 10:32 AM

Hi John,

 

Some thoughts, FWIW:

 

After collimation using any collimation device, I always fine tune collimation with the camera in place.  Significant changes are always necessary.  Even the slightest tilt in how the camera sits in the focuser or in sensor mounting within the casmera can result in problems.  I start with the defocused star method, making sure the star is in the center of the field. Then I fine tune that again looking for side to side differences in the corners near focus.

 

Loose focuser screws can create frustration.  I once chased collimation/tilt problems for 6 months only to discover that the problem was loose screws.

 

Some focusers provide screws for adjusting focuser tilt.  My 16in Sky Watcher has them.  My 6in Astro Tech doesn't.  I don't know about yours.

 

Coma occurs because light rays in the outer part of the field of view are not parallel to the telescope axis, and therefore don't reflect with the same geometry as the on axis rays in the center of the FOV.  This is an unavoidable law of physics.  So called coma free optics are not truly coma free.  They are coma corrected.  Coma correctors attempt to put Humpty Dumpty back together again.  They do this by sacrificing quality in the center of the FOV for improved quality in the outer FOV.  To be better than just good enough, they need to be designed for the exact OTA that they will be used on.  And even then, they are only effective when focus and alignment are just right. 



#7 elpajare

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Posted 26 January 2020 - 12:43 PM

You could try the option of a coma-free RC for less price. I use it with an IMX294 and I have no coma problems.

The RC design is different from the Newtons and SC

 

https://www.astrosho...624-ota/p,53810

 

This is what the manufacturer says:

 

Omegon RC telescopes provide a wide, coma-free field of view - perfect for round stellar images right out to the edge of the field of view.



#8 Stars4jon

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Posted 27 January 2020 - 12:22 PM

Hi John,

  Coma correctors attempt to put Humpty Dumpty back together again.  They do this by sacrificing quality in the center of the FOV for improved quality in the outer FOV.  

This is a false statement about sacrificing quality at the field center, see Paracorr Spot Size Graphs. Even at f/3.5, the spot sizes at 1/3 the size of the Airy Disc at field center, and equal to the Airy Disc at 20mm field off axis(about 40 times smaller than the spot size without the Paracorr). 

 

Paracorr_2_chart.jpg



#9 cmooney91

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Posted 28 January 2020 - 12:51 PM

I read that the MPCC does not work too well at F4.

 

After reading this comparison, I decided to go for the Sky-watcher F4 Quattro (same as the 2" GPU tested)

 

It has been working great with my 8" F3.9 ASI294 combo.

 

The down side is you would have to get creative if you wanted to try to use it visually. On the plus side, it is cheaper than the parcorr and it does not slow the scope down by 15% (32% longer exposures) which is good for EAA. 


Edited by cmooney91, 28 January 2020 - 01:33 PM.


#10 cmooney91

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Posted 28 January 2020 - 01:27 PM

Here are my corners  with the Skywatcher quattro + ASi294 + 8" F3.9.

 

It is spaced to 55mm using the ASI Spacers (it supposedly wants 53.66mm spacing at 800mm f.l.).

 

I've never checked for tilt.

 

I use a barlowed laser for collimation so it should be close, but it could be slightly off.

 

 

I think it is plenty good enough for EAA.

Having such a large fully corrected fully illuminated FOV at 1.17"/px is a blast. It makes finding DSO easy, and it gives many options for framing. 


Edited by cmooney91, 28 January 2020 - 01:31 PM.

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#11 geminijk

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Posted 30 January 2020 - 04:57 PM

You could try the option of a coma-free RC for less price. I use it with an IMX294 and I have no coma problems.

The RC design is different from the Newtons and SC

 

https://www.astrosho...624-ota/p,53810

 

This is what the manufacturer says:

 

Omegon RC telescopes provide a wide, coma-free field of view - perfect for round stellar images right out to the edge of the field of view.

I thought of the RCs, but at their native F8, I would still need a reducer, and concerned that I would have similar issue of that introducing issues dropping to F4. Still on the table, perhaps as an upgrade this year. It depends on how much more tweaking I have to do with the fast newt. 

 

General question arises however, if I get a reducer for the RC to drop to F4-5, do others battle issues with spacing from that as well? I would imagine yes, but never owning a RC not sure. 

 

John



#12 geminijk

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Posted 30 January 2020 - 05:04 PM

Here is my latest tweak. I purchased a SvBony variable spacer off of amazon. Took an image, moved either in/out, retook image etc. This is my best results with the current Baader MPCII. Very close. This image is JUST for the comparison of the coma....so I'm not leaving much details besides its a live grab from SC, its NGC_2420 2020-01 and is 12 frames for a total of 369s, no post processing. 

ES8"f3.9, ASI 294mcPro, MPCII, Baader MSGF

 

 

 

I have an ES FR/C arriving tomorrow and will test. Will do a side by side vs. the Baader MPCII. That link for comparing reducers really was informative. Depending on the tests, thats the one i'm going to keep. 

 

 

Attached Thumbnails

  • CN_NGC_2420_PostSpacing_2020-01-27T20_56_42_12frames_369s_WithDisplayStretch.png

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#13 geminijk

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Posted 30 January 2020 - 05:09 PM

Link to full res 

 

https://flic.kr/p/2in6dwv


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#14 geminijk

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Posted 03 February 2020 - 10:40 AM

Got the ES Coma Corrector Friday, and guess what, clear weather! 

 

1st Impression - High quality accessory, as one would expect for the $320 price. I am excited too on how it will perform with eyepieces on the AD12 I have, and really like the 2" adapter it includes with integrated helical adjustment for the purpose of visual. 

 

Below is one of my first shots with it. I'm impressed, and I think I found my solution, it does exactly what I want. The MPCC II did well with adjustments as well, but I'm sold on the fact that I can use this ES visually, and I also like the solid tube on it, as it permits me to get more of a solid connection without sag on my optical train (not verified, just my opinion).

 

M 53 and NGC 5053, ES f3.9 w/ES CC, 294mc Pro, 10x60secs, saved with display stretch via Sharpcap. 

 

CN_M_53_NGC_5053_2020-02-02T02_08_26_15frames_900s_WithDisplayStretch.png


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