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

Can a Coma corrector enhance contrast on galaxies like m33?

DSO Reflector Eyepieces Observing
  • Please log in to reply
20 replies to this topic

#1 dilip13

dilip13

    Lift Off

  • -----
  • topic starter
  • Posts: 9
  • Joined: 15 Sep 2020
  • Loc: India

Posted 11 May 2021 - 11:17 PM

Using a 16 inch F4. 4 go to Dobson, bortle 5 sky, wondering adding an ES HR coma corrector would enhance the views of fainter face on galaxies etc.

#2 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 9,715
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 11 May 2021 - 11:39 PM

It will help resolution over a wider field, but not at center field and has no effect on contrast. The Coma Corrector will benefit a lot at lower mag / wider fields. Other than that --- darker skies --- most especially for fainter face-on galaxies. Light pollution really kills those face-on spirals.   Tom


  • izar187, Jim Waters, ShaulaB and 5 others like this

#3 EricSi

EricSi

    Vostok 1

  • -----
  • Posts: 139
  • Joined: 16 Apr 2021
  • Loc: Seattle, WA

Posted 12 May 2021 - 12:18 AM

+1 on going to a site with darker skies, that makes all the difference. M33 and M101 and other face on spirals have low surface brightness, and are barely brighter than the sky itself in light polluted skies.


  • dave253, TOMDEY, MellonLake and 1 other like this

#4 spereira

spereira

    Fly Me to the Moon

  • *****
  • Moderators
  • Posts: 5,820
  • Joined: 21 Apr 2017
  • Loc: Bedford, NH

Posted 12 May 2021 - 07:26 AM

Moving to Equipment.

 

smp


  • dilip13 likes this

#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 92,505
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 12 May 2021 - 07:57 AM

It will help resolution over a wider field, but not at center field and has no effect on contrast. The Coma Corrector will benefit a lot at lower mag / wider fields. Other than that --- darker skies --- most especially for fainter face-on galaxies. Light pollution really kills those face-on spirals.   Tom

 

At F/4.4 , the "coma free", diffraction limited field is 1.9 mm in diameter. In a 16 inch, that's a field of 3.6 arc-minutes.  There's still coma present inside that field, that's the field where a perfect mirror is at least diffraction limited, quarter wave...  It's tiny.

 

I use a Paracorr with my 16 inch F/4.4 nearly all the time. I use well corrected eyepieces. it doesn't affect galaxies a lot but what it does is make it easier to distinguish small, faint galaxies that are not centered from the comatic/astigmstic stars.

 

Bortle 5 covers a wide range, 19.50 mpsas to 20.49.  At 20.49, numerous galaxies are visible, at 19.50, far fewer.

 

The ES coma corrector requires a great deal of inward focuser travel, make sure you have enough.

 

Jon


  • desertstars, havasman, MellonLake and 1 other like this

#6 sevenofnine

sevenofnine

    Viking 1

  • -----
  • Posts: 915
  • Joined: 16 Apr 2016
  • Loc: Santa Rosa, California

Posted 12 May 2021 - 01:44 PM

Other more knowledgeable members have answered your question but I will just add that M-33 was clearly visible from a Bortle 4 sky using 15x70 binoculars. Look for something larger than the full Moon and very faint like a thin smoke ring. Good luck! waytogo.gif


  • Jon Isaacs, ShaulaB and dilip13 like this

#7 MellonLake

MellonLake

    Apollo

  • *****
  • Posts: 1,445
  • Joined: 16 Dec 2018
  • Loc: Toronto, Canada

Posted 12 May 2021 - 01:47 PM

Your telescope is ideal for galaxies.  Will a coma corrector help, marginally as Jon indicated above, it will help find small galaxies in the edges of the field as you scan looking for them.  There are 3 main keys to finding galaxies with your 16"

 

1) Dark skies.  Many many more galaxies will be visible in Borlte 1 or 2 than Bortle 5.  I found 67 galaxies in 2 hours scanning Virgo with my 10" last summer (from Bortle 1).  These were just the large galaxies that clearly had a non-stellar appearance.  I bet I actually saw more than 100 but simply did not identify them as galaxies. 

2) Dark adaptation.  No man made lights, no phones, no laptops, nothing for 1 hour.  This will greatly help with finding galaxies.  Dark adaptation is really important.

3) Averted Vision.  You really need to use averted vision to find the faint galaxies.   

 

I have experience looking for galaxies with my 10" Dob in Bortle 1, 2, 5, 6 and 7.  In Bortle 7, I can find the core of M31 and not much else.  In Bortle 5 I can find quite a few galaxies (most of the Messier galaxies and a few NGC).  In Bortle 1 or 2 I can find hundreds and hundreds of galaxies.   Nothing will have a bigger impact than dark skies.  Your $ are better spent on gas than on a Coma corrector.

 

 

Rob


Edited by MellonLake, 12 May 2021 - 01:48 PM.

  • izar187, ShaulaB, sevenofnine and 2 others like this

#8 SteveG

SteveG

    Cosmos

  • *****
  • Posts: 9,949
  • Joined: 27 Sep 2006
  • Loc: Seattle, WA

Posted 12 May 2021 - 01:50 PM

My very short answer would be no. 



#9 areyoukiddingme

areyoukiddingme

    Fly Me to the Moon

  • *****
  • Posts: 6,627
  • Joined: 18 Nov 2012

Posted 12 May 2021 - 03:31 PM

A possible implicit question here is seeing M33 specifically.

 

A major impediment to seeing it is that it's almost certainly bigger than you think it is (more than twice the size of the full moon). Using a lowest power you can get in a 16" scope will be very helpful, because if you are using even medium powers there's a good chance that it is taking up most of the field (quite possibly all and more) and you are looking only at a part of the spiral structure.

 

When I first tried with an 11" SCT I tried multiple times before working out that M33 was essentially filling the entire field of view. A 16" F4.4 should be a bit better for field.

 

A coma corrector in this case probably will help, because at F4.4 with a 16", a large portion of M33 would be at the edges of the field at all but the lowest powers/widests fields.

 

Of course, what was said above applies to more typical sized galaxies. 


  • Jon Isaacs likes this

#10 brentknight

brentknight

    Gemini

  • *****
  • Posts: 3,038
  • Joined: 29 Dec 2014
  • Loc: Foley, Alabama

Posted 12 May 2021 - 03:48 PM

Using a 16 inch F4. 4 go to Dobson, bortle 5 sky, wondering adding an ES HR coma corrector would enhance the views of fainter face on galaxies etc.

I agree with others who say the CC won't help much with contrast on galaxies near center field.  But with that 16" it will help distinguish multiple objects in the same field.  Most improvement will be on open clusters on the edge of the field that are near other objects.  A perfect example is the pair of clusters M35 and NGC2158.  Without a CC, the smaller but more compact NGC2158 is almost invisible because none of the stars will focus and likely only show up as a faint haze.


  • dilip13 likes this

#11 Vic Menard

Vic Menard

    Cosmos

  • *****
  • Posts: 8,167
  • Joined: 21 Jul 2004
  • Loc: Bradenton, FL

Posted 12 May 2021 - 05:58 PM

As Jon noted above, at f/4.4, your coma "free" field diameter (without correction) is less than 2mm. To fit M33 in a single field of view, you'll need about a 1-degree field of view. The field stop in a 17mm Nagler is 24.3mm (13X your coma free field diameter), and a 17mm Ethos field stop is 29.6mm--16X your coma free field! Adding a coma corrector (ES HR) should extend your coma "free" field diameter by a factor of 6, still only 1/2 the field of the 17mm Nagler. If you're already using Ethos eyepieces with your 16-inch f/4.4, I would highly recommend the Paracorr 2--which will extend your coma "free" field diameter by a factor of 14!

 

Of course, you'll also want a dark sky with reasonably good seeing. When I was looking for M33 in my 22 f/4 StarStructure at Chiefland with Paracorr 2 and 26 Nagler, I could clearly see the outer halo surrounding the galaxy before I had the much brighter galaxy centered in the field of view. With the (double) spiral arms centered, there's a wealth of detail, spiral structure, numerous HII regions and lots of tiny stars. So I'll be the odd man out and say, "Yes, a coma corrector with a moderate to large aperture f/4 to f/4.5 go-to Dob will help you to see more detail in extended deep sky objects like M33."


  • Jon Isaacs and a__l like this

#12 havasman

havasman

    Voyager 1

  • *****
  • Posts: 12,702
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 12 May 2021 - 06:34 PM

Paracorr 2 is better than ES HR and easier to both fit into your system and use.


Edited by havasman, 12 May 2021 - 06:35 PM.


#13 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 92,505
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 12 May 2021 - 07:30 PM

So I'll be the odd man out and say, "Yes, a coma corrector with a moderate to large aperture f/4 to f/4.5 go-to Dob will help you to see more detail in extended deep sky objects like M33."

 

 

I'm with Vic.  With a coma free region of 2 mm and M33 covering a field of about 30 mm, to say that a coma corrector won't help with M33 is simply saying crappy optics, crappy eyepieces are just fine for viewing the bright nebulae that make M33 so interesting.

 

Now there is some real truth in saying  you can get by will less than ideal optics but the other side of the coin is one nearly always sees more with well corrected eyepieces in coma free fields.

 

I have two F/4.4 Dobs, One F/4.06, the Paracorr (2) lives in the focuser.

 

Jon

 

Jon



#14 Mitrovarr

Mitrovarr

    Skylab

  • *****
  • Posts: 4,039
  • Joined: 12 Sep 2004
  • Loc: Boise, Idaho

Posted 12 May 2021 - 10:47 PM

The improvement will be minor. Coma is a relatively small effect. It matters on point sources like stars, but on extended objects? I think you'd be hard pressed to tell the difference.

 

I'd probably still get one, because you'll want it on other stuff like star clusters.


  • dilip13 likes this

#15 a__l

a__l

    Mercury-Atlas

  • -----
  • Posts: 2,827
  • Joined: 24 Nov 2007

Posted 13 May 2021 - 12:31 AM

Adding a coma corrector (ES HR) should extend your coma "free" field diameter by a factor of 6, still only 1/2 the field of the 17mm Nagler. 

Please give me a link to this information.


  • dilip13 likes this

#16 bjkaras

bjkaras

    Mariner 2

  • -----
  • Posts: 285
  • Joined: 24 May 2019
  • Loc: Back and forth between Santa Clara, CA and Las Vegas, NV

Posted 13 May 2021 - 10:04 PM

I have a 10” f/5 and Don’t use a coma corrector. For galaxies they are not necessary, except maybe for the edge of the field. I make up for that by nudging the scope slightly (thank goodness for dual

axis drives!). Dark skies and good alignment are far more important IMO. They might be useful for point objects near the edge of the field and star clusters, but for galaxies or other extended objects, not so much.


  • ShaulaB and dilip13 like this

#17 dilip13

dilip13

    Lift Off

  • -----
  • topic starter
  • Posts: 9
  • Joined: 15 Sep 2020
  • Loc: India

Posted 15 May 2021 - 04:45 AM

Thanks for the answers. I read somewhere in cloudynights, that the coma corrector greatly enhanced the visibility and details of M33. Hence this doubt. Will wait for the coma corrector to arrive and check this theory
  • Jon Isaacs likes this

#18 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 92,505
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 15 May 2021 - 05:43 AM

Thanks for the answers. I read somewhere in cloudynights, that the coma corrector greatly enhanced the visibility and details of M33. Hence this doubt. Will wait for the coma corrector to arrive and check this theory

Greatly enhanced seems like hyperbole to me.  The 15% added magnification of the Paracorr probably makes the most difference.

 

Jon


  • dilip13 likes this

#19 Voyager 3

Voyager 3

    Apollo

  • -----
  • Posts: 1,189
  • Joined: 20 Jul 2020
  • Loc: Near Bangalore, India

Posted 15 May 2021 - 10:35 AM

Greatly enhanced seems like hyperbole to me.  The 15% added magnification of the Paracorr probably makes the most difference.

 

Jon

I agree it isn't a huge enhancement , but what if the coma corrector in question was something like a ES or Baader ones ? 


  • dilip13 likes this

#20 areyoukiddingme

areyoukiddingme

    Fly Me to the Moon

  • *****
  • Posts: 6,627
  • Joined: 18 Nov 2012

Posted 15 May 2021 - 04:04 PM

If you are at F4.4, and looking for Hii regions in M33, then I think a coma corrector will be important. 

 

Whether it's a night and day difference, probably not, but I can imagine those regions going above and below detection when they get to a comatic region at the edges.


  • dilip13 likes this

#21 brentknight

brentknight

    Gemini

  • *****
  • Posts: 3,038
  • Joined: 29 Dec 2014
  • Loc: Foley, Alabama

Posted 15 May 2021 - 04:46 PM

If you are at F4.4, and looking for Hii regions in M33, then I think a coma corrector will be important. 

 

Whether it's a night and day difference, probably not, but I can imagine those regions going above and below detection when they get to a comatic region at the edges.

And probably only on a night with good seeing.


  • dilip13 likes this


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





Also tagged with one or more of these keywords: DSO, Reflector, Eyepieces, Observing



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics