Coma corrector for 12” dobsonian

[bosun21]

Would I get any benefit from one for visual observing ? Later next month when I get my equatorial platform delivered from David Lukehurst I may be tempted to stick a camera asi294MC (provisionally) and do a bit of EEVA etc. I know that for this a coma corrector will be required. Any recommendations for ones that don’t break the bank Thanks  

     
Ian 

Edited January 19, 2023 by bosun21
Typo

[AstroKeith]

It's a question of f number of the scope mostly. What is yours?

[bosun21]

3 minutes ago, AstroKeith said:

It's a question of f number of the scope mostly. What is yours?

It’s the 12” F5

[AstroKeith]

Normally, at F5, for visual use I would say try it first. Most won't see coma, although with ultra wide fov eyepieces it can be seen if you look for it.

Similarly with the camera, its a medium sized chip, and for EEA with an dobsonian equatorial mount, its again marginal.

[bosun21]

1 minute ago, AstroKeith said:

Normally, at F5, for visual use I would say try it first. Most won't see coma, although with ultra wide fov eyepieces it can be seen if you look for it.

Similarly with the camera, its a medium sized chip, and for EEA with an dobsonian equatorial mount, its again marginal.

Thanks for the heads up. What about capturing a few images like 5-30 second exposures and stacking them? Thanks again 

[bosun21]

My smallest FOV eyepieces are my Morpheus 76 degrees and my Nirvana range at 82 degrees.

[Guest]

As long as your eyepieces are well corrected a coma corrector isnt really going to be needed.  My Q70s benefit from them but the pricier eyepieces dont need them.  As far as a cheap coma corrector goes the GSO coma corrector is around a hundred bucks US.  There are numerous threads about how to "properly" set one up, I myself just drop it in and it does a nice job 

[AstroKeith]

2 hours ago, bosun21 said:

Thanks for the heads up. What about capturing a few images like 5-30 second exposures and stacking them? Thanks again 

I'm going to doubt that the Dob and EQ platform will be stable enough for such exposures. Some have been very successful doing planetary imaging but that is very short exposures.

[bosun21]

2 hours ago, AstroKeith said:

I'm going to doubt that the Dob and EQ platform will be stable enough for such exposures. Some have been very successful doing planetary imaging but that is very short exposures.

I disagree, have you seen what the guy in Portugal is doing with his 12 & 8” dobsonians on an EQ platform? He has made some fantastic images. Check out his YouTube channel called Dobsonian Power. I didn’t think it was possible before seeing his results. For some of the larger targets he uses a Starizona coma corrector/reducer.

[Louis D]

I use a GSO coma corrector with a 25mm spacer tube between the optics nosepiece and the eyepiece holder.  With my ES-92 eyepieces, it's quite obvious even at f/6 when I forget to put the CC in the focuser.  However, at high powers, I have to remove it because it induces a bit of spherical aberration on axis which turns fine planetary details to mush.  The Tele Vue Paracorr II reported doesn't have this issue.

[Whistlin Bob]

Yeah- I tried my SW 0.9x CC, that I use when imaging, with an Aero 30mm eyepiece and there was a nice improvement in my 14" f4.6 dob.

[Mr Spock]

When I use my 30mm UF I don't really see any coma. That's x51 and 1.4°. 

[bosun21]

1 hour ago, Mr Spock said:

When I use my 30mm UF I don't really see any coma. That's x51 and 1.4°. 

I’m just going to wing it. I will only consider a CC if my views are being badly affected by coma, if not then it’s one expense I can avoid.

[Louis D]

Coma is one of those things that you don't notice until it's gone, and then you notice it when it's back.

I went without a CC for 15 years in my Dob quite happily.  Then, a GSO CC came up for sale on CN classifieds for $75, so I went for it since it was a fraction of what I was paying for individual eyepieces during that period of upgrading.  BOOM!  The outer regions of 70+ degree eyepieces were noticeably sharper.  When I removed it, I couldn't unsee the slight unsharpness I had been living with.

Admittedly, if your eyepieces have loads of astigmatism or field curvature in the outer field, you won't notice a difference.  This was the case for me prior to my eyepiece upgrading spree.  You also might not notice with long eye relief eyepieces with powerful Smyth lenses that slow down the incoming light cone for the image forming upper group(s).  This is the case with my 10mm Delos.  I absolutely cannot detect any change in the image with or without a CC.  This is not the case with the 12mm ES-92, however.  It desperately needs a CC for best performance.

Low power eyepieces like Panoptics and their clones probably benefit the most from a CC.  I've also found that some long focal length eyepieces have sharper field stops with the CC than without.  I have never been able to pinpoint what is the optical cause of that artifact.

[bosun21]

4 minutes ago, Louis D said:

Coma is one of those things that you don't notice until it's gone, and then you notice it when it's back.

I went without a CC for 15 years in my Dob quite happily.  Then, a GSO CC came up for sale on CN classifieds for $75, so I went for it since it was a fraction of what I was paying for individual eyepieces during that period of upgrading.  BOOM!  The outer regions of 70+ degree eyepieces were noticeably sharper.  When I removed it, I couldn't unsee the slight unsharpness I had been living with.

Admittedly, if your eyepieces have loads of astigmatism or field curvature in the outer field, you won't notice a difference.  This was the case for me prior to my eyepiece upgrading spree.  You also might not notice with long eye relief eyepieces with powerful Smyth lenses that slow down the incoming light cone for the image forming upper group(s).  This is the case with my 10mm Delos.  I absolutely cannot detect any change in the image with or without a CC.  This is not the case with the 12mm ES-92, however.  It desperately needs a CC for best performance.

Low power eyepieces like Panoptics and their clones probably benefit the most from a CC.  I've also found that some long focal length eyepieces have sharper field stops with the CC than without.  I have never been able to pinpoint what is the optical cause of that artifact.

How do you set up the GSO CC ?, is there any back focus measurements etc. I apologize if this is a stupid question but I have only used reducer/flatteners previously.

[Louis D]

I believe the GSO has a 75mm +/-5mm separation requirement.  So, if you're roughly in the ballpark, you're good to go.  Since most of my eyepieces focus within 5mm of the shoulder, I've found that adding a 25mm M48 spacer ring between the optics section and the eyepiece holder removes 95%+ of all visual coma for nearly all of my eyepieces.  The lone exception is my 12mm Nagler T4 which focuses 20mm below the shoulder.  I could clearly see residual coma, so I did the following to parfocalize it using five 4mm-thick, 48mm-ID O-rings and a 20mm M48 extension tube:

I say visual coma because a camera would most likely still detect some uncorrected coma, but our eyes are not so adept at noticing slight ovalness to stars in the far outer field.

The Tele Vue Paracorr II and ES HR CC both have tunable tops, but I think their designs are less forgiving of being off a few mm from the correct spacing.  That, or their owners are less forgiving of slight imperfections.

I will warn you to remove the pot metal thumbscrews GSO supplies with their CC and replace them with M4 steel cap head hex screws.  One of their cheapo screws sheared off in one of the two thumbscrew holes and I can't remove the remnants without drilling it out and retapping the threads, so I've been living with using a single steel thumbscrew since then.  Why do they do that?  A dozen steel screws bought in bulk would probably cost no more than a dollar in Taiwan.

[bosun21]

Thanks for the advice, and I also can’t understand the penny pinching that the manufacturers make just to save pennies. I would replace them post haste now.

[Captain Scarlet]

In my 12” f/5 newt I was on the point of selling my 35mm Panoptic (then my widest eyepiece), the stars in the outer field being so bad (I’m visual only). Views were very far from the “field of fine points” described by so many for such as the Double Cluster and Pleiades. Then, separately, I got a Paracorr2, and it all magically sharpened up! I found the CC made a huge difference.

Magnus

Edited January 20, 2023 by Captain Scarlet