Has anyone used an Explore Scientific Coma Corrector with a SW 12" Collapsible Dob?

[mandragara (Richard)]

Thinking of getting one but I'm worried it might not be compatible with the rather high-profile focuser that's included.

I know I could lower my trusses to play with the light cone, but that'll be a nightmare for collimation.

[raymo]

Why not use the SW one, is the ES one cheaper?
raymo

[mandragara (Richard)]

Originally Posted by raymo

Why not use the SW one, is the ES one cheaper?
raymo

ES one has a tuneable top, bit like a helical focuser, SW one does not. So I'd have to muck about with spacers\parfocalising rings etc.

I should clarify that this is for visual use, not AP.

[Outcast (Carlton)]

Hi Richard,

I'm curious as to why you feel you need a coma corrector for your SW 12" dob for visual use?

Have you observed abberations that you feel need correcting with a coma corrector? Are you sure your collimation is tight? Are you perhaps using eyepieces that aren't well suited to a Newtonian?

I have two Newtonians, a custom built 12" dob that uses SW optics & a 5" Vixen. Both scopes are f5.

I use them regularly with a range of eyepieces (see my signature) & simply have not observed any visual abberations that would warrant using a coma corrector.

I do own one which, I use for astrophotography with my Vixen Newt & I have used it on both Newts visually & noted zero difference... ie: there were no detectable abberations when not using one visually.

At the end of the day, you may indeed need one but, I just thought I'd pose the question?

[Nikolas (Nik)]

Coma correctors are they not more suitable for astrophotography?

[mandragara (Richard)]

It is good that you question my desire for a coma corrector, after all money doesn't grow on trees. I was inspired to get one by some posts I read over on Cloudy Nights, coma correctors seem popular for visual use below f/5

I use a mix of Explore Scientific 68 and 82 degree eyepieces, plus a Televue 32mm wide field

I observe distorted stars at the edge of my FOV, somewhat like the following image:



This distortion is apparent across all of my eyepiece focal lengths, so I assume it is coma. It is noticeable even in my Tele Vue eyepiece, which I know will be very well corrected.

I believe my collimation is reasonably tight as I use a Farpoint laser collimator and fine-tune with a cheshire. Skies haven't been steady enough recently to do a proper star test, but the centre of my FOV seems pretty crisp.

If I'm wrong please let me know! As I'm writing this I realise that it might be field curvature, need to check that when the skies clear over Sydney.

[astro744]

The user manual for the ES unit mentions a 13.5mm distance. It is not very apparent but I believe this is the extra inward travel you will need on your focuser. The Tele Vue Paracorr 2 has a similar distance of 14mm.

A coma corrector can be vey beneficial visually not just photographically. As to whether you need one is a personal preference depending on your tastes (which if like mine can vary night to night).

I gave no experience with the ES unit but have used the Paracorr with tunable top (model just before P2). I often observe with a 16” f4.5, 10.1” f6.4 and 6” f5.5 without the Paracorr in place simply because the coma doesn’t bother me. However when I do use a Paracorr, I can immediately see the difference even at f6.4. Note, I also have a Skywatcher 10” f4.7 but I cannot use a Paracorr with it because Skywatcher in their wisdom, (or lack thereof) put a stop inside the draw tube preventing full insertion of the Paracorr and thereby preventing me to reach focus with a Paracorr in place. The same stop also prevents my 2” Astro-systems Barlowed laser from fully inserting so I am unable to see the return beam spot? It is the black focuser not the white one.

Anyway back to whether you need a coma corrector and I say first get some quality eyepieces that are well corrected that will show you only primary mirror coma and then decide if that coma is bothersome. A coma corrector corrects primary mirror coma; it won’t do much for eyepiece aberrations.

The Tele Vue unit has a 1.15x amplifying factor and whilst some may say a unit with 1x factor is better, there is a reason for the 1.15x and that is to flatten the field ever so slightly. This is beneficial for some eyepieces such as the 22mm type 4 Nagler which in a TV-101 is pristine and fiat to the edge of field but in a typical Newtonian, say 1500 to 2000mm focal length, there is very slight field curvature visible with the 22mm which the Paracorr cleans up very nicely.

[Outcast (Carlton)]

Richard,

I'm by no means an expert or particularly experienced but, what you show in that image does not fit with what I understand coma to look like.

My understanding of coma is that stars in edge of fov will have little comet tails where as your image shows almost a double star or if I was imaging, what I would call a star trail from imperfect PA & tracking.

I'm wondering if you have some other distortion going on here rather than excessive coma?

It might pay to check your primary mirror & see if your mirror clips are a little tight.. pinched optics can induce aberrations too....

Again, no expert but, you comment on using a farpoint laser for collimation then using the cheshire to 'fine tune'. You should really be using the cheshire for collimation with the laser to 'fine tune'.

An excellent guide to collimation can be found here:

http://www.astro-baby.com/astrobaby/...ian-reflector/

It's a common misconception that a laser is the be all and end all of collimation & in truth.. whilst it will line up your optics it can still result in poor colliimation... have a read of the guide I've linked to, it describes a process of getting optics aligned.. firstly with your focuser since this is the reference point required for viewing...

Apologies if any of this is 'suck eggs' material, I don't mean it to be... it may just be that your aberration is not coma but, rather another issue that can be fixed without additional expense & it might be worth checking some things & perhaps revisiting your collimation process before you part with your hard earned cash...

[astro744]

The diagram you show is not coma as coma is a radial aberration, i.e. stars will appear comet like with their tails radiating away from the centre. Coma is linear and increases in size further away from centre. The aberration you have shown looks like sagittal astigmatism and more likely a combination of aberrations.

See https://www.handprint.com/ASTRO/ae4.html (scroll down)

I do not know enough about aberrations to be able to comment further but I can tell you what you are seeing is not pure coma from the primary mirror for once you see it and it alone you may like me be surprised how small it is off axis and even call it beautiful as I did the first time I saw it and it alone.

Ask at Cloudy Nights too and show your diagram if you don’t get a satisfactory answer here. Note not every answer will be entirely factual because unfortunately people tend to copy what others say even if what others say is flawed and without having any experience with the problem at hand. The urban myth about needing a coma corrector below f5 is just that an urban myth. You don’t magically need a coma corrector below f5 and not need one above f5. It is a personal preference and if you find coma objectionable you may want but not necessarily need to use a corrector at higher f ratios or choose not to use one at lower f ratios.

I think first you need to determine the cause of the aberration you are seeing before investing in a coma corrector which is design to correct for primary mirror coma only. Perhaps others can help you determine this cause.

[mandragara (Richard)]

Thanks for the extra info guys. I like the grounded nature of these replies, so many of the CN replies seem to encourage people to buy things for the sake of it.

I'm going to wait for a night of good seeing over Sydney and then tease out what this optical artifact is.

Some research shows it might be astigmatism. If so, given the effect is distributed radially about the center of the field of view, that indicates that it would be eyepiece astigmatism. Yet I perceive it in my Tele Vue eyepieces, which should be well corrected for that.

So another culprit would be field curvature, which would explain the radial distribution and the abberations presence in both. I wear glasses but do not observe with them on, as I only suffer from spherical abberation (no astigmatism) and can tune the focus wheel to compensate for it. However, I might be lacking the ability to focus on the stars at the centre and edge of the FOV at the same time.

So it's not an easy diagnosis, but my suspicions are now more directed at field curvature than coma. This is going to take some detective work, if it is field curvature I should be able to tune the focuser to get the edge stars sharp and the centre of the FOV blurry.

I found this website handy by the way: http://umich.edu/~lowbrows/reflectio...scobel.27.html

Cheers,

Richard

[raymo]

In all the many years I have owned/used SW reflectors, I have never come
across one that came from the factory with a faulty primary mirror; some
a bit better than others, but none that I would consider unacceptable. Did
you buy it new, or used? I don't think that a scope would ever be allowed to leave the factory showing the aberration in your image.
raymo

[Outcast (Carlton)]

Originally Posted by mandragara

Thanks for the extra info guys. I like the grounded nature of these replies, so many of the CN replies seem to encourage people to buy things for the sake of it.

I'm going to wait for a night of good seeing over Sydney and then tease out what this optical artifact is.

Some research shows it might be astigmatism. If so, given the effect is distributed radially about the center of the field of view, that indicates that it would be eyepiece astigmatism. Yet I perceive it in my Tele Vue eyepieces, which should be well corrected for that.

So another culprit would be field curvature, which would explain the radial distribution and the abberations presence in both. I wear glasses but do not observe with them on, as I only suffer from spherical abberation (no astigmatism) and can tune the focus wheel to compensate for it. However, I might be lacking the ability to focus on the stars at the centre and edge of the FOV at the same time.

So it's not an easy diagnosis, but my suspicions are now more directed at field curvature than coma. This is going to take some detective work, if it is field curvature I should be able to tune the focuser to get the edge stars sharp and the centre of the FOV blurry.

I found this website handy by the way: http://umich.edu/~lowbrows/reflectio...scobel.27.html

Cheers,

Richard

I too wear glasses but, detest observing with them on. I do actually suffer from astigmatism in one eye but, it doesn't seem to effect my viewing enough to bother me.. well, I don't notice anyways...

Might be worthwhile exercise observing with & without your glasses to see if the abberation is in fact.. your eyes... try each eye, with & without glasses to see if there is any improvement.. you might be surprised...

I find I can extract more detail using my right eye to observe Jupiter... whereas, I naturally use my left eye for observing, shooting, etc.. but, I tried my right eye on Jupiter one night & found I saw more detail... or at least, thought I did...

I own a number of ES 82* EPs & have no issues with them in any of my scopes, certainly nothing like the image you posted. Sure, there is probably some minor aberration right out on edge of fov but, I can't say I notice it plus.. I can't say I spend a great deal of time observing right out on edge of fov anyways. Don't own the 68* series but, have Vixen LVWs which are 65* afov & they are absolutely pin point to the edge in every scope I own...

Can't speak definitively for TV's as I don't own any... but, at the risk of being branded a heretic... perhaps they may not be as perfect or 'well corrected' as everyone makes them out to be... Also, I'll add that all our eyes are different so.. what seems perfect to one set of eyes may not be all that perfect to another set...

My suggestions FWIW are, have another look at your collimation perhaps revisit your routine using the astrobaby guide to see if your laser might be misleading you... check mirror clips, etc.. in case they've been over tightened & pinched the optics perhaps.. & try with & without glasses to see if any change in your view...

Certainly no point in spending money, rather large sum for a decent coma corrector if it doesn't actually improve your image.. I think you have a few things to check before splashing any cash...

Another thing to try perhaps is to see if you can borrow any eyepieces from a friend.. see if the problem persists.. even try some minimalist glass in the form of an ortho... if the problem persists no matter the eyepiece chosen, that would suggest the problem lies within the scope or your eyes rather than eyepieces..

Good luck with finding the source of your problem, please keep us posted on how you go... your troubleshooting could be very informative to others...

[astro744]

Note the Tele Vue 32mm Widefield eyepiece is an early design that did suffer from mild astigmatism. Consider the Wide Field as a better corrected Erfle eyepiece. Consider the Panoptic as a better corrected Wide Field eyepiece with the residual astigmatism of the Wide Field series removed. Tele Vue took the Wide Field eyepiece series to Nagler like performance levels in their Panoptic series of eyepieces. You can use the Panoptic and Nagler as a baseline to test your telescope for aberrations down to f4 as Tele Vue designed in these large apparent field offerings. I would not be so confident in testing with a ‘clone’ and in your case the Wide Field although a much improved design over anything else offered at the time (I think around 1982), is not as well corrected as the Panoptic and Nagler series with the Nagler with its wider field being an excellent test eyepiece.

Note also that you are observing with a larger exit pupil with the 32mm and if the sky is dark your eye will open up and any edge defects in your eye will me more pronounced. I think you are correct in thinking that this is an eyepiece and perhaps eye combination problem rather than a primary mirror issue. If there were astigmatism in the mirror then what you would see is stars that are elongated across the field and this elongation would shift 90 deg when going in or out if focus. At best focus stars would be square.

There could be field curvature but this would not cause distortion only a focus shift between centre of field and out field. I detest field curvature in cheap wide apparent field ‘clone attempt’ eyepieces. What is the point of a wide field if the outer portion is out of focus. It’s useless as a finder eyepiece since any faint fuzzy DSO at the edge of field will most likely be invisible. It was a cheap 80 deg ‘clone’ that got me onto Tele Vue and I have never looked back.

To test if your eye is causing an issue look through the eyepiece and note any aberration. Then turn your head 90 deg, not the eyepiece or the telescope and see if the aberration rotates. If it does it is your eye. Note though it will be most people’s eye especially at a wide exit pupil so try this at different magnifications. Pick a target full of stars such as a large open cluster in Carina. This way you can observe from centre to edge from lower to higher powers, (preferably tracked so the object doesn’t move).

What is the longest focal length ES eyepiece you have? Of the ‘clones’ the ES are well regarded by many and you may find the 82 deg ES perhaps better corrected as the Wide Field but unless it is of long focal length probably not a good comparison with the 32mm because of exit pupil differences.

I have a couple of ES 82 but only in the very short focal length and I bought them used to compare against the Tele Vue (curiosity) and because eyepiece reviews online are often given by people who have never used the eyepiece they are praising or not praising as the case may be nor stating which telescope they used the eyepiece in. (For the record it was not an ES clone that got me onto Tele Vue but a cheap 30mm/80 deg offering that is sold under many names).

I would like to add that for the record I have not looked through a 32mm Wide Field but have looked through a 24mm Wide Field quite a few times but that too was in the late 1980s, (6” f5.5 and 14” f5 Newt). I just remember the views were WOW and if there was any astigmatism I cannot remember seeing it and certainly not like in my 32mm Celestron 2” Erfle. Information I have stated on the mild astigmatism of the Wide Field and correction in later series, I remember reading on the Tele Vue web site a few years ago. Note the Tele Vue Wide Field was a considerable improvement over other offerings at the time and still regarded as a fine eyepiece amongst today’s offerings on the market.

[mandragara (Richard)]

Hi all,

Finally managed to get a night of good seeing after months of clouds, what strange summer weather aye?

Anyway, I've determined that the aberration I'm seeing in the eyepiece is not due to field curvature, however when I defocus a starfield in the eyepiece I see the following distortion of the telescope aperture and secondary shadow.



Am I correct in interpreting this as being astigmatism within the eyepiece and not coma?

[tempestwizz (Brian)]

If it is astigmatism, the axis of elongation should should change by 90° as you move either side of focus. (Don’t let any astigmatism present in your viewing eye confuse what you are actually viewing either.)
Also, can try rotating the primary in its holder to see if the aberration follows the rotation.
If the primary cannot be rotated, be wary of the mounting ‘system’. There have been many examples where the use of silastic as an adherent has put enough strain onto the primary to cause astigmatism in an otherwise good mirror.
HTH