Comacorrector to eyepiece backfocus distance for visual use?

[ONIKKINEN]

I have a TS Maxfield 0.95x comacorrector in my VX8 that advertises a 55mm working distance for the backfocus to a camera sensor.

But which part of the eyepiece is the 55mm to, if it even applies to eyepieces like it does for sensors? The corrector does not advertise visual use in any way but i would assume there is some way to make it work. Just thread in an adapter or two and something to hold the eyepieces and should work?

[Louis D]

The back focus distance would refer to the focal plane of the eyepiece which is generally close to the shoulder or reference surface of the eyepiece where the lower insertion barrel widens out to the upper barrel.

You could simply add M48 spacer rings screwed into the bottom of the eyepiece that would then screw into the top of the CC as one approach to get the required 55mm of spacing.  This is how the Baader MPCC is generally used with eyepieces.

Alternatively, you could try to use it with an eyepiece holder such as the Baader ClickLock with M48 female thread, again with M48 spacer rings as needed to reach the proper spacing.

[ONIKKINEN]

9 hours ago, Louis D said:

The back focus distance would refer to the focal plane of the eyepiece which is generally close to the shoulder or reference surface of the eyepiece where the lower insertion barrel widens out to the upper barrel.

You could simply add M48 spacer rings screwed into the bottom of the eyepiece that would then screw into the top of the CC as one approach to get the required 55mm of spacing.  This is how the Baader MPCC is generally used with eyepieces.

Alternatively, you could try to use it with an eyepiece holder such as the Baader ClickLock with M48 female thread, again with M48 spacer rings as needed to reach the proper spacing.

I have some unused spacers and a 2 inch to 1.25 inch threaded adapter so sounds easy enough. I also have a few different types of eyepieces so i assume these could have slightly different placements of the focal plane as well.

I would also assume that its not a millimeter science for visual as it is for imaging. I can tolerate coma somewhat in visual but not at all for imaging, so a somewhat uncorrected field might not be noticeable.

[Louis D]

I use the GSO CC with a fixed separation of about 75mm.  I simply added a 25mm M48 spacer ring between the included eyepiece holder and the optics nose piece.  As long as you're within 5mm of the design distance, it's hard to perceive much residual coma.  Generally, eyepiece field curvature and edge astigmatism drown it out.  I've only had to parfocalize one eyepiece with it, the 12mm Nagler T4.  It focuses about 20mm below the shoulder, so I added a 20mm M48 ring to the 2" skirt along with 5 4mm thick O-rings of 50mm ID.  When you're 20mm out, the residual coma is quite apparent.

I have no idea if the TS Maxfield CC will be as forgiving as the GSO CC, but it might be.

Keep us informed of your experimentation in case someone else wants to try it in the future as well.

[ONIKKINEN]

11 hours ago, Louis D said:

I use the GSO CC with a fixed separation of about 75mm.  I simply added a 25mm M48 spacer ring between the included eyepiece holder and the optics nose piece.  As long as you're within 5mm of the design distance, it's hard to perceive much residual coma.  Generally, eyepiece field curvature and edge astigmatism drown it out.  I've only had to parfocalize one eyepiece with it, the 12mm Nagler T4.  It focuses about 20mm below the shoulder, so I added a 20mm M48 ring to the 2" skirt along with 5 4mm thick O-rings of 50mm ID.  When you're 20mm out, the residual coma is quite apparent.

I have no idea if the TS Maxfield CC will be as forgiving as the GSO CC, but it might be.

Keep us informed of your experimentation in case someone else wants to try it in the future as well.

Lacking clear skies at the moment i figured out an actually decent way to test coma from home. I pointed my telecope to a balcony about a kilometer away that conveniently has Christmas lights put out on a pretty wide area. My 19mm 65 degree Omegon flatfield eyepiece nicely covers the entire line of christmas lights end to end and makes it quite easy to judge coma at a glance. Sure im looking through dirty windows that are far from optically good but better than nothing.

I found that its not all that clear what the effect is with small variations. With the adapters i have on hand and just by lifting the eyepiece a bit i found that anywhere from 45-60mm from the corrector threads to the end of the eyepiece barrel is similar. 70mm was clearly not good, as was under 40. Ill have to test again with real stars as even at 44x power the Christmas lights were not point sources. Still good to know im in the roughly right area with the adapters i have. And of course just to test if this method is good at all i took the comacorrector out and there is the normal f4.5 strength of coma even quite close to the center of the field which is to be expected.

[ONIKKINEN]

Just tested under the stars and found the same to still be true. 50mm and thereabouts works fine. Coma is still present at the edges but im not sure whether thats the eyepiece or the corrector or both. Also worth mentioning that looking at the Maxfield 0.95x spot sizes you can see that there just is residual coma that is not corrected. Still i think this nicely frees up the inner 50-70% of the view to be visually coma free at a glance.

Curiously i found that i could put a barlow in the eyepiece holder and the barlowed eyepiece was also much better than the barlow without a comacorrector, coincidence perhaps? For whatever reason i had thought that comacorrector + barlow is a big no no.

[Louis D]

On 26/11/2021 at 21:24, ONIKKINEN said:

For whatever reason i had thought that comacorrector + barlow is a big no no

Not really, it's just that some CC/Barlow combinations simply won't come to focus in some telescopes due to limited in or out focus.