Coma corrector

[Adam1234]

Could anyone help me understand whether I can use a coma corrector with my current scope or whether I need a different scope?

I've got the Meade Lxd75 6-inch newtonian (D = 152mm, F = 762mm, f5) that I use for astrophotography with my DSLR. 

However in order to get focus I've got to have my focuser racked pretty much all the way in with perhaps only a few millimeters spare (couldn't tell you the exact amount but it's not much).

Does this mean that I wouldn't be able to use a coma corrector with my current setup, as due to the extra distance this would create between the focuser and the camera, I would not then have enough inward focus to get the camera in focus?

Also what exactly does it mean when a coma corrector requires say, 55mm backfocus? I did read a post from a few years ago on this but found it quite confusing.

[wimvb]

With a dslr on a newt you really do need a coma corrector. You won't get decent stars otherwise.

Back focus means that the distance between the back of the cc and the sensor should be 55 mm. Most dslrs have their sensors at 45 mm behind the front flange. Only a few Nikons have a different flange to sensor distance. The t2 camera adapter is 10 mm thick. 45 + 10 = 55, so you're good.

[Adam1234]

42 minutes ago, wimvb said:

With a dslr on a newt you really do need a coma corrector. You won't get decent stars otherwise.

Back focus means that the distance between the back of the cc and the sensor should be 55 mm. Most dslrs have their sensors at 45 mm behind the front flange. Only a few Nikons have a different flange to sensor distance. The t2 camera adapter is 10 mm thick. 45 + 10 = 55, so you're good.

So will the coma corrector change the position where focus is achieved with the camera? 

My concern is whether I'll still be able to achieve focus with the coma corrector attached? Currently to achieve focus I need to have my focuser racked almost all the way in with maybe about 7mm inward travel left. With the added length of the coma corrector, I don't want to risk not being able to get focus and wasting my money.

[wimvb]

The corrector adds only one or two mm to the mechanical distance between the focuser and the T2 adapter, and doesn't influence the back focus of the corrector. But I can't tell you if it will change the focus position. This may depend on the brand of cc. Better check that with the supplier, or ask if you can buy a corrector under the provision that it will work, and if not, that you can return it for a refund.

[Nicola Hannah Butterfield]

If you look on top of the camera there should be a circle with a line through it, excuse the graphic art work or lack of, but something like below. Measure from this line to focus tube, if as @wimvb says and it's only a couple of mm if you have more than that free to focus from tube to T-ring, you should be ok, it is something I am about to look into as it's one of my next steps a coma corrector on my SW200P and off the top of my head I seem to have about 10mm, other option is a low profile focuser as this should enable the camera to obtain prime focus with more wiggle room.

I can obtain prime focus connecting the camera direct onto the T2 thread, but not with the 1 1/4" nose piece, mind you vignetting bad enough as it is.

 

[Adam1234]

1 hour ago, wimvb said:

The corrector adds only one or two mm to the mechanical distance between the focuser and the T2 adapter, and doesn't influence the back focus of the corrector. But I can't tell you if it will change the focus position. This may depend on the brand of cc. Better check that with the supplier, or ask if you can buy a corrector under the provision that it will work, and if not, that you can return it for a refund.

What do you mean by mechanical distance, is that something different from actual distance?

I'm so confused. I've been looking on the internet at coma correctors but they look more than one or two mm in length, unless I am missing something and totally not understanding what you are saying.

[wimvb]

1 minute ago, Adam1234 said:

What do you mean by mechanical distance, is that something different from actual distance?

I'm so confused. I've been looking on the internet at coma correctors but they look more than one or two mm in length, unless I am missing something and totally not understanding what you are saying.

Coma correctors are inserted into the focus tube, so they don't add to the focus distance. Only a mm or 2 protrude.

Glass also has an optical thickness, which is different from its mechanical/physical thickness. But that should be irrelevant here.

My guess is that a coma corrector such as the Baader or Skywatcher, will work ok. But to be on the safe side, ask the supplier before you order.

[Adam1234]

1 hour ago, Nicola Hannah Butterfield said:

If you look on top of the camera there should be a circle with a line through it, excuse the graphic art work or lack of, but something like below. Measure from this line to focus tube, if as @wimvb says and it's only a couple of mm if you have more than that free to focus from tube to T-ring, you should be ok, it is something I am about to look into as it's one of my next steps a coma corrector on my SW200P and off the top of my head I seem to have about 10mm, other option is a low profile focuser as this should enable the camera to obtain prime focus with more wiggle room.

I can obtain prime focus connecting the camera direct onto the T2 thread, but not with the 1 1/4" nose piece, mind you vignetting bad enough as it is.

 

I'll check for that circle with a line when I get home from work. 

I couldn't get prime focus with the nose piece either and had to find a very thin adapter to go from the 2 inch thread on the focuser to the T ring.

I want to try and avoid having to get a low profile focuser at the moment, maybe when I get a new scope at some point in the future if I need one.

[alacant]

15 hours ago, Adam1234 said:

I wouldn't be able to use a coma corrector with my current setup

Hi

Assuming you have a 2" focuser, you could use all but reducing coma correctors. The Baader Mk3, the GPU or the GSO will work fine, indeed the latter can be used as a cure when you haven't enough inward focuser travel. The commonly used sw 0.9 cc will not come to focus unless you move the mirrors closer to each other.

HTH

Edited March 20, 2020 by alacant

[Nicola Hannah Butterfield]

@Adam1234it's to the right and rear of the penterprism on my two Canons.

[Adam1234]

40 minutes ago, alacant said:

Hi

Assuming you have a 2" focuser, you could use all but reducing coma correctors. The Baader Mk3, the GPU or the GSO will work fine, indeed the latter can be used as a cure when you haven't enough inward focuser travel. The commonly used sw 0.9 cc will not come to focus unless you move the mirrors closer to each other.

HTH

Hi alacant, yes I have a 2" focuser, but it's one with screw thread on the outside rather than one where you slot in and tighten. Would this still work with a cc?

[alacant]

1 hour ago, Adam1234 said:

screw thread on the outside

Hi. Could you post a photo? There's gotta be an adaptor methinks...

[Adam1234]

10 minutes ago, alacant said:

Hi. Could you post a photo? There's gotta be an adaptor methinks...

Sure, will do that when I get home from work

 

[Adam1234]

2 hours ago, alacant said:

Hi. Could you post a photo? There's gotta be an adaptor methinks...

So here's photo of my focuser:

 

Here's my camera attached, roughly at the focus position (have not moved focuser since last imaging session):

 

And if helpful, I use this adapter for attaching T Ring to focus tube:

 

[fozzybear]

Do have any other adapters supplied with scope? like how would you fit an eyepiece as there is an adapter 2" with thumbscrews that would screw onto the focuser drawtube to secure the CC

Edited March 20, 2020 by fozzybear
photo added

[alacant]

1 hour ago, Adam1234 said:

photo of my focuser

Hi. I think this is it:

https://www.firstlightoptics.com/adapters/astro-essentials-compression-ring-adapter-for-sky-watcher-refractors-m56.html

In any case, measure the diameter of the thread. If it's 56mm, then that's the one. If it isn't then you know what you're looking for...

HTH.

[Adam1234]

11 minutes ago, alacant said:

Hi. I think this is it:

https://www.firstlightoptics.com/adapters/astro-essentials-compression-ring-adapter-for-sky-watcher-refractors-m56.html

In any case, measure the diameter of the thread. If it's 56mm, then that's the one. If it isn't then you know what you're looking for...

HTH.

Brill thanks, I'll check the diameter and get one of those.

Thinking of maybe getting this cc, and just hope I have enough manoeuvre for getting focus. https://www.firstlightoptics.com/coma-correctors/baader-mpcc.html

[alacant]

21 minutes ago, Adam1234 said:

getting this cc

Yeah, it's populsr and works ok if you can get the spacing right. I found it gave astigmatic edges over DSLR sensors. The GPU (or SW aplanatic) are double the outlay but are capable of giving corner to corner without having to jump through hoops. 

27 minutes ago, Adam1234 said:

enough manoeuvre

If not, it's a simple job to move the mirrors closer.

HTH

 

[Adam1234]

I think it's M54 I need as the adapter I use to connect to the t ring is M54, but on the M54 version the thread is on the outside 😫😫

[Adam1234]

2 hours ago, fozzybear said:

Do have any other adapters supplied with scope? like how would you fit an eyepiece as there is an adapter 2" with thumbscrews that would screw onto the focuser drawtube to secure the CC

 

This is what I use to fit eyepieces (1 1/4inch), I couldn't achieve prime focus with this so I'd have no hope with a cc in line with this.

[alacant]

50 minutes ago, Adam1234 said:

21 minutes ago, Adam1234 said:

thread is on the outside

 

https://agenaastro.com/blue-fireball-360-camera-angle-adjuster-rotator-with-m54-male-female-thread-r-03.html

Dunno. Probably cheaper to get one made...

50 minutes ago, Adam1234 said:

 

 

Edited March 20, 2020 by alacant

[Adam1234]

11 hours ago, alacant said:

https://agenaastro.com/blue-fireball-360-camera-angle-adjuster-rotator-with-m54-male-female-thread-r-03.html

Dunno. Probably cheaper to get one made...

 

No idea how I'd get one made, but if I had to get this one I'll probably get it. I'll try and do some working out and see what room it gives me

[alacant]

6 hours ago, Adam1234 said:

No idea how I'd get one made

https://www.preciseparts.com/ppmain/index.html

Or simply replace the focuser. This would probably be easier and cheaper:

https://es.aliexpress.com/item/32955763408.html

HTH

Edited March 21, 2020 by alacant

[Adam1234]

Hmm that seems to work out more than the agenastro adapter.

I think I might just get an astrophotography dedicated scope, was going to get one sooner or later anyway. Now I got a decide which scope to get.

[Endolf]

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