Coma Corrector Question

[mynameiscd (Andy)]

Its probably been asked a thousand times before but I'm not sure on what way to connect my camera.
I bought a Bintel coma corrector and tried my camera configuration in the picture and it seems worse with it than without it on.
I just use the 2 inch adapter on the camera but I've read all different ways to connect the camera and different spacings. One is 45mm, another is 75mm and another is 20 mm between the camera lens mount to the 1st element.
Anyone else here with a similar setup?
Canon 450d
Skywatcher 150mm Newt
Cheers
Andy

[Outcast (Carlton)]

Andy,

Bintel's site states 'Back focus is 80mm +/- 2mm from Corrector Lens to image plane'.

In this I would read this as 80mm between your coma corrector lens to the sensor.

You should be able to find sensor depth for your camera, add in the T2 adapter thickness & then make up the remainder to 80mm with spacers.

From talking with more experienced imagers on this forum, I believe most DSLR's are around 50 - 55mm sensor depth including T2 adapter. This indeed proved quite correct when I was looking at achieving back focus of 105mm when using an f6.3 focal reducer on an SCT.

Thus, I would suggest you are looking at 50 - 55mm for your camera & looking for 30 - 25mm distance to your corrector lens to achieve the 80mm back focus recommended by Bintel (assuming there number is correct).

You may then need to do some minor tweaking of exact spacing because, none of that information above allows for threads, shoulders on corrector in relation to where the corrector lens is, etc...

However, this should give you a pretty good place to start.

Hopefully, someone with way more experience will jump in soon & either shoot me down or back me up... I don't mind; just thought I'd jump in with what little knowledge I have to see if I could help..

Cheers

[mynameiscd (Andy)]

Hi Carlton,
Thanks for the response, it's made a few things clear.
My camera ffd (flange to film distance) is 44mm for a 450d
My T adapter is 13mm.
And my coma corrector is 44mm to the first element with the connecting tube (which I find out is for visual only)
So total that's 105 from sensor to corrector.
No wonder it's out of focus!!
After a bit of a search I found these.
https://www.bintel.com.au/product/bi...v=6cc98ba2045f
And this
https://www.bintel.com.au/product/t-...v=6cc98ba2045f

At the moment my camera adapter hasn't a thread so I need a new adapter with a 42mm thread, then a T48-42 adapter and hopefully these together will be 36mm. Then with my distance to sensor at 44mm it will make up the difference.
Knowing my luck it will still be about 10mm short !!.
I'll wait and see what comes in the mail.
Maybe I need a spacer??
It's a bit hard being about 350km to the closest telescope shop but that's why I've got zero light pollution.
Cheers
Andy

[Outcast (Carlton)]

No worries Andy,

Hopefully, you get it sorted.

Although I haven't put it to use yet, the issues you describe is why I decided to go down the Baader MPCC route. Not the cheapest option but, as I understand it, there is no change in back focus distance with the MPCC.

I managed to pick one up secondhand for considerably less than new price.

http://https://www.baader-planetariu...-newtons).html

[mynameiscd (Andy)]

Thanks Carlton,
I was also looking at a Baader here on IIS a while ago but I missed it (probably the one you bought) and jumped in and bought a Bintel one but never used it because I had too much else to work out with guiding etc.
Hopefully I'll finally get to use it when I get the right adapter.
Cheers
Andy

[Startrek (Martin)]

Andy
I originally looked the Bintel CC but after researching a bit I chose a Baader MCCP mk111 coma corrector for my Canon 600D and no issues at all with focus in either of my 6” or 8” newts
It’s a quality optical accessory, pin point stars to edge of field , no coma what so ever , they are pricy but they work exceptionally well
If you still have trouble with back focus , maybe try some 4mm thick x 15mm diamter round cork spacers ( Bunnings ) under your primary mirror to bring it forward , it may give you a bit more focus ( I did that to my 6” F6 newt prior to buying a coma corrector)
Martin

[Outcast (Carlton)]

Originally Posted by mynameiscd

Thanks Carlton,
I was also looking at a Baader here on IIS a while ago but I missed it (probably the one you bought) and jumped in and bought a Bintel one but never used it because I had too much else to work out with guiding etc.
Hopefully I'll finally get to use it when I get the right adapter.
Cheers
Andy

Actually, I missed that one too...

I stumbled across one on Fleabay, brand new in the box, never used... took a chance... & when it arrived the description was spot on, the threads didn't even show any sign of use...

One gets extremely lucky occasionally, I rarely buy Astro gear from Fleabay; too much of a gamble...

Cheers

[etill (Elliot)]

I was lucky enough to get an MPCC with the scope I bought here, I've never used one before and had no problem with it on a 60D. Being very short the camera sits nice and close to the OTA which I think helps with better balancing also.

[glend (Glen)]

Andy, can you be clear about what kind of Newt you have, there are some setup for visual use and some for imaging. The visual configured newts may not achieve camera focus, because there is not enough in-travel on the focuser position. An imaging configured Newt works well focusing a camera, but may need a focuser extension tube to be used visually.
There will be all sorts of advice about reconfiguring your scope but we really need to know what we are starting with.

[doppler (Rick)]

The Baader Mccp screws straight into the t mount, your Bintel coma corrector should unscrew in the middle. The thick end looks like it is a visual adapter for eye pieces, some Baader kits come with such a visual adapter.

Rick

[mynameiscd (Andy)]

Thanks for your responses Elliot, Glen, and Rick.
Sorry for not being clear before.
My setup is a Skywatcher Black Diamond 150mm Newt. and the camera works fine.
I've also got some assorted extension tubes so I can do visual as well.
The Coma corrector I've got comes apart in the middle and I'm waiting for some adapters from Bintel so hopefully it will work then.
Thanks
Andy