Back Focus

[Oisinmcc16]

Hi everyone,

So I've been looking to buy a Skywatcher 130PDS to go along with my Rowan Modded HEQ5 and Nikon D5600, but I'm struggling to grasp what sort of back focus I need. I appreciate the concept behind back focus, but I don't know what equipment to buy. I know I need a coma-corrector at the least and an M48/T2 adapter for the camera, but how much back focus that provides, and whether or not that's enough I don't get. 

If anyone has any advice they could give me it'd be greatly appreciated.

 

Thanks,

Oisin

[vlaiv]

27 minutes ago, Oisinmcc16 said:

I know I need a coma-corrector at the least and an M48/T2 adapter for the camera, but how much back focus that provides, and whether or not that's enough I don't get. 

I'm not really sure you do get it correctly. CC does not provide back focus - it "removes" it sort of.

Before you start, maybe have a look here:

I'm sure someone will come along to advise on actual back focus of 130PDS and rest can be found online (check Nikon flange focal distance on wiki: https://en.wikipedia.org/wiki/Flange_focal_distance) and CC and T-ring usually have their optical path requirements / properties listed so that won't be much of a problem to check out.

[barbulo]

I use the 130PDS with a Canon 600D and a Baader MPCC. The back focus of the CC is 55mm to the sensor. 
The problem with some newtonians (not the 130PDS) is that they don’t have enough inward focus distance to reach focus. PDS series have a shorter focuser to prevent that. 

[Oisinmcc16]

7 hours ago, vlaiv said:

I'm not really sure you do get it correctly. CC does not provide back focus - it "removes" it sort of.

 

Hi,

Thanks for the reply!

When I said "I get it" it was more so in the terms of what's actually happening for focus to be reached i.e., how the light focuses and such, I just recently finished my module in my astrophysics degree on Light & Optics so I appreciate that, I was more so confused as to whether the coma corrector will help me reach - or in this case, remove, as you've said, - the required back focus - per say.

What I'm grasping from Barbulo is that I don't need to worry too much as the coma corrector he's mentioned is the one I was looking to get, so if his works fine and he uses a camera with an APS-C sensor, then they should act fairly similarly.

[vlaiv]

8 minutes ago, Oisinmcc16 said:

 

Hi,

Thanks for the reply!

When I said "I get it" it was more so in the terms of what's actually happening for focus to be reached i.e., how the light focuses and such, I just recently finished my module in my astrophysics degree on Light & Optics so I appreciate that, I was more so confused as to whether the coma corrector will help me reach - or in this case, remove, as you've said, - the required back focus - per say.

What I'm grasping from Barbulo is that I don't need to worry too much as the coma corrector he's mentioned is the one I was looking to get, so if his works fine and he uses a camera with an APS-C sensor, then they should act fairly similarly.

In that case, you'll need few bits.

MPCC III has both M48 and T2 threads.

Working distance from M48 is 58mm (as per TS website info):

Nikon F mount flange distance is 46.5mm

And this Nikon F mount M48 adapter from TS has optical length of 8.5mm:

https://www.teleskop-express.de/shop/product_info.php/info/p3629_TS-Optics-T-Ring-from-M48-Thread-to-Nikon-F-Mount.html

So you have 46.5 + 8.5 = 55mm and you are 3mm short for M48 distance of 58mm - this means you'll need 3mm M48 extension:

https://www.teleskop-express.de/shop/product_info.php/info/p6435_TS-Optics-3-mm-Extension-with-M48---2--Filter-Thread-and-2--Diameter.html

You can try other Nikon F adapter - like this one:

https://www.teleskop-express.de/shop/product_info.php/info/p195_TS-Optics-Optics-T2-Adaptor-for-NIKON-Cameras.html

but unfortunately it does not say how much of optical path it uses up.

You can also use this one:

https://www.firstlightoptics.com/adapters/ts-ultra-short-t2-adapter-for-nikon-dslr-1mm-length.html

but I would avoid putting anything shiny in optical train, and since it is only 1mm long - you'd need very specific extension of 7.5mm (not sure if anyone makes extensions that are not whole number of mm long). You can always use distancing rings though - like 7mm extension and 0.5mm distancing ring, but that adapter is shiny and can cause reflections.

 

[Oisinmcc16]

20 hours ago, vlaiv said:

 

 

Okay, so after doing a bit of reading I'm beginning to realise its not so much about "reaching" that 55mm, its more a case of not exceeding that 55mm mark because then you'd end up not being able to rack the focuser in enough to be able to reach focus!

Where you mention "flange", I'm getting that its the distance from the sensor of the camera to the "lens", or in this case I'm guessing the scope? So does this 55mm of back focus in the coma corrector include the 46.5mm of the flange for the Nikon F mount and then also however long the M48/M42 adapter is?

[vlaiv]

Just now, Oisinmcc16 said:

Where you mention "flange", I'm getting that its the distance from the sensor of the camera to the "lens", or in this case I'm guessing the scope? So does this 55mm of back focus in the coma corrector include the 46.5mm of the flange for the Nikon F mount and then also however long the M48/M42 adapter is?

Since you'll be using body of Nikon camera with Nikon F mount, you need to account for how "deeply" embedded sensor is inside the body. That is flange focal distance:

Different cameras have different this distance.

Coma corrector needs sensor at some distance from its thread - in case of Baader MPCC and M48 thread that is 58mm, so image looks like this:

Blue line is total distance from MPCC to sensor and it is 58mm (for M48 - it is 55mm for T2 connection). Flange focal distance is red - it is 46.5 for Nikon F.

You have one or two more bits in between - T/M48 ring (depends which one you choose) and any extender. These two need to be of combined length that is equal to 58 - 46.5 if you opt for M48 connection or 55 - 46.5 if you opt for T2 connection.

In first case (M48 connection) you need 11.5mm of distance and in later (T2 connection) you need 8.5mm distance.

M48 ring has optical length of 8.5mm (for some reason), while there is no data for regular T2 ring (could ask TS or other supplier for that info, depending where you want to purchase), shiny T2 ring has optical path of 1mm so you need additional 7.5mm of extension to reach 8.5mm needed for T2 connection.

Makes sense?

[Oisinmcc16]

I think so...

which would you recommend between getting the M48 or the T2?

I'm going to assume the T2 as it will require less spacers to reach the additional 8.5mm required?

[vlaiv]

10 minutes ago, Oisinmcc16 said:

I'm going to assume the T2 as it will require less spacers to reach the additional 8.5mm required?

Usually I would recommend T2, but my only concern is that two T2 rings I found available don't have good specs. One is all shiny which I would avoid and second does not have specified optical length.

Maybe someone who owns Nikon F mount T2 ring can advise on optical length of the item?

[alacant]

On 04/01/2022 at 12:56, Oisinmcc16 said:

how much back focus that provides

Try not to overthink it. The focal plane on a 130pds is about 6.5cm above the front face of the fully retracted focuser.

The Baader cc comes with an adjustable adapter to place the camera sensor 55mm -plus a few mm wiggle room- from the front face of e.g. this t2 ring. Stick the whole assembly -the cc slides into the focuser- into the telescope and away you go. No need to measure anything. There is ample room either side of focus.

That's it. 

Cheers and HTH

 

 

Edited January 6, 2022 by alacant

[alacant]

16 hours ago, vlaiv said:

M48 ring has optical length of 8.5mm (for some reason)

The distance flange - sensor on a Nikon F is 46.5mm

try: 8.5 + 46.5

Ring any bells?

clue: it's an infamous back-focus number

Cheers and HTH

Edited January 6, 2022 by alacant

[vlaiv]

19 minutes ago, alacant said:

The distance flange - sensor on a Nikon F is 46.5mm

try: 8.5 + 46.5

Ring any bells?

clue: it's an infamous back-focus number

Cheers and HTH

Yes, it makes sense for T mount as T mount has 55mm flange distance, but this is M48 connection

Baader MPCC has ~58mm with M48 connection and some other CCs have larger distance with M48

[alacant]

2 minutes ago, vlaiv said:

Baader MPCC has ~58mm with M48 connection

Let's try to help the OP if we can.

He doesn't need anything other than a m42 t2 adapter and what is supplied by Baader.

Thanks.

[alacant]

10 minutes ago, vlaiv said:

this is M48 connection

I placed the diagram for you here.

 

[vlaiv]

18 minutes ago, alacant said:

Let's try to help the OP if we can.

He doesn't need anything other than a m42 t2 adapter and what is supplied by Baader.

Thanks.

I completely agree with you.

OP needs T2 adapter for Nikon F that has declared 8.5mm of optical length.

I only pointed out that I was not able to find such T2 adapter. One that has specified optical length - has 1mm of optical length. Other one that I was able to find - does not have any specification on optical length.

Neither does one that you linked on AliExpress.

I honestly don't want to recommend something that at least does not have proper specifications (it might as well have 8.5mm of optical length - but how do we know unless someone using the item comes forward and says that it is so).

[alacant]

2 minutes ago, vlaiv said:

I completely agree with you.

OMG vlaiv. Surely not!

 

[Oisinmcc16]

hey guys...

 

So they've just delivered 5 new 130pds, 1 new SW 0.9x coma corrector and there's a load of 44x Nikon T2 Adapters.

At this point I'm thinking of getting those, what do you guys think?

[alacant]

18 minutes ago, Oisinmcc16 said:

Nikon T2 Adapters

No. The SW 0.9 cc has only the m48 option. This one for example.

What is '44x Nikon T2 Adapters'.

HTH

 

 

Edited January 6, 2022 by alacant

[Oisinmcc16]

1 minute ago, alacant said:

No. The SW 0.9 cc has only the m48 option. This one works fine.

What is '44x Nikon T2 Adapters'.

HTH

 

 

Just meant there were 44 of the T2 adapters left... wasn't really necessary looking back at it.

Astro-Essentials supply M48 adapters so I'm assuming they're just as good as the one you've linked from Ali-Express. I'm more inclined to buy everything from the same company in the UK so that delivery times aren't all over the place.

Thanks for your help

[vlaiv]

18 minutes ago, Oisinmcc16 said:

hey guys...

 

So they've just delivered 5 new 130pds, 1 new SW 0.9x coma corrector and there's a load of 44x Nikon T2 Adapters.

At this point I'm thinking of getting those, what do you guys think?

You can get that combination:

https://www.firstlightoptics.com/reflectors/skywatcher-explorer-130p-ds-ota.html

+

https://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html

+

https://www.firstlightoptics.com/adapters/astro-essentials-m48-camera-adapter.html

(Nikon version)

Provided that @FLO confirms that Astro Essentials M48 camera adapter for Nikon F mount has 8.5mm of optical path, because Skywatcher x0.9 CC has 55mm backfocus requirement and M48 thread

That setup will work.

[alacant]

T and T2 rings have a fixed length for a given camera. The thickness is that which gives 55mm from the front face of the ring to the sensor.

[vlaiv]

1 minute ago, alacant said:

T and T2 rings have a fixed length for a given camera. The thickness is that which gives 55mm from the front face of the ring to the sensor.

Not all do.

https://www.firstlightoptics.com/adapters/ts-ultra-short-t2-adapter-for-nikon-dslr-1mm-length.html

[alacant]

An ultra-short t2 adapter is not a t2 adapter. It is an ultra-short T2 adapter.

The OP does not need an ultra-short T2 adapter. The OP does not require a T2 adapter of any sort.

Cheers

[Oisinmcc16]

Just thought I'd let you guys know that I just bought all the gear...

 

Skywatcher 130pds

Baader MPCC III

Nikon M48 Adapter

 

The SW coma corrector was out of stock so went with the Baader one instead, hopefully all should be fine. Thanks for all your help and clear skies!!

[alacant]

Well done. You'll love it.

Don't forget to remove the adapter and screw an extension onto the cc. A thin 2" filter -without the glass- is usually just right.

Good luck.

Edited January 11, 2022 by alacant