Nice calculator for back focus

[powerlord]

I've not come across this before, so apologies if it's egg sucking:

https://cloudbreakoptics.com/blogs/news/calculating-back-focus-metal-back-distance

assuming it's accurate I've had the totally wrong back focus with my altair 0.8x and image train for example.. so gonna try correcting it for tonight!

stu

[teoria_del_big_bang]

Just looking at this calculator, but I assume it just is programmed with the correct back focus for a certain setup of camera, FW with a certain corrector or flattener, then adds on 1/3 thickness of the filter and then gives you the remaining spacers needed to make up that back focus, so nothing different to manually adding up all the elements and doing a bit of maths.

Best way (I thought anyway) to check is to look closely at your images, hopefully well focussed images, and see if there is any distortion of stars at the corners of the image.

How far out do you think your BF is ?

Steve

[teoria_del_big_bang]

I have tried the calculator and it is right but I would be a little cautious because it assumes a set thickness of the filter wheel (I guess the manufacturers specification) but does not include any adapters on the filter wheel that connect it to the camera and any spacers on the scope side. Also it is best to measure the actual width taken up by the FW and adapters as I have found these to be up to 1 mm different from the manufacturers specification.

Steve

Edited February 4, 2022 by teoria_del_big_bang

[AstroNebulee]

I like the look of this indeed, shame though it doesn't have my ovl field flattener on it as with my sw 72ed with rotator, 2 inch  zwo ir uv filter and my 294MC pro shoukd be 55mm but I think it's more like 56mm with the star pattern I'm getting. 

[teoria_del_big_bang]

1 minute ago, AstroNebulee said:

I like the look of this indeed, shame though it doesn't have my ovl field flattener on it as with my sw 72ed with rotator, 2 inch  zwo ir uv filter and my 294MC pro shoukd be 55mm but I think it's more like 56mm with the star pattern I'm getting. 

If you have a filter after the flattener then it will be more than 55 as you need to add 1/3 thickness of the filters.
Normally filters are between 2 and 3 mm so usually BF is around 55.7 to 56 mm, so you are probably right about it being 56 mm.

If its a LP filter screwed into the flatttener and before the flattener then you don't add this 1/3.

Steve

[AstroNebulee]

29 minutes ago, teoria_del_big_bang said:

If you have a filter after the flattener then it will be more than 55 as you need to add 1/3 thickness of the filters.
Normally filters are between 2 and 3 mm so usually BF is around 55.7 to 56 mm, so you are probably right about it being 56 mm.

If its a LP filter screwed into the flatttener and before the flattener then you don't add this 1/3.

Steve

In the image my ir uv filter screwed onto the nose piece of the FF then inserted into the focus tube along with the 55mm spacers then 294mc pro. 

Since that image I've added the rotator instead of the original visual back to have a secure imaging train (think the bevel of the FF nose piece was giving me tilt) , the filter will screw onto the rotator. So it'll be rotaor/filter then FF then spacers then 294mc pro. So will try 56mm bf. 

Sorry for hijacking your thread Stu. 

Edited February 4, 2022 by AstroNebulee

[powerlord]

No bother. What I find a pain is mostly trying to work how how I can get Bf of X with the multitude of 42mm adapters I have. You know.. Its say 36..i have a 10, and a 20, but no 6. I have a 15 but no 1..i have a 21 and a 15..ah there we go. Etc.

[teoria_del_big_bang]

36 minutes ago, powerlord said:

No bother. What I find a pain is mostly trying to work how how I can get Bf of X with the multitude of 42mm adapters I have. You know.. Its say 36..i have a 10, and a 20, but no 6. I have a 15 but no 1..i have a 21 and a 15..ah there we go. Etc.

Is this with the ASI1600 and filter wheel ?

36 mm seems a lot of spacers if it is, the camera will have 6.5 mm between the front of the camera and image plane that means the filter wheel is only  55 - 6.5 - 36 = 12.5 mm wide - is that right ?

Steve

Edited February 4, 2022 by teoria_del_big_bang

[powerlord]

No just a random number example. It says with efw, and my 1600 for altair o. 8 it's 30.33, but i put 25 for who knows why. Zwo says 29.5 + 20mm efw.

So dunno why they are saying 0.83 more. But I was way off last time.. It comes from trying to get the right things out in the dark at midnight. Duh

 

[teoria_del_big_bang]

27 minutes ago, powerlord said:

No just a random number example. It says with efw, and my 1600 for altair o. 8 it's 30.33, but i put 25 for who knows why. Zwo says 29.5 + 20mm efw.

So dunno why they are saying 0.83 more. But I was way off last time.. It comes from trying to get the right things out in the dark at midnight. Duh

Ah I see 🙂 

What is a little odd though is that if you select the ZWO filter wheel but no filter the figure is 28.50

But then add the LRGB it then is 30.33, so +1.83 mm for the filter, which from what I have been told means the ZWO filters are 5.5 mm thick, seems a lot.

And if they were ZWO NB filters its different again at 29.5, so if you have NB and LRGB you cannot get the correct BF for both ?????

The 29.5 could be right as it is adding 1mm and I guess the filters are 3 mm thick so that's the 1/3 width of filter I have been told to add by others on SGL.

But just looking on FLO now at ZWO filters it says they are all 2 mm thick.
I must admit this calculator seems right until you add the filters, unless this adding 1/3 thickness of them has been wrong advice but when I look at my images it seems to work for me.

TBH I am not convinced this calculator is right, but I don't profess to be any kind of expert .

Steve

 

[teoria_del_big_bang]

And regarding getting the right BF then I fine THESE are essential.

I would think getting it to the nearest 0.5 mm is going to be good enough.

Steve

[powerlord]

yeh my camera or maybe it was the efw came with a set of spacers, so ok there.

anyway, tonight was not to be as somehow I found out I'd damaged two of my power lead plugs, ripping them off. 🙄

i have rigged up the 6d for the first time in ages with the SY135 for some widefield - at least no BF issues there.

 

[teoria_del_big_bang]

No chance of any imaging up here tonight, thick with cloud and very wet

 

[powerlord]

forecast clear all night here, so attempting a poor mans asi2600 with the 6d -orion in full. we'll see if worthwhile...

obsy should be finished tomorrow - ran out of time tonight.