Backfocus C8 + Barlow : Help. - Forums

Topic starter

Oct 11, 2020

Hello everyone,
i have a question about the C8 back focus.
Indeed, for planetary acquisition, I use a Crayford 13cm long, a X-CELL X2 Barlow, an extension tube 50mm and my 1100D but the results are not very good, as you can see… https://www.astrobin.com/uwqcwf/?nc=user

Is my back focus not good ? How can I upgrade my picture ? I find the planetary very difficult …

Help me, astrobin community, you're my only hope.

Engaging

Giovanni Fiume

Oct 11, 2020

Hi Darktytanus, I think the weak ring in your rig is your Canon, not backfocus (that's important only in DSO imaging).You should try with a planetary camera, like ASI o QHY, color version.Check collimation every night and focus with a Bathinov mask on a star.Use Firecapture or Sharpcap to capture, AS!3 for stacking, Registax 6 for wavelets and Gimp for finishing touch.

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Michael Timm

Oct 11, 2020

Hi Darktytanus, did you get the spacing between the Barlow and the camera right? Every Barlow has a working distance it needs between itself and the attached CCD. I do not know the right spacing for your Barlow, but you should make sure that you hit it. Also - like Giovanni wrote - a Canon is not the perfect cam for doing planetary.

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Darktytanus

Topic starter

Oct 12, 2020

Giovanni Fiume:
Hi Darktytanus, I think the weak ring in your rig is your Canon, not backfocus (that's important only in DSO imaging).You should try with a planetary camera, like ASI o QHY, color version.Check collimation every night and focus with a Bathinov mask on a star.Use Firecapture or Sharpcap to capture, AS!3 for stacking, Registax 6 for wavelets and Gimp for finishing touch.

i Know that a apn is weak, but i thought the backfocus is week too !

Michael Timm:
Hi Darktytanus, did you get the spacing between the Barlow and the camera right? Every Barlow has a working distance it needs between itself and the attached CCD. I do not know the right spacing for your Barlow, but you should make sure that you hit it. Also - like Giovanni wrote - a Canon is not the perfect cam for doing planetary.

I think this is the reason of the bad images.. but how could i know the good space between my barlow and my cam ? Now the space between the barlow and my cam is 50mm .. and i think the backfocus is maybe to much...

Giovanni Fiume

Oct 12, 2020

Yes, you are correct, but backfocus play different role in hires and dso imaging.In dso imaging backfocus distance is critical because is related with corrector or reducer specification.If it is wrong, you may have the field not perfectly correct, with distorted stars on the field edge.In hires instead backfocus distance between Barlow and sensor has impact on real Barlow power.All Barlow are declared for certain magnification, 2x, 3x and so on.But this power is calculated from Barlow lens and ocular field stop.If you increase this distance you get a higher magnification.This in general is true for normal Barlow, not telecentric model as for example Televue Powermate.https://www.televue.com/engine/TV3b_page.asp?id=53&Tab=_appNow, in hires, camera sensor stay further of an ocular, therefore you could have much magnification.Normally just few mm are not problem, but if it is more you will go over your ideal sampling, generating an oversampling … your image will be bad.Dummy rule is that every 10 mm of increase distance the power is increased by 1.Your distance of 50mm are really too much.

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Giovanni Fiume

Oct 12, 2020

I add that another dummy rule states that the ideal sampling is reached with a focal ratio equal to 5 x pixel dimension or, when the seeing is very good, 7 x pixel dimension.
Your Canon has pixel size of 5.2 microns, so in the first case you have an f / 26 (with a 2.6x Barlow) and in the other case f / 36 (with a 3.6x Barlow).
These are only theoretical calculations, just to give some indications.
I use a Barlow Televue 2x on my Edge 800 HD, and recently bought a Baader Q-Barlow that give me 2.25x or about 1.3x when screwed directly onto the camera's 31.8 nose (ASI385MC or ASI120MM-S).

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Darktytanus

Topic starter

Oct 12, 2020

Giovanni Fiume:
I add that another dummy rule states that the ideal sampling is reached with a focal ratio equal to 5 x pixel dimension or, when the seeing is very good, 7 x pixel dimension.Your Canon has pixel size of 5.2 microns, so in the first case you have an f / 26 (with a 2.6x Barlow) and in the other case f / 36 (with a 3.6x Barlow).
These are only theoretical calculations, just to give some indications.
I use a Barlow Televue 2x on my Edge 800 HD, and recently bought a Baader Q-Barlow that give me 2.25x or about 1.3x when screwed directly onto the camera's 31.8 nose (ASI385MC or ASI120MM-S).

thanks. ;)
Your answer really help me. I thought well that 50mm is too much. I think the correct distance will be 30mm so the magnification will be good at2x.

Giovanni Fiume

Oct 12, 2020

Well Darktytanus, now the problem is how get right magnification with your seup.
Canon EOS 100D, as all others dslr, have a 44 mm backfocus.
With a standard T2 adapter (11 mm) you get a 55 mm backfocus (magic number for astroimagers

).
Also if you use a low profile T2 adapter (1 mm) you will get around 45 mm of backfocus.
Therefore I doubt you can get to 30 mm distance, and even if you succeed, at 30 mm of distance of Barlow from sensor, you will get 5x power (2 + 3).
If you put your 2x Barlow at 45 mm distance from sensor (best case), you will have about 6.5x power (2 + 4.5).
Your C8 will work at f/65.
To make it work at f / 30 you should put your sensor at 10 mm from the 2x Barlow.
I fear this is not achievable with the Canon but only with a dedicated planetary camera.

Sorry if I didn't explain myself well, but I'm Italian and I don't speak English well.

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Scott Alber

Oct 12, 2020 · Edited: Oct 12, 2020

My two cents would be to get a planetary camera. I use a ZWO 178 MC with 2.4 micron pixels. You may find that you do not even need a barlow and can shoot at a lower gain and/or higher frame rate. However, if that is not an option, I have taken planetary images with my Canon 550 T2i using the 5x setting with Backyard EOS software.

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Darktytanus

Topic starter

Oct 12, 2020

Giovanni Fiume:
Well Darktytanus, now the problem is how get right magnification with your seup.Canon EOS 100D, as all others dslr, have a 44 mm backfocus.
With a standard T2 adapter (11 mm) you get a 55 mm backfocus (magic number for astroimagers ).
Also if you use a low profile T2 adapter (1 mm) you will get around 45 mm of backfocus.
Therefore I doubt you can get to 30 mm distance, and even if you succeed, at 30 mm of distance of Barlow from sensor, you will get 5x power (2 + 3).
If you put your 2x Barlow at 45 mm distance from sensor (best case), you will have about 6.5x power (2 + 4.5).
Your C8 will work at f/65.
To make it work at f / 30 you should put your sensor at 10 mm from the 2x Barlow.
I fear this is not achievable with the Canon but only with a dedicated planetary camera.

Sorry if I didn't explain myself well, but I'm Italian and I don't speak English well.

your english is good

I understand everything but not the meaning sorry ...
But i think i have to buy a camera or shooting without my barlow...
but i see images of planetary pic with the exact same setting except the barlow but they don t give the good adaptator.. i don t understand how they make ?

Scott Alber:
My two cents would be to get a planetary camera. I use a ZWO 178 MC with 2.4 micron pixels. You may find that you do not even need a barlow and can shoot at a lower gain and/or higher frame rate. However, if that is not an option, I have taken planetary images with my Canon 550 T2i using the 5x setting with Backyard EOS software.

yep, i think i will buy one.

Darktytanus

Topic starter

Oct 12, 2020

Will this accessory is good to be nearer the barlow ?https://www.amazon.fr/dp/B07VHR5X9F/ref=cm_sw_r_cp_api_fab_24iHFb2RQYTCX

Giovanni Fiume

Oct 12, 2020

It's a Nikon standard T2 adapter with a 31.8-T2 nosepiece. Very very good price.
Check here for examples (for Canon EOS):
https://www.teleskop-express.de/shop/product_info.php/info/p7079_TS-Optics-1-25--Prime-Focus-Adapter-for-Canon-EOS-DSLR.html
With this accessory you put Barlow lens 55 mm away from sensor.
I thought you already had such a thing. How did you connect your Barlow to the Canon?
And what capture software are you using ?
I have also seen Astrobin users with setup like your … and i've seen also your shots.
I begin to think that the problem is not backfocus, but only poor focus, collimation to be improved and (perhaps) the use of an inadequate software.
You might as well take Scott Amber's advice and use the Canon without Barlow and with BYEOS software.
p.s.: I like your dso shots, especially m82 (my favourite galaxy). Congratulations !

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Darktytanus

Topic starter

Oct 12, 2020 · Edited: Oct 12, 2020

Giovanni Fiume:
It's a Nikon standard T2 adapter with a 31.8-T2 nosepiece. Very very good price.Check here for examples (for Canon EOS):
https://www.teleskop-express.de/shop/product_info.php/info/p7079_TS-Optics-1-25--Prime-Focus-Adapter-for-Canon-EOS-DSLR.html
With this accessory you put Barlow lens 55 mm away from sensor.
I thought you already had such a thing. How did you connect your Barlow to the Canon?
And what capture software are you using ?
I have also seen Astrobin users with setup like your ... and i've seen also your shots.
I begin to think that the problem is not backfocus, but only poor focus, collimation to be improved and (perhaps) the use of an inadequate software.
You might as well take Scott Amber's advice and use the Canon without Barlow and with BYEOS software.
p.s.: I like your dso shots, especially m82 (my favourite galaxy). Congratulations !

i use this kind of adaptator with the 55mm accessory ..
my collimation seems to be good when i look to stars.. maybe it’s not enough précise..
i capture with my camera only no other software...
thanks for the like !

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