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Old 05-06-2021, 07:07 AM
bluesilver (Peter)
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Working out best focal length for planetary imaging with a Dobsonian

I am not sure if i have got this posted in the correct section, so apologies if i have got it in the incorrect place.

So i am planning to do some planetary imaging with a Skywatcher 16" Dobsonian which has GOTO tracking.

I realize it might not be the best ideal scope for imaging with, but i think it will work ok for planetary imaging.
So i got the book Planetary Astronomy for a bit of a read and research http://planetary-astronomy.com/

It is a very well detailed book but the math's that they get involved with is where i get completely lost no matter how many times i try to re read to sort out what they are going on about.

I have a ASI224MC camera was was hoping i might be able to use this with the Dobsonian.

I am trying to work out the best imaging ratio or what size barlow or not barlow to use or what they refer to as the best focal length for planetary imaging.
In the book they mention that it is uncommon in Planetary Astronomy not to use a barlow for imaging the planets.

But no matter how i look at it even with the Astronomy .tools site https://astronomy.tools/calculators/ccd
It tells be that adding a barlow will make things less compatible.

From what i can see so far, the imaging camera is 3.75µm and the scope focal length is 1800mm which all works out to be 0.43 pixels.

Just not really sure what i am suppose to do with these numbers unfortunately.
Then in the book they talk about taking an image of the planet, then open up WinJupos to get an image size pixel of the planet to work out the correct focal length.
All very confusing for myself .

So i was hoping someone might be able to offer a bit of advise or some information to help me get on the right track here.

I have all the processing programs, just trying to get tings setup correctly if possible with this setup.
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Old 05-06-2021, 09:24 AM
Startrek (Martin)
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Peter,
To work out my focal ratio for planetary imaging I use the following -

Focal Ratio for Lunar and Planetary Imaging

General rule of thumb to determine the “best focal ratio” of your image train for Lunar and Planetary imaging is -
Poor night of seeing 3.5 x pixel size of your camera
Average night of seeing 5 x pixel size of your camera
Good night of seeing 7 x pixel size of your camera

Canon 600D pixel size is 4.3 uM
Poor seeing 4.3 x 3.5 = 15
So focal ratio F15 required
Average seeing 4.3 x 5 = 21.5
So focal ratio F21.5 required
Good seeing 4.3 x 7 = 30.1
So focal ratio F30.1 required

My 6” f6 Bintel scope ( 900mm focal length )I use a 4 x or 5 x Televue Powermate
My 8” f5 Bintel scope ( 1000mm focal length ) 4 x or 5 x Televue Powermate

Seems to work Ok in the past few years

Just remember planetary imaging is more luck than skill
If your focal length is ok , Collimation is spot on and your well focused , the rest is up to how kind the atmosphere is to you and your nights imaging , hence the term “ Lucky Imaging”

Some really good planetary processing software to use is Autostakkert 3 for stacking your video files and Registax 6 for final sharpening , contrast and colour etc... they are very popular and do a great job

Cheers
Martin
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Old 05-06-2021, 12:29 PM
bluesilver (Peter)
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Thanks for the reply Martin.
That definitely simplifies things a whole heap better than what they were trying to say and go through in the book for sure.

So i am using a ASI224MC camera which is 3.75µm
I do have a modified Canon 600D but being modified it would be no good for planetary imaging without getting a dedicated UV filter to sort of bring it back to standard again.

So with the ASI224MC it should then work out to:
Poor seeing 3.75 x 3.5 = 11.25
So focal ratio F11.25 required
Average seeing 3.75 x 5 = 18.75
So focal ratio F18.75 required
Good seeing 3.75 x 7 = 26.25
So focal ratio F26.25 required

Therefore to reach F11.25 i would have to use a 2.5X Barlow or Powermate
To reach F18.75 i need a 4X Barlow
To Reach F26.5 i would need a 6X barlow.

Dose that all read and sound correct so far?

I take it that the Powermates are a far better option than a Barlow for planetary imaging?
I do have ZWO Atmospheric Dispersion Corrector, but yet to have the change to give it a try.
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Old 05-06-2021, 12:46 PM
bluesilver (Peter)
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I probably should of started another post on this next question , but didn't want to clutter things up too much.

But with the ASI224MC Camera i would need a UV filter as i am sure that these cameras not have them built it, I could be very well wrong here though.

Looking through a few filters that i currently have, i have a baader neodymium ir cut filter
https://www.firstlightoptics.com/lig...um-filter.html

Would this type of filter be ideal for imaging the planets with the ASI224MC camera or would i be better of with a dedicated IR cut filter?

I see that Bintel has a few Astronomik ProPlanet filters ranging from 642 BP IR Pass filter, 742 IR pass filter 807 IR pass filter.
The 642 BP IR Pass filter appears to be R+IR so the red must be a required as well as IR.

But still trying to research on all this also.
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Old 05-06-2021, 01:37 PM
Startrek (Martin)
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Quote:
Originally Posted by bluesilver View Post
Thanks for the reply Martin.
That definitely simplifies things a whole heap better than what they were trying to say and go through in the book for sure.

So i am using a ASI224MC camera which is 3.75µm
I do have a modified Canon 600D but being modified it would be no good for planetary imaging without getting a dedicated UV filter to sort of bring it back to standard again.

So with the ASI224MC it should then work out to:
Poor seeing 3.75 x 3.5 = 11.25
So focal ratio F11.25 required
Average seeing 3.75 x 5 = 18.75
So focal ratio F18.75 required
Good seeing 3.75 x 7 = 26.25
So focal ratio F26.25 required

Therefore to reach F11.25 i would have to use a 2.5X Barlow or Powermate
To reach F18.75 i need a 4X Barlow
To Reach F26.5 i would need a 6X barlow.

Dose that all read and sound correct so far?

I take it that the Powermates are a far better option than a Barlow for planetary imaging?
I do have ZWO Atmospheric Dispersion Corrector, but yet to have the change to give it a try.
Peter,
You need to use your scopes native focal ratio ( f5 ??? ) to determine what size Barlow or Powermate to use to achieve those calculated planetary focal ratios
So scopes native focal ratio x Barlow
or Powermate = Calculated focal ratio for planetary for respective seeing condition
Does that make sense ?

Your calculated planetary imaging focal ratios look ok ( I didn’t check the maths )

IMO Powermates are superior to Barlow’s for planetary imaging ( 5 times the price too )
I bought a full set ( 2 x , 2.5 x , 4 x and 5 x ) with all adapters to suit my EOS T ring about 4 years ago , prices have gone through the roof but they are supreme optical instruments, you pay for what you get

Cheers
Martin
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Old 05-06-2021, 02:18 PM
bluesilver (Peter)
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Yes makes perfect sense in what you are saying.
Sorry i forgot to put that number in the calculations.
Scopes focal ratio is F4.4

So F11.25/4.4 = 2.6 so i just round it down to a 2.5X barlow
F18.7 / 4.4 = 4.25 so round that down to 4X Barlow
F26.5/4.4 = 6.02 so round that down to 6X barlow.

Sorry, just forgot to put that i the original post, sorry for any confusion there.

But thanks heaps, makes it really easy to understand now, at least i can now go back and re read over the that part of the book to know what they are on about now.
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Old 05-06-2021, 06:18 PM
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Tulloch (Andrew)
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As was said above, aim for a focal ratio around 5x the pixel size of the camera. However, also be aware that barlows don't usually have a fixed "magnification" when it comes to imaging. This is explained well in the Teleview website here - you may well be better off with a 3x barlow (much more common that 4x).
https://www.televue.com/engine/TV3b_...=52&Tab=_photo

To get the correct colours for the planets, you need an IR cut filter, I just use the ZWO one on my 224 which just screws into the camera nosepiece.
https://www.bintel.com.au/product/zw...v=322b26af01d5

The 16" Dob should be a real planet killer, if you get some good seeing you should expect excellent results. As well as Christophe's book, have a look at these tutorial videos, I find them to be very helpful.
http://planetaryimagingtutorials.com/

Andrew
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Old 09-06-2021, 03:36 PM
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spacetff (Steve)
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Great thread, I also have a 16" dob and appreciate the info here. Thanks
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Old 09-06-2021, 07:27 PM
DJT (David)
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This is a great article in sky and telescope from a few years back. Nice image showing the relationship between sensor and focal length and target type

http://www.skyandtelescope.com/astro...educers/4/?c=y

Cheers
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