What would be the best Powermate for my scope?

[Gordy]

Hi all.


I have a 5 inch refactor f7.7 (1000mm FL) which is used for visual and deep sky imaging. I already use a 1.5x focal extender but it is not enough for those smaller objects. For this reason I am thinking of buying a Powermate to increase the image size. With the Powermate range, is there a need to match the magnification type with a scope's FL? If so, what would be the best one which would be compatible with my scope's native FL? I know whatever is chosen the f number will increase accordingly.



Thanks in advance.

[Merlin66]

Gordy,
Which camera are you using?
Normally I work to plate scale...if, say, you have a DSLR (5.7micron pixel) then the Plate scale at prime focus would be 1.22 arcsec/pixel

With the x1.5 you get around 0.81 arcsec/pixel, which is pretty good for average seeing conditions.


I use the CCDCalc to give a review of the possible results.
http://www.newastro.com/book_new/camera_app.html

[Startrek]

I use the Jerry Lodriguss guide below for planetary imaging -

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

My 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

Therefore I mostly use my 4 x Powermate and 5 x Powermate on my 6” f6 and 8” f5 newtonian reflectors

Hope the above helps

[Merlin66]

Interesting....
Your Average seeing "rule of thumb" seems to give a plate scale of 0.25 arcsec/pixel.
Not sure the average mount and guider for the OP's DSO requirement would compliment such a plate scale...
For planetary/ solar/lunar using a fast frame camera and "lucky imaging" it may be feasible.....

[Startrek]

Ken
Your optical maths is obviously correct but here’s some of my recent images captured with BYEOS Planetary mode from Bortle 8 Sydney average seeing with my 6” f6 newt , Canon 600D , 4x and 5x Powermate and only 45 to 60% of the best 1500 AVI frames chosen in Autostakkert 3 and some fine sharpening in Registax 6. My Canon 600D set at 5 x zoom achieves close to 1:1 pixel resolution then I add the Powermate to increase my focal ratio accordingly to the seeing
Note: Mars was in the middle of a huge dust storm so poor clarity
Cheers
Martin

[Merlin66]

Martin,
Very nice results!


For "lucky" planetary imaging the rule of thumb is OK.


Not sure it can be applied to extended exposures required for DSO's.

[Startrek]

Ken
Here’s a close up image of the Carina Nebula ( featuring the Keyhole Nebula and Eta Carina Star ) captured in March with my 8” f5 newt , Canon 600D with a 2 x Powermate ( newt has a focal length of 1000mm and with Powermate is effectively 2000mm ) 32 x 5 minute exposures
Stacked in DSS , processed in Startools

You do lose a great deal of detail as soon as you increase focal length at the same aperture
This was an experiment to see if I could image DSO’s with a Powermate and achieve reasonable results, which seemed to work out ok ( however Carina is a bright object at mag 3.5 ) Considering the quantum efficiency of my Canon 600D is 40% and my new ZWOASI2600MC has a QE of over 80% , I’m going to experiment on some galaxies and other objects in the near future in my new obs when’s it’s finished in March at my dark site. I’ll be using my 8” f5, a 2 x Powermate and the new ZWO camera capturing plenty of 5min subs and hopefully see if I can process some decent images out of it all.
Cheers
Martin

[Gordy]

Hi Ken.


I am using a Canon 6D. At 1500mm focal length it gives me 0.89 arc/pixel.

[Merlin66]

Gordy,
Unless you ‘re interested in planetary imaging, I’d stick with what you have.

[gregbradley]

For DSO 1-2 arc second per pixel is the usual range. 1 arc sec s considered ideal by Roland Christen of AP fame based on average seeing of around 3 arc secs.

At my dark site I find on nights of good seeing .42 arc secs gives sharp images but on weaker seeing its better to go to .89 arc secs.

But this is for DSO not planetary where long focal length is used.

Greg.