Mars 18 April

[Alan]

Spent quite some time capturing Mars between the clouds with good but highly variable transparency and seeing.

Quite pleased with this one but still a few more to process.

C9.25 F40 DBK21

[StarlightKnight]

Nice image Alan

Don

[Alan]

Thanks Don, much appreciated

[mattytheshark]

Amazing! Wish I could get my images to look as sharp!  Well done sir!

[PeterCPC]

Ditto Matts comment.

Peter

[oldpink]

nice capture

[si@nite]

Very nice indeed Alan!

[Alan]

Many thanks Matt, Peter, Oldpink and Simon. Regarding the sharpness I've found an electric focuser really helps a lot

[Barv]

Nicely done Alan! 

[Virgoman]

A very nice image indeed Alan. The seeing has been very trying. You have captured a nice image there with good detail!

                                                           Best regards,

                                                                                  Ralph

[JamesF]

Lovely

James

[Alan]

Appreciate your comments Marv, Ralph and James. Looks like the clouds are here for a few days so time to catch up on the image processing!

[Alan]

OOOps Barv NOT Marv

[JamesF]

Appreciate your comments Marv, Ralph and James. Looks like the clouds are here for a few days so time to catch up on the image processing!

Indeed.  I have piles of data to process.

James

[Steve 1962]

Really very nice Alan - I'm very envious.

Incidentally - how did your achieve f40?

Steve

[Space Cowboy]

Nice capture Alan.

[Pete Presland]

very nice sharp image!

[JamesF]

Incidentally - how did your achieve f40?

I think an external focuser could potentially push the focal ratio up to around f/13 depending on the camera and whatever else is in the optical train.  Add a 3x barlow on that and you're pretty close.

I find however that if you want a specific focal ratio it's easier with a Mak/SCT to get an image, calculate back to the actual focal ratio and then adjust the imaging train accordingly.

James

[Alan]

Thanks James, Steve, Stuart and Pete

I have measured the F-ratio in an image scale calculator provided to me by Chris Garry in 2012 image_scale_calculator.xlsx

The measured ratio is F43! I used a 4X imagemate barlow and as James points out the actual focal ratio of the telescope has been increased a little by the addition of a fairly low profile JMI focuser.

In 2012 I often imaged at F40-F50 and it seems to work now although the nights when the seeing allows this are much rarer. I focus by dropping the gamma to 70% or so to increase the contrast, then return it to 100%, DBK21 - gain around 800, 1/60s and only run half histograms. From many of the excellent images here on SGL I am sure that other imagers have much better methods but this seems to work OK for me at the moment

Cheers

[JamesF]

I think doing such a calculation is the best way to determine effective focal length and thus focal ratio with an SCT (or Mak).

Because such scopes focus by moving the focal plane to the camera by repositioning the primary mirror rather than moving the camera to the fixed-position focal plane, and the effective focal length of an optical system changes depending on the distance between the components (the primary and secondary mirrors in this case), it's almost impossible to find any other way.  This effect also means that the scope only has the claimed effective focal length when the camera (or eyepiece) is a given distance from the back of the OTA, information which is not usually provided.

I seem to recall that I calculated the effective focal length of my 127 Mak to be 1400mm when my DSLR was put straight onto the visual back for instance, despite the stated 1500mm focal length.

James

[Alan]

Indeed James.   I read somewhere that the scope works at F10 with the eyepiece about 4 inches from the physical back of the scope, about the same distance as the visual back plus diagonal.  It would appear that my set up makes the scope about F11 which sounds about right.  All things considered, large obstruction and only fully aberration optimised at F10 its amazing that these designs consistently produce such damn good results !!

[theo]

That is a lovely shot Alan. Nice work!!

Pete

[Alan]

Much appreciated Pete