SBIG ST8300C - First Light

[dcalleja]

Hi
Been a while since I've posted anything (or done any imaging for that matter) due to work pressures. Anyway my new SBIG 8300C arrived last week and I managed to get out last night for a few hours.

Heres the first image (yes its Eta). 3x5min at -15c through the ED80 (scale is reduced but I'll put the full size up on www.dancalleja.com ).

I love the camera; easy to use, precise cooling, high sensitivity, low noise and simple dark management .

Processing was a quick one so any critique is welcome.

I note some vignetting - do I need a flattener - and what are people using on these 80mm scopes?

[JD2439975 (Justin)]

Not qualified to comment on the flattener but as far as camera data goes I can see why you've fallen in love with it, 15min of first light is showing a lot of detail there.
Should be nice through the 8" scope.

[gregbradley]

Wonderful ist light and bodes well once its all sorted.

Yes you need a flattener. All RC scopes are corrected for coma only. They still suffer to some degree from spherical aberration meaning off axis stars will be to some degree out of focus. Only corrected Dall Kirkhams or corrected compund scopes like corrected Casasegrains adjust for this. In the RC a field flattener handles the bulk of this. In your image you can see the stars near the centre are sharply in focus yet the edge stars are clearly out of focus. It is also worse on the right side than the left. This may be some other issue like tilt in the camera or sag in the focuser under weight.

Basically the bottom line is to get ideal performance from any 2 mirrored scope it requires a corrector lens of some description usually a doublet sometimes a triplet to correct for the spherically induced aberrations.
Even your $50,000 RCOS ideally still needs a flattener (although not as badly needed).


Ask Paul his posts say he uses a Tak flattener.

Greg.

[Tilt (Michael)]

Originally Posted by dcalleja

I note some vignetting - do I need a flattener - and what are people using on these 80mm scopes?

Nice image, very smooth. It must be great to be imaging with a cooled CCD

As for the curve in the field, that can be corrected by using a flattener such as a William Optics 0.8x field flattener/reducer. I use the Pflat2 with my ED80, this seems to work fairly well.

Michael

[multiweb (Marc)]

Originally Posted by dcalleja

I love the camera; easy to use, precise cooling, high sensitivity, low noise and simple dark management .

Looks great. Glad to see you're having fun with it. I have the exact same scope as yours, an Orion ED80. I have used the WO P-FlatIV with some success here and here.

[dcalleja]

Gents - thanks. Looks like the WIlliams may be getting the nod here. Any experiences with ordering these from OS or is there a local supplier?

Marc
Those look like some good results so I feel another purchase coming on to use with my ED80 imaging rig!!

[mill (Martin)]

Niccceee!!!
The Pflat you can get from http://www.andrewscom.com.au/site-section-10.htm

[telecasterguru (Frank)]

Dan,

I think you will find Andrews at Greystanes in Sydney are a Williams supplier. Have a look at their website.

By the way, great first light.

Frank

[dcalleja]

Thanks Chaps - another toy for me!!

[dugnsuz (Doug)]

Great first results Dan - thread seems to have diverted to the search for a flattener. Why did you go with -15 degrees, doesn't the CCD get down to -40?
Very impressive first light
Doug

[Hagar (Doug)]

Nice first light image... As for a flattener. I used a WO PFlat2 on my ED80 but now believe the flattener of choice would be the PFlat4 as it is adjustable and still works well with large sensors. Avail from Andrews I believe.

[dcalleja]

Doug & Doug - Yep will be placing an order tomorrow. I'm sure it could get down to -40. The issue is that it was already ambient 25 and to maintain the temp accurately you need to set the temp so the peltier is not working at 100% all the time. I probably could have set it lower though.

[Peter Ward]

Originally Posted by gregbradley

All RC scopes are corrected for coma only. They still suffer to some degree from spherical aberration meaning off axis stars will be to some degree out of focus. ......

Greg.

Que???? I beg to differ

Last time I checked a text on telescope design, RC's only suffer field curvature and some astigmatism. If the astigmatism is contained within the airy disk, you'll never see it in an image.

Spherical aberration is a fabrication error and is only present if the telescope has been made incorrectly.... Checking my RCOS inteferogram is has a 1/25th wave spherical error...i.e. essentially perfect.

Field flatteners, be they for RC's, SCT's APO's or DK's do just that, put all the stars onto a flat focal plane....a handy accessory if you can't bend your CCD to the required curve

P.S. Nice first light...focus a little soft...everthing else all heading in the right direction

[Satchmo]

Originally Posted by Peter Ward

Spherical aberration is a fabrication error and is only present if the telescope has been made incorrectly.... Checking my RCOS inteferogram is has a 1/25th wave spherical error...i.e. essentially perfect.

We need to be careful quoting numbers here.

The 30-40% central obstruction throws light out of the airy disc and in to the surrounding fresnel rings reducing an essentially good `theoretical' wavefront to one producing effectively at around 0.8 Strehl , similar to an unobstructed scope with 1/4 wave spherical aberration, so quoting 1/25 wave for the scope is a bit academic. In fact it can't work any better than any 1/4 wave wavefront 'scope with a small obstruction. Certainly an instrument with 1/4 wave spherical and a .3 obstruction will perform worse, but only reduced to around 0.7 Strehl in the presence of obstruction ( what is called the EER or Encircled Energy Ratio which takes in to account the central obstruction) .

[Peter Ward]

Originally Posted by Satchmo

We need to be careful quoting numbers here.

I don't recall quoting strehl ratio's. Just the spherical error of the system.

Paul Jones gives a spherical and astigmatic certification of the optical sets he ships to RCOS. As I said, the spherical error is this case near 'nuff nil

BTW didn't mean to hi-jack the thread......it's a great first light Daniel

[Satchmo]

Iv'e reread Gregs and your post and can see the sense in which you were quoting this SA figure ( ie no `inherent' spherical aberration in an RC) .

I would be interested to know some time the quoted total system RMS accuracy. . As I've expressed the sharpness of your photos is not down to a small SA term or otherwise as the central obstruction will significantly degrade the finest optical system but it all helps.

Sorry for the hijack Daniel, very promising first light.