Anyone knows how to change or shorten the Starblast focuser?

Chris · Post by **Chris** » Tue Jul 06, 2010 12:45 am

Hi all,

Sorry for the typo, I bought an imaging starblast several months ago but am not able to do anything with it as being a newtonian, the focus can't reach my DSLR chip.

Kind of need to get it 1-2cm closer. Any ideas that only mess with or change the focuser without shifting the position of the mirrors?

I am aware the smaller ccds like the DSI pro and orion starshoot are compatible with the starblast. But the rack and pinion focuser is crap anyway and orion apparently don't sell focusers for 4.5 inch reflectors. Or should I just get the cameras mentioned above? My main concern is the lack of cooling, small megapixel count and poor resolution.

MooEy · Post by **MooEy** » Tue Jul 06, 2010 5:02 pm

resolution is a function of pixel size and focal length, not pixel count. to call a puny chip camera poor resolution is a little unfair.

what t-adapter/t-ring combo are u using? would there be anything shorter? also, if it's really that 1-2cm away, any chance of shifting the primary mirror up just a little more to reach focus?

orly_andico · Post by **orly_andico** » Tue Jul 06, 2010 5:05 pm

starblast has too small a diagonal to fully-illuminate a DSLR. you would have huge amounts of vignetting with such a setup.

that's probably why orion designed it for the StarShoot crowd.

the StarShoot and most modern DSLR's have almost the same pixel pitch (about 5 - 6 micron) so the resolution would be the same.

the DSLR would have a much wider field, but if you can't illuminate that field.. then it's not so useful.

Chris · Post by **Chris** » Tue Jul 06, 2010 10:40 pm

I see,

I am not so proficient in camera stuff. I got an imaging starblast, and the secondary is HUGE. So maybe can, maybe cannot...

@Mooey: I using t-ring and antares T-adapter. Checked with Joo Beng, that is the shortest possible.

Hmmm, it seems pretty hopeless now. I think I will just use it as guide scope then.

timatworksg · Post by **timatworksg** » Tue Jul 06, 2010 11:19 pm

Sorry to hear that Chris. The StarBlast suits CCD cams like the starshoot and DSI ccd cams. DSLR's need a wider FOV. Even if you did find a way you would get vignetting due to the small secondary. Also it may not be possible to midify the Starblast's spider to hold a larger secondary....size ratio and all that!

I think I have read some either risking it and moving the primary or shortening the focusser tube. All done at risk I must add!
Perhaps use it as a guidescope/visual for now and fool around with afocal for planetary...till a good deal for a ccd comes up. But yeah, the pixel count is less compared to what you get with a DSLR.

In the end, whatever you decide to do,...as long as it works for you,...then Hell Yeah! HAve Fun!!

orly_andico · Post by **orly_andico** » Wed Jul 07, 2010 1:04 am

according to this thread the imaging starblast (even with the larger secondary) barely covers a DSI-type sensor so would definitely be too small for a DSLR.

http://www.cloudynights.com/ubbarchive/ ... o=all&vc=1

Post by **weixing** » Wed Jul 07, 2010 10:09 am

Hi,
IMHO, you don't need the 100% illuminating circle to cover the whole DSLR sensor for casual imaging... you won't see the different normally.

I think the main issue for using DSLR to do imaging on StarBlast is coma and I don't think there is any 1.25" coma corrector in the market at the moment. That's why this scope is design to use on a small CCD chip when coma is still acceptable.

Anyway, Newtonian is a DIY friendly design, you can basically change any components "easily" (compare to other design), such as secondary mirror, focuser (but you need to enlarge the focuser hole on the OTA) & etc.

By the way, I think your best bet is to shorten the tube a bit and shift the primary mirror up.

Good luck and have a nice day.

orly_andico · Post by **orly_andico** » Wed Jul 07, 2010 10:21 am

According to Mel Bartel's diagonal calculator, the 2" diagonal on the StarBlast will fully-illuminate a 12mm radius image circle. This should cover an APS-C sensor (which is 16mm x 24mm in size). However the StarBlast is f/4 so as weixing pointed out, the coma will be quite severe.

The usual suggestion to reach focus with a DSLR is to shorten the tube. I think it's possible to unscrew the mirror cell from the back of the tube. Then use a dremel with carbide cutting wheel or similar and shorten the tube by 1 - 2 cm, then drill new retaining screw holes for the cell.

Chris · Post by **Chris** » Wed Jul 07, 2010 1:19 pm

I see. Thanks for the advice guys.

I think I will be using this with the meade DSI pro, then the set can move on to being a guidescope when I ungrade to a better imaging scope.

Meng Lee · Post by **Meng Lee** » Wed Jul 07, 2010 1:38 pm

Hello Mr Chris,

I owe u this formula.

Resolution, or more correctly, sampling is given by,

Sampling (arcseconds per pixel) = (206 x pixel size in microns) / (focal length in mm)

so it has nothing to do how many mega, giga, tera pixels.

Meng Lee