William Optics GT81 APO Triplet

[scoobee]

I'll confess there have been times when I feared WO had lost their way (i.e. DDG focusers) but I am very impressed with what I am currently seeing. The GT102 box has a hand-written QC sentence with the date and signature of the person responsible. And sure enough it was faultless

My 71 ED is outstanding even without a reducer or flattener and to be honest this months S@n mag review gave it  4 out 5 stars, maybe an extra half star is need

Back on track, after scrapping enough for the GT81, the GT102 comes along and now looks like a longer wait!!

So, what will be the difference between an f-ratio of f5.52 as compared to F6.9? as I cannot decided between the WO or the Hotech.

Thanks

[FLO]

So, what will be the difference between an f-ratio of f5.52 as compared to F6.9? as I cannot decided between the WO or the Hotech.

In traditional photographic terms the f6.9 option should require nearly double the exposure time. But (and this is a big 'but') the aperture is unchanged so the telescope will not collect more photons at f5.52 than it does at f6.9. Aperture determines how many photons are collected (larger apertures collect more light) and the f-ratio determines how those photons are distributed across the CCD pixels. In the real world the f5.52 option will require less exposure time but the difference is not as significant as you might think. 

For more on this subject please see Craig Stark's excellent article. 

HTH, 

Steve 

[scoobee]

In traditional photographic terms the f6.9 option should require nearly double the exposure time. But (and this is a big 'but') the aperture is unchanged so the telescope will not collect more photons at f5.52 than it does at f6.9. Aperture determines how many photons are collected (larger apertures collect more light) and the f-ratio determines how those photons are distributed across the CCD pixels. In the real world the f5.52 option will require less exposure time but the difference is not as significant as you might think. 

For more on this subject please see Craig Stark's excellent article. 

HTH, 

Steve 

Many thanks for the link and explanation, this is what I presumed to be the case but is hotly debated at our club hence my confussion

[DrRobin]

In terms of imaging, especially deep sky objects, I have found that exposure times follow the F/number rather than the aperture.  So if you go from a F/7 to F/5 exposure times approximately half.  If your F/7 scope has a much bigger aperture then the F/5 scope, then exposure times don't quite double.

If I change from my C11 @ F/10 to an 8" newt at F/4 exposure times come down around 4-5 times, just about what you expect from the F/ratio, but slightly modified by the bigger light capacity of the C11.

HTH

Robin

[DrRobin]

GT102 - Quick first impressions

As I have had my GT102 for a week now, I thought I would write a few first impressions.

Overall I am very pleased, the build and finish looks good and the hardware in gold looks sharp as well.  I have only used it for one white light solar imaging session, but it was easy to reach focus with my wedge with a DMK41.  It will also focus with a 3x TV barlow and a small focal reducer thanks to the very generous focus travel.

The rotatable focuser works really well and the focus action is very smooth and precise.  The 10:1 focus knob is hidden behind an aluminium cap, which with the temperature gauge on the other side are no use what so ever.  The screw on cap could be drilled to hold a stepper motor if I ever want to motorise the focus but otherwise they could have not fitted these and save a few quid.  Although I only had a DMK on board, with my wedge and TV 3x I would be easily over the weight of the average DSLR.

My one and only solar session was through cloud and as most people are going to be interested for wide field night time, I can't really comment yet on the optics, but expect they will be superb.  It is going to be several weeks until it is fitted to a mount for night time and then I will test the scope and flattener/Reducer and post again.

I bought this scope with for both solar (white light to start with), but also wanted something for night time use and didn't want to buy two scopes, it's a bit over-specified for solar.  So far it has reached all expectations.

Robin