Unable to reach focus on Skywatcher ED80 APO

[ozboyz1]

I just got this tube, but unable to reach focus with my Canon 550D. I suppose I need an extension tube? If so, could you please advise which one should I get, 50mm or 80mm?

Many thanks in advance.

[Alien 13]

You will probably need a 2" (50mm) extension if you are not using a FR/FF to allow for the extra lightpath of the removed diagonal I use something like this http://www.firstlightoptics.com/adapters/skywatcher-2-inch-t-adapter.html but a normal extension would work just as well.

Alan

[Ant]

Does the camera come to focus with a diagonal in place? If it does you can have a rough measure.

But I think the 80mm would be better, as the focuser can be wound out less with the [heavy] camera attached.

But do try and have a measure.

Ant

[xtreemchaos]

hi mate i get round it by useing 2inch wedge but i have sony a37 camera not cannon but it should  work with cannon.all the best charl.

[hjw]

When I bought mine I was told that I need a 50mm extension, which I bought and which does do the job (on a Pentax). But I also noticed that I would need a flattener pretty much after my first imaging session. I bought the Orion flattener without focal reduction and have been happy with it. It was only twice as much as the extension tube and I could have saved the money for the extension.

Clear skies!

HJ

[Steve Ward]

Forget an extension and put the cash towards the essential 0.85x FR/FF and the FLO adapter for it that allows you to use 2" filters .

http://www.firstlightoptics.com/reducersflatteners/skywatcher-85x-reducer-flattener-for-ed80.html

http://www.firstlightoptics.com/adapters/flo-adapter-for-skywatcher-focal-reducers.html

[bruno]

I agree with the above posts on FF's, when i got my extention I got focus took some shots & posted on here & although the comments were positive it was pointed out to me that my picture would have been better if i had used a FF as it removes the vignetting from the edges of the shot, so now i have both, point being is i could have saved £15.00 if i had bought the FF first  here is a link http://www.firstlightoptics.com/reducersflatteners/skywatcher-field-flattener.html

[Steve Ward]

The dedicated FF/FR that I linked to is the better option , made specifically for the ED80 , as well as flattening it 'speeds up' the ED80 from f7.5 making it much better suited for imaging.

[lensman57]

You have 3 choices.

1- An Orion / SW Field flattener without reduction ratio @ £65.00~£70.00, this already takes a 2" filter on the nose.

2- A dedicated SW FF/FR with a reduction ration of 0.85X @ approx £150.00 and about £40.00 extra for the end nose piece to take 2" filters,  and the M48 to T2 adapter. Total cost about £190.00.

3- An extension tube of 50~80mm length, about £25.00 to £30.00,  to compensate for the lack of a diagonal, If you go down this route get the longer extension tube so the focuser's outward travel is minimised and this helps rigidity as SW Crayford focuser fitted to its budget range are not too hot.

The third option is rather a redundant one as you would need to flatten the field at some stage so your choices are effectively down to 1and 2.

BTW, an FF/FR does not make F number of the scope faster, this is a myth. What it does is reduce the Focal Length and give you a wider flat field. As the Focal ratio is defined as FL/Aperture the apparent FR is reduced otherwise there would have been no need to design mega box fast F scopes, we would just use a focal reducer of 0.5X instead.

Regards,

A.G

[Steve Ward]

The effective focal ratio is reduced from f7.5 to f6.375 (600/80 - 510/80) ... ? 

[lensman57]

The effective focal ratio is reduced from f7.5 to f6.375 (600/80 - 510/80) ... ? 

Putting an FF/FR in the imaging train does not bring more photons through the 80mm aperture of the scope from the target , the only way of speeding the imaging. You will get more photons through because the FOV is wider so if the target entirely fills the reduced field one may argue for the reduction at the expense of resolution of course but if using the FF/FR makes the target even smaller no gain is achieved besides the flat reduced field, the latter is the case in the majority of widefield DSO imaging.

Regards,

A.G

[ozboyz1]

HI all,

Much appreciate the advice    Which FR is the best choice for my ED80 F7.5, I'm using it mainly for AP. I've seen 0.63, 0.75. 0.8 etc.

I have Baader coma corrector for my Newt, is it similar to FF ?

Thanks again

[Uranium235]

HI all,

Much appreciate the advice    Which FR is the best choice for my ED80 F7.5, I'm using it mainly for AP. I've seen 0.63, 0.75. 0.8 etc.

I have Baader coma corrector for my Newt, is it similar to FF ?

Thanks again

If I were in your shoes, I'd go with the dedicated SW 0.85x FF/FR. It screws directly to the focuser drawtube via its M54 thread, making it ultra-solid and reliable (rigid connections are a sort of "prime directive" in imaging...lol).

Your MPCC is basically just a flattener designed to counter the coma/curvature you get with a newtonian..... But I have often thought... "has anyone actually tried one of these in a frac?".... my guess is that it would probably overcorrect and look pretty nasty.