f6.3 focal

[zakkhogan]

I intend to get one for my CPC800 for imaging purposes.

Will it also help when view faint objects though the eyepiece as well?

[scarp15]

I have only used my f6.3 focal reducer on my C8 to create a wider field for when visually framing views of larger objects such as some open clusters, M31 and M81/M82 etc, for this it is quite effective. I have read accounts from some visual users that it also appears to make stars a bit brighter, although I haven't used mine enough to personally comment on this.

[John]

The focal reducers, as their name suggests, reduce the effective focal length of the scope so enable a wider field of view to be seen. As they add glass to the system (3 elements I think) they might have a slight reduction in light throughput although the ones I've used are very well coated so I've not noticed this. I don't think they could actually increase light throughput for visual use though.

[alan potts]

I have one of these for my LX 200 and I was one of the brain dead that thought you could put it on slam in a 40mm eyepiece with wide field and see behind me. Wrong.

For imaging they reduce the amount of exposure that you require in the case of F 6.3 from F 10 I would say by about a third to a half. I am sure imaging experts will jump in and give you the correct info.

I got mine for visual and they cannot make the stars brigther as the F/R has 3 lens elements so there is a reduction in brightness. What will occur is a wider field with less magnification for any given eyepiece as the F/L goes down. I think it is because it gives a slightly tighter field of view and sharper stars. Your limit of eyepiece is 28mm 2 inch without vignetting.

The other thing they are good at is sealing the optic tube in the case of SC scopes, stops dust fairly well.

Alan.

[Dunkster]

Your limit of eyepiece is 28mm 2 inch without vignetting.

Just curious, how is this calculated? 28mm 2" EP at f6.3 is pretty wide for SCT

[alan potts]

Dunster,

No idea what so ever, 28mm gives a faction over 1 degree. I tried it with a 30mm UWA but it vignetts, so to a 32mm S Plossl with a 60 degree FOV. They say you can only use 27mm, but I can't see any vignetting with 28mm.

I want to try my 24mm UWA when I get it from Enland to see if that is OK. I would then like to try a 26mm Nagler, this would give i.i degrees, but to the very best of my knowledge there isn't one in Bulgaria.

I will have to buy one to try that.

Alan.

[John]

I don't think the F/6.3 focal reducers were designed to be used with long focal length 2" format eyepieces - thats when you seem to get the vignetting issues. For visual they seem to work better with 1.24" eyepieces in a 1.25" diagonal.

When I had a C8 I used a 2" 40mm eyepiece with a 70 degree field of view to get my widest views. No focal reducer involved.

[alan potts]

John, maybe you are right, but this is what I have found possible. I bought it with the idea of getting a 70 degree field and a magnification of around 50 on my LX 200. I was of course woefully wrong, this is what I mean when I say I have no one to talk to in a shop. Now of course I have all of you members on the forum, never occured to me to join before. When you are new to a hobby you make mistakes, this one was my only mistake so far.

Alan.