Meade/Celestron SCT focal reducers on refractors

kayheem · Post by **kayheem** » Mon Jan 09, 2006 10:41 pm

Under the appendices, does the 'NO' mean 'No, he did not try it out' or 'No, the stated combination does not work'?

I thought he meant the latter i.e. cannot work 'cos cannot come to focus.

Post by **rcj** » Tue Jan 10, 2006 12:04 pm

hmm....my sentiments exactly. little ambiguity there..perhaps the reviewer should provide more details.

Tachyon · Post by **Tachyon** » Thu Jan 12, 2006 8:08 pm

Ok. Just tried the Celestron 0.63 reducer on the 66 ED APO. Eyepieces used were Edmund Plossl 28mm, generic Plossl 40mm, Lomo 8-24x zoom. The results are ('Yes' = able to focus; 'No' = unable to come to focus):

1. Scope - EP: No.

2. Scope - WO SCT diagonal - EP: Yes.

3. Scope - 0.63x - WO SCT diagonal - EP: No.

4. Scope - 0.63x - EP: Yes, but you have to rack the focuser all the way out and pull the eyepiece out. The plossls left the focuser tube entirely, but the Lomo zoom was able to retain a bit of the eyepiece barrel within the focuser, enough for the compression ring to hold the EP. This is due to the longer EP barrel of the Lomo zoom (35mm) vs the shorter plossl barrels (25mm).

Cheers!

Post by **Airconvent** » Fri Jan 13, 2006 11:27 pm

ok...I also did some similar tests. my refractor has a shorter focal length as compared with Tachyon (400mm I think but can't confirm as their website is down now)

Here are the results.

Equipment Under Test: 66mm refractor, 388mm FL with Celestron f/6.3 focal reducer (FR), 32mm, 40mm, 15mm super plossl, x2 barlow, x2.5 TV barlow., N8 SCT Visual Back (VB)

Scope + FR = Visual back too short. Cannot insert diagonal

Scope + FR + SCT VB = Can only focus if eyepiece is about 1mm from Visual back.

Scope + FR + SCT VB + Diagonal = Cannot focus

Scope + FR + SCT VB + Diagonal + barlow = cannot focus

Scope + FR + SCT VB + barlow = cannot focus

Overall confirms what everyone is saying here, the FR cannot work on a refractor (using standard components anyway). thanks to remus for that detailed description on why it performed this way...

Off topic :
BTW, pleasantly surprised to note that the image is erect, so with a 40mm ep, I can use the scope as a finder without disorienting myself!

I trained the scope on a lantern one block away and played with 2 barlows and a 4mm eyepiece. At x220, pleasantly surprised to see the image only softened a little and I could see the lit filament through the lantern fabric.
Using the fine focus, I can still get a pretty good image. Then ridiculously pushed it to x440 and that's when the image broke down and was unable to get a focus on the filament. Not a technical assessment but I am pleased with its perfirmance albeit not too happy with the small aperture...

kayheem · Post by **kayheem** » Sat Jan 14, 2006 8:11 am

Airconvent wrote:
I am pleased with its perfirmance albeit not too happy with the small aperture...

The answer to that is simple. Get a 4" or 5" Apo!

BTW, when you mention that the various combos were not able to come to focus, was it due to insufficient in or out-travel of the focuser?

Kay Heem

Post by **rcj** » Sat Jan 14, 2006 6:34 pm

good observations!

Post by **Airconvent** » Sat Jan 14, 2006 9:47 pm

kayheem wrote: The answer to that is simple. Get a 4" or 5" Apo!

BTW, when you mention that the various combos were not able to come to focus, was it due to insufficient in or out-travel of the focuser?

Kay Heem

thanks Kay Heem but the intention was to get a setup that was so light, you would not know you are carrying it! so stability takes second place to portability...
as to your question, some are insufficient out travel while others I can't even tell as I see no focus for the entire focussing range...

btw, just confirmed the focal length for Tachyon's scope is 460mm against my 388mm....but seems both models also unable to focus using with FR....

Tachyon · Post by **Tachyon** » Sun Jan 15, 2006 1:14 am

Actually, mine can reach focus with the 0.63x focal reducer, as long as I do not use a diagonal, and use an EP with long barrel.

Cheers!

Post by **Airconvent** » Sun Jan 15, 2006 1:26 am

Tachyon wrote:Actually, mine can reach focus with the 0.63x focal reducer, as long as I do not use a diagonal, and use an EP with long barrel.

Cheers!

hi andrew,
how do you define an eyepiece with a long barrel?
my 40mm is actually quite long but slender compared with pentax's and naglers...heh heh
actually if I had tried to put a t-adapter between the visual back and the eyepiece, likely I will be able to get a focus....

Canopus Lim · Post by **Canopus Lim** » Sun Jan 15, 2006 6:51 pm

The problem is inward travel. This is because the focal reducer shortens the telescope focal length and hence the eyepiece have to be brought nearer (inward travel) to the objective lens. By using a diagonal (which adds to the optical path), it extends the physical length of the telescope and prevents the eyepiece from reaching focus. I suppose longer focal length eyepieces will be easier to reach focus.