Celestron Nexstar 8SE SCT

[cloudsweeper]

It's arrived!  The excitement is palpable.  Can't wait to use it and make some observations concerning performance compared with the 127 SLT.

Bigger than I thought - it's like an oildrum on three short telegraph poles, but I don't say that in a negative way of course!

A couple of things you experienced folk might be able to assist with, please:

1. The specs are 2032mm and 203.2mm, but round the rim at the front it says 2000mm.  Is that just a nominal figure, and is it really 2032mm?

2. You can't carry the assembled unit, so am I right in assuming the normal practice is to remove the tube for carrying, rather than the whole mount?

Thanks a lot.

Doug.

 

[Peter Drew]

Hello Doug.  You are going to enjoy this telescope, I have one and have been very pleased with it. The primaray mirror is nominally 203mm diameter and at f10 the focal length is nominally 2032mm, I say nominally as SCT effective focal lengths vary a few % depending on what accessories are in use, a diagonal for instance increases the fl due to refocusing of the primary mirror. I usually remove the OTA each time although I can lift the whole setup. The only slight downside I have found with the 8se is that it is the maximum size for the mount and needs carefull handling to avoid vibrations. 

[Bino Pete]

36 minutes ago, cloudsweeper said:

It's arrived!  The excitement is palpable.  Can't wait to use it and make some observations concerning performance compared with the 127 SLT.

Bigger than I thought - it's like an oildrum on three short telegraph poles, but I don't say that in a negative way of course!

A couple of things you experienced folk might be able to assist with, please:

1. The specs are 2032mm and 203.2mm, but round the rim at the front it says 2000mm.  Is that just a nominal figure, and is it really 2032mm?

2. You can't carry the assembled unit, so am I right in assuming the normal practice is to remove the tube for carrying, rather than the whole mount?

Thanks a lot.

Doug.

 

Exciting times indeed Doug!

Looking forward to a first light review.

Peter

Edited April 2, 2016 by Bino Pete

[cloudsweeper]

Thanks @Peter Drew - one other little thing: there is no shoe for my finderscope, so can I easily get one that will fit?

Cheers,  Doug.

[Peter Drew]

I binned the RDF and modified the base of a standard 9x50 RACI finder to fit the holes left by the RDF.

[cloudsweeper]

Thanks, Peter - I'll try that!

[laudropb]

It's very nice scope. You will have a very enjoyable time learning its capabilities.

[YKSE]

The focal length of SCT 2032mm is said to be correct if you use the stock 1.25" diagonal with SCT mount(with minor variations depending on EP's focus postion since primary mirror's move changes the focal length), that's about 100mm light path length.

If you add accessories in the light path, e.g. using 2" diagonal instead, or add a focuser, the focal length will increase. The number I've seen mentioned is that every 10mm above 100mm adds 31mm focal length, besides, every 25mm above 100mm adds 1/23 spherical aberration. So if you add a focuser(some 90mm long) and a 2" mirror diagonal (some 60mm longer than the stock diagonal), focal length with be arond 2032+15*31=2497mm, that's about a f12.5. Besides, 150mm over 100mm adds 6/23 spherical aberration, assuming that the scope was original diffraction limited (1/4 spherical aberration), the accessaries have made it closer to 1/2 spherical aberration, tthe performance deterioation should possible be seen.

My C8 (a SE OTA) didn't come with RDF, the finder shoe is suitable for either SCT RACI or any finder with Vixen style mounting shoe, Skywatcher and many others have finders with this shoe, not Williams Optics, I think.

[cloudsweeper]

Thanks @YKSE - so if I get this, the mirror has a FL of 2000mm, the 1.25" diagonal (extra 100mm path) adds 32mm, and a 2" diagonal (160mm path) would add a further 19.2mm making a total of 2051.2mm?

Cheers,

Doug.

[YKSE]

The measurement I read is 2032mm FL with 1.25" diagonal, that 100mm is the length from diagonal's connecting plate to EP's shoulder through axis center. a 2" mirror diagonal with its connetions (both to the scope and to EP) will be about 60mm longer than 100mm. the increased FL will be (60/10)*31 =186mm, results in 2218mm FL.

Prism diagonal will have shorter light pass than mirrors, for 2" diagonals, only prism diagonal with direct or SCT connection to the scope can have the specified 2032mm FL.

if you have a 2" diagonal, you can easily verify it: using a same 1.25" EP, looking at a target in daytime with both diagonals, there'll be noticeable narrower FOV with 2" diagonal, that's the effect of longer FL.

[cloudsweeper]

Thanks again @YKSE - I've adjusted my calculations accordingly and now they match your figures.  Just when I thought I was getting the mag down with long FL 2" EPs, this effect reverses it again!

Doug.