New f/6.3 SCT - First Light

[cloudsweeper]

Well, not exactly, it’s the familiar f/10 fitted with a new Focal Reducer / Field Flattener.

I have toyed with the idea of getting one of these for my 8SE SCT on GoTo for a few years, and finally took the plunge with an Antares version – cheaper than the Celestron, and reportedly just as good. Intended use: visual only.

It does not give greatly improved max FOV (more later), but the main reason for buying it was to hopefully get sharper star images, especially at high power. Collimation was checked, and so poor collimation was not the reason for soft images. Celestron publish data/charts which show “spot diagrams” for a basic SCT, a coma-free version, and the Edge HD model, and on axis, the basic model is clearly softer (not tight). (Off-axis, aberrations are apparent, least of all for the Edge HD.)

The Focal Reducer (FR) is reported to perform best with the normal Visual Back, and a 1.25” diagonal. (It is also believed to work well with a 2” SCT-fitting diagonal (and no Visual Back), plus an adaptor for 1.25” EPs, and I did try this. It worked OK, without evidence of vignetting, and stars were generally sharp across the view.) However, I did the main test with the 1.25” diagonal (high reflectivity model).

An extra advantage is that the FR blocks off the tube against dust ingress.

 

MAGNIFICATION

With the FR, the max useable mag is about the same as for the f/10 SCT: a little over x400. (This is expected, being a function of aperture.)

 

SHARPNESS & FLATNESS

Specs removed – I use varifocals.

‘Scope well cooled – no spiky stars in the central field.

I chose to work in a region in Orion’s Belt, with several stars, none very bright.

Stars were sharp across the field for Baader Aspheric 36, Baader Hyperion 10, Morpheus 17.5, ES (82deg) 14, 11, 8.8, and 6.7, and TV Radian 6 - covering a range of x36 to x213.

They were generally a bit less sharp, but no worse at the edges, for Cel XCel LX 5 and TV Delite 4, with mags of x256 and x320 – as might be expected.

Plossls:

The Meade Super Pl 32 and 15, and the Meade 3000 Plossl 25 all showed signs of focus dropping off towards the edge of field, but the TV Plossl 20 didn’t. It was a pleasure to use. (Or did I just expect this?!)

 

FIELD OF VIEW

This is of course a big consideration for many users, so I offer my findings on how the FR affects FOV.

The best FOV is about the same with or without the FR:

1.34° without, using a 42mm 2” EP, increasing very slightly to 1.42° using a 36mm 1.25” EP. (Assumed AFOVs: 65° and 50°.)

The improvement is much more noticeable when only 1.25” EPs are used in each case. With the FR, the FOV increases in all cases by about 58% on average:

0.62° without, 0.98° with, when using a 25mm 50° EP. This is quite a significant increase.

0.15° without, 0.23° with, when using a 5mm 60° EP. This is much less significant.

If we take a quarter of the central Finder Circle (0.5°, 30arcmin) as a nominal measure of least worthwhile increase in FOV, that is 7.5arcmin or 0.13°, and this is achieved (decreasingly so) as mag increases. It is no longer achieved with the 6mm 60° EP and shorter.

 

SUMMARY

So now I believe I have what I have been striving for: a decent aperture, GoTo, and finally – nicer, sharper stars with most EPs. (It’s not going to match the Edge HD, but is I think an improvement on the straight SCT.) And, of course, there is greater FOV for all 1.25” EPs, most noticeably at low power.

I would recommend the Focal Reducer to SCT owners who seek sharper stars on- and off-axis, but if you only want to raise the best FOV figure for your telescope, the FR will not satisfy that need!

Doug.

Edited March 1, 2022 by cloudsweeper

[coopman827]

What was the FOV with the 2" 42mm eyepiece, 2" SCT diagonal and the FR?  It seems like this would have provided the max. FOV.  

Edited March 3, 2022 by coopman827

[cloudsweeper]

1 hour ago, coopman827 said:

What was the FOV with the 2" 42mm eyepiece, 2" SCT diagonal and the FR?  It seems like this would have provided the max. FOV.  

Yes, you might expect that, but as I understand it, long focus 2" EPs simply don't perform well in that configuration.  The gain in FOV is limited, and vignetting can arise.

Guess I'll have to try it out!

I'd be interested to hear what others have found here.

Doug.

[Louis D]

You'll have to see what affect the FR has on planetary details at high powers when some planets are better positioned.

Did you try looking for the E and F Trapezium components with and without the FR?  That would also be a good test.

[Mr Spock]

Maximum field size is determined by the exit aperture of the scope. Most RFs don't match that and can actually reduce maximum field size.

My old C9.25 could manage 1.25° with a 42mm LVW, which theoretically should reach 1.28°. There was just a bit of vignetting holding it back.

[cloudsweeper]

26 minutes ago, Louis D said:

You'll have to see what affect the FR has on planetary details at high powers when some planets are better positioned.

Did you try looking for the E and F Trapezium components with and without the FR?  That would also be a good test.

I've seen E and F in a Dob using just x127, so the "new improved"  SCT should be able to manage that I hope.  I'll have a look next time out, while Orion is still around!

Doug.