ED120 and Skyliner 12" comparison in varying seeing

[MartinB]

In a recent thread Russ reminded me that when Ade Ashford reviewed the Skywatcher ED range he found that the ED120 added little to the view compared with an ED100. This puzzled me since I found a significant difference in resolving power between a WO FLT 110 and an ED120. I guess that if the seeing is poor the resolution is seeing limited rather than aperture limited. I'm sure it's much more complicated than this but that was my simple take on it.

Last night I had a surprise break in the cloud so decided to do a bit of observing. Normally I would have got out the Skyliner but this time decided to set it up alongside my ED120. The seeing was poor at first, evident at >x130

Looking at Saturn with the ED120 and a Pentax 7mm XL (x130) gave a stable view. One band was seen clearly on the disc, Cassini was clearly visible as a distinct black ring at either side but not all the way round. The ring was sharp and clear in front of the disc with a crisp shadow. Disc colour was a warm yellow.

Initially I found the best views with the Skyliner were to be had with a 10mm Pentax XW (x150) Saturn was just too bright so I had to use a neutral filter. This improved things no end. Again I could see a single band on the disc but Cassini was a real struggle to make out. The ring had none of the crispness of that shown with the ED120. I could see the ring as it passed in front of the disc but there was none of the 3D effect provided by the 120. I tried barlowed 25mm plossl (x120) but this didn't improve things. Things looked sharper and smaller with a 15mm plossl. It seemed that the seeing was hammering the Newt much more than the smaller refractor.

The moon gave a similar picture. I found that I could make out similar detail in and around Clavius with both scopes however the contrast was vastly superior with the ED120. This made it easier to pick things out. I was finding things easily with the apo and then had to search for them in the newt.

All of a sudden, within a minute, the seeing improved markedly. I was now able use the ED120 with a 5mm Baader orthoscopic (x180). The colour with this EP was much cooler than the Pentax (less glass) but it gave one of the finest views of Saturn I've ever had. Now the ring was very cleanly seperated by Cassini up to the point where it crossed the disc. Additional subtle banding was present on the disc. I pushed things further by barlowing a 9mm orthoscopic and then the 7mm Pentax but this didn't improve matters.

Now I could power up the 12" Newt with the 7mm Pentax (x210) - finally the scope got to show what it can do. Everything that was lacking before became clear. Saturn had excellent contrast. I was surprised to see that the colour was probably slightly warmer than with the apo. The subtle extra banding on the disc, evident with the ED120, was present, it just lacked a little of the crispness of the refractor but this was a marginal difference.

I finally pushed things up to x300 with the 5mm Baader orthoscopic. Now this is hard work with a dob! However, finally the resolving power of the extra aperture started to show. The banding on the disc was definitely easier to make out. However, aesthetically, the ED120 with the 5mm orthoscopic was the most pleasing.

I finished off by having another look around Clavius. The extra contrast of the ED120 was still evident and this helped me pick out tiny features which I might have missed with the Newt. I was about to have a look through the Skyliner at x300 when the cloud rolled in.

So from this single, unrepresentative night I would say that, in poor seeing, the ED120 out performed the 12" Skyliner on Saturn and the moon primarily because it seemed to be less affected by the seeing. This might be very different on other nights and from other locations since seeing is a complex business and it's effects on different scopes is very variable. In good seeing, on a motorised mount the 12" Newt will reveal more than the 120mm refractor but the optics of the ED120 go some way to make up for deficiencies in aperture when lunar observing around the terminator (shadows detected more easily). I guess I've just managed to convince myself of the truth of something which is common knowledge!

[durr]

Martin I have been humming and arring about buying the Skywatcher ED PRO 120mm but your report has made me think very seriously. I want a refractor and was not too sure whether to go for the 100mm or 120mm. I also thought about the Equinox version but it does not seem worth it for the extra bit that you get with the finish. What sort of image do you get on wide field and are the star images point like? Steve

[moondog]

Interesting report Martin.

Refractors usually seem to have the edge over reflectors in terms of contrast and that is why I liked the TAL 125R I used to have - even though this wasn't an APO.

I too read the original Ade Ashford review of the ED120 and his comments seemed a bit odd. I found a marked difference between the 100 and 125mm TAL and would have expected this between the ED100 and the ED120. Your report seems to vindicate the ED120.

MD

[GazOC]

My experience tallys with yours Martin. Smaller refractors can show more contrast on planets than larger Newts or CATs, esp in average or poor seeing. Mars and Saturn seem to benefit the most, Jupiter less so.

[lunator]

Martin

Interesting report. In my experience from London most nights my scope's performance is limted by the seeing and it is when the seeing is excellent that you can really find out what the scope is capable of.

Cheers

Ian

[MartinB]

What sort of image do you get on wide field and are the star images point like? Steve

So far durr, I've done very little imaging with the ED120 and what I have done was messed up by guiding problems and wind. However, I don't see why it shouldn't perform very well. I have found the colour correction to be very good once well focussed so stars should come out well.

it is when the seeing is excellent that you can really find out what the scope is capable of.

The interesting thing about this session was the rapid change in the seeing conditions half way through - from poor to pretty good - 4/10 changing to 7/10. The ED120 performed brilliantly in the poor stuff but was still matching the 12" reflector when it got better except for the resolution at very high powers. I might try splitting some doubles next!

[lunator]

Martin

Have a go at gamma Virginis (Porrima) It about 1" apart now so should be within the abilities of the 120

I would be interested to hear how the 2 images compare.

Cheers

Ian

[MartinB]

Will have a go sometime when the imaging isn't running Ian. Do you have any co-ordinates?

[MikeP]

Looked it up on SkyMap -

Apparent RA: 12h 42m 07.27s, apparent Dec: -1° 29' 54.0"

I'm sure Ian will correct me if its wrong.

Mike

[lunator]

It's in the middle of Virgo........................

[MartinB]

Right, think I can find that!