Scope for more power on DSO's - 120mm refractor?

[kirkster501]

Hi all, I am a happy user of a CPC1100 for visual and planetary and a ED80 for wide field visual and imaging. I'd like to start and get more zoomed in on small DSO stuff like Ring Nebula, Crab etc when I need equatorial mounted kit.

I was thinking C9.25 (I do like SCT scopes) to go on my HEQ5 pro. But I have a slight concern if it will work well for DSO on that mount.

So would a Equinox 120 fit the bill you think? How much "closer in" would this be compared to the 80ED? They come in used at about the right price for me - £800 odd. Or even a ED120 for even less?

Thanks, Steve

[ollypenrice]

You can model different focal lengths and chip sizes with this free software. http://www.newastro.com/book_new/camera_app.php It, or another version of it, is indispensable. You can see which combinations will frame which objects satisfactorily. This is always step number one for every image we do here.

The size of a target on your chip depends on focal length (and chip size.) I work between 200mm and 2.4 metres (with some big gaps in between!) and find that the big refractor of FL980mm falls a bit between two stools being a bit short for the galaxies and a bit long for the nebulae. The images below are at 450mm and 980mm to give you an idea of scale.

Be aware that long FL instruments are not just heavy but very demanding of tracking accuracy. Imaging with a long FL is a serious undertaking. If you don't get the tracking right you are not really gaining any resolution over what you'd achieve more easily at a shorter FL because it will be lost to guiding error. I would recommend an OAG for SCTs every time.

I know that Fay images well with a 120 on an HEQ5. Try the CCD calc with that plugged in to see if it would meet yur needs.

Olly

[swag72]

I had a 120ED and found that it was at a very much inbetween focal length for me and certainly didn't suit what I wanted. Olly's images illustrate the difference between the ED80 and the 120ED well as will the fov calculators out there. My 120ED was used soley for imaging and I used it with a small chipped camera, so I was effect getting as close to the smaller stuff as I could get. I found that it just wasn't close enough. Of course a few galaxies were too big for the fov, but generally the smaller DSO's were still small.

At a focal length of 900mm I learnt that it was something of an uncomfortable length for me, never really getting me close enough to what I wanted. Of course, something bigger would have been better, but then that moved up to a whole different price bracket as my HEQ5 would almost definitely not have been up to much more.

[Vlad]

The SCT is the one. Plenty of focal length for the DSO's, yet with a focal reducer, the wider objects are easily captured. Also an aexcellent visual scope, showing fainter stars than the 120, but having said that, the visuals of an equinox are stunning!

[Capricorn]

It is a 9.25 inch scope of course it will work well on DSO's, just keep the magnification down, use 32mm, 30mm and 25mm eyepieces.

It is not really the scope type that makes one better or otherwise for a particular area of observation it is the ease of getting decent magnification that is why we use different scopes.

Getting 300x on an f/5 10 inch newtronian is not easy, on your 9.25 it is, and as planets supply enough reflected light for high magnification it is just easier to use an SCT

Conversly the long focal length of the 9.25 will tend to supply larger magnifications, the longer focal length, so obtaining a small image of a DSO which is bright enough is more difficult on yours but easier on the 10" newtonian. Bet that getting 50x on your 9.25 is not really possible as you would need about a 50mm eyepiece, on the 10" f/5 just drop in a 25mm eyepiece and you have a small but bright image of the DSO - makes finding them easy.

[ollypenrice]

The OP mentioned the Edge series. They have great imaging optics apart from the disastrous focal ratio. This would need super long exposures at a super long focal length. Tim has shown that it can be done but I doff my hat to him. There is a Lepus reducer but it has a very small image circle which rather defeats the object of the large flat field. Celestron don't seem to be quoting an image circle for their reducer for the Edge though they may have done so by now. On the other hand the Meade ACFs do work with the AP reducer.

Olly

[kirkster501]

Hmmm... Thanks for the input guys but now I am confused...! What shall I do then? Get a 200P or something perhaps? My requirements are to stick with the HEQ5 if possible (at least for the next year or so), and get a OTA that can get closer to smaller DSO's like the Crab. I like SCT's, but then again I like my small ED80 as well. I have no ideological opposition to any particular OTA.

[Crunchard]

I really don't know much about imaging having only dabbled, but what about a Ritchey Chretien Astrograph?

[kirkster501]

I am also tinkering with the idea of wedge mounting my CPC as well - irrespective of any new scope I might get. I am a strong enough chap so the task of mounting the scope like this is a not an issue for me.

steve