Galaxy busting with a refractor...?

[ollypenrice]

Since I already spend a lot of time attending to kit I'm not really in the market for an open tube imaging reflector. I'd like a closed tube instrument so as not to add more maintenance to my life.

One possibility would be to go for a C8 Edge and have two metres FL at F10 and 0.91 "P/P using my existing Atik 11000 (so 11 meg.)

Alternatively I could stick with my TEC140 and achieve 0.78 "P/P with an Atik One-nine or 490. (Still plenty of pixels at 9 meg for final screen size.)

The targets would be galaxies, so small, in which case my feeling is that F ratio is not important. On the optical side it's about aperture. But then the Sony chips are faster than the Kodaks. (I recently worked alongside SGL member Horwig and his Sony camera on M31 and it's clear that his camera was significantly faster.) The One-nine has a peak QE of  about 75% as compared with about 50% for the 11000. Binning colour would be a possibility.

Why not Sony chip, focal reducer and C8 Edge? Nope, too expensive! It's new scope or new camera.

The framing, resolution and final screen size are all remarkably similar and don't seem to argue one way or the other.

I confess to an irrational urge to see what kind of small-object images I could tease out of a refractor.

What do you reckon?

Olly

[Tinker1947]

I'm certainly no expert but i did buy a C11(new) and found a couple of things that needed sorting, first for imagine its needs to be the "Edge" variant to save space for the stuff that hangs on the back, the other is more to the design, collimation is a real pain a £350 HoTech really needs to be bench mounted to use, do it by the airy disk seemed better but the end result always seemed a soft image, if you like adjusting a scope go for it, buy second hand so the cost can be re-gained and of course mount it on something better than a NEQ6......

[ollypenrice]

I'm certainly no expert but i did buy a C11(new) and found a couple of things that needed sorting, first for imagine its needs to be the "Edge" variant to save space for the stuff that hangs on the back, the other is more to the design, collimation is a real pain a £350 HoTech really needs to be bench mounted to use, do it by the airy disk seemed better but the end result always seemed a soft image, if you like adjusting a scope go for it, buy second hand so the cost can be re-gained and of course mount it on something better than a NEQ6......

Thanks. I'd only consider the Edge version because I'd want it clean across the chip and my chip is enormous. I know what you mean about a slight SCT softness, and star sizes tend to be rather large. Of course the TEC is very sharp.

[ChrisLX200]

I went through that thought process myself and decided my best bet was a 12" class instrument with about 2000mm f/l (a compromise between speed and enough f/l). If it were to compete with the 5" Televue then optically it needed be excellent offering good resolution (over and above what the refractor could provide) and a well-corrected field large enough to cover a 52mm diagonal sensor. I also wanted something light in weight, stable in terms of holding collimation, easy to mount (both onto the GEM and for carrying a camera/focuser), offering quick thermal equillibration,  and all in a low-maintenance tube. The ODK12 is 2040mm f/l @ f/6.8 with 52mm field and tiny spot size on and off-axis (well, that's what they claim anyway). The carbon tube is thermally stable, has internal heaters to keep dew at bay and 3 fans get the temperature down quickly. The OTA weighs just 19kg bare so reasonably manouverable. A big flat rear face offers plenty of space to mount stuff and the optics are non-moving (so hopefully stable tracking unguided).

Whether it actually works as advertised is something I have yet to discover... ;-)

ChrisH

[Peter Drew]

I would try your refractor first before taking a further decision. It's what you wanted to hear wasn't it.

[ollypenrice]

I would try your refractor first before taking a further decision. It's what you wanted to hear wasn't it.

Yes!

[Tinker1947]

You could go the other route and get a Reflector, Orion Optics with a 1/10 wave mirror, i am likely to do this in the summer drop the SW 250P-DS and look at the 150 or 200mm tube, i have e-mailed them as to can i just buy the tube and optics as i have a Focuser ect, no response yet....A lot cheaper than the wonderful bit of kit ChrisH purchased but should out shine a SW Reflector.......

http://www.orionoptics.co.uk/VX/vxrange.html

[ollypenrice]

You could go the other route and get a Reflector, Orion Optics with a 1/10 wave mirror, i am likely to do this in the summer drop the SW 250P-DS and look at the 150 or 200mm tube, i have e-mailed them as to can i just buy the tube and optics as i have a Focuser ect, no response yet....A lot cheaper than the wonderful bit of kit ChrisH purchased but should out shine a SW Reflector.......

http://www.orionoptics.co.uk/VX/vxrange.html

 A case can be made for Newts but I don't want the maintenance involved in an open tube reflector. If you only have one to look after it's OK but I have five rigs of my own and five robotic installations... I do want a closed tube. 'Wipe it down with a damp cloth...'    

Olly

[Tinker1947]

I will put some cling film over the end of mine, let you know how it works out.......

[swag72]

After talking you to about this Olly when I was waiting for my ODK, I did .....stupidly ..... pump the figures into CCDCalc of a Tak 150 and a 4120 sensor...... what you said made sense, but the £15k or so price tag certainly didn't!!!

[ollypenrice]

After talking you to about this Olly when I was waiting for my ODK, I did .....stupidly ..... pump the figures into CCDCalc of a Tak 150 and a 4120 sensor...... what you said made sense, but the £15k or so price tag certainly didn't!!!

Heheh. Scary, isn't it.  The difference is that I already have the TEC140 and a set of currently unused 1.25 filters...

Olly

[Thalestris24]

TEC140 + ED/APO Barlow? Seems like a cheap option  

Louise

[andrewluck]

The targets would be galaxies, so small, in which case my feeling is that F ratio is not important

Small, but not point sources. Therefore the light would be spread over many pixels and spiral arms are notoriously faint to start with. A higher f-ratio will deliver less light into each pixel and you'll need to expose for longer to get above the read noise.

My feeling, and I'm ready to be corrected here, is that spatial resolution is the most important consideration. Don't over-sample if your local conditions won't support it.

Andrew

[ollypenrice]

Small, but not point sources. Therefore the light would be spread over many pixels and spiral arms are notoriously faint to start with. A higher f-ratio will deliver less light into each pixel and you'll need to expose for longer to get above the read noise.

My feeling, and I'm ready to be corrected here, is that spatial resolution is the most important consideration. Don't over-sample if your local conditions won't support it.

Andrew

Yes. I really made this point in regard to the Edge focal reducer which, on chip-fitting targets, would make no difference since it's object photons which count. I think my seeing can, on occasion, support quite fine sampling. I ran 0.66 "PP for a couple of years with Yves' 14 inch. Going a little coarser would be ideal, I think.

Olly

[philj]

I went through this for galaxy and planetary nebula Olly with a C11 some time ago. I eventually gave up due to the many variables with long focus imaging with an sct and switched to a well behaved f7.5 refractor. I found the focus drift on the sct due to thermal contraction to be a pain on long imaging runs.

[ollypenrice]

I went through this for galaxy and planetary nebula Olly with a C11 some time ago. I eventually gave up due to the many variables with long focus imaging with an sct and switched to a well behaved f7.5 refractor. I found the focus drift on the sct due to thermal contraction to be a pain on long imaging runs.

Was this a carbon or a metal tube, Phil?

Olly

[mikeyscope]

Olly

Like you I was considering the same things as yourself only my interest is more in planetary nebulae and other small targets but didn't want to exclude trying some galaxies too.

As I previously owned and used a Takahashi FS-128 for many years and a CN-212 I was able to try the best of both worlds, for myself personally the refractor was the most enjoyable and versatile bearing in mind my setup is portable.

Assuming you are considering the 8" Edge because you prefer refractor like stars rather than having diffraction spikes then a cassegrain would seem the most likely option beyond a refractor provided little or no distortion/mirror shift during long subs.

I'm a bit out of touch with what makes and models are out there but if I had a home observatory I would probably consider something with a little more aperture than 8" to be a significant enough step up from your TEC-140 probably 10" to 12" with an f-ratio not more than f10.

Saying that it's hard to get past the versatility of a refractor, can be reduced, can be used at native, can be barlowed/extended and usually doesn't need collimated and can produced nice stars in flat fields.

Mike

[ollypenrice]

Olly

Like you I was considering the same things as yourself only my interest is more in planetary nebulae and other small targets but didn't want to exclude trying some galaxies too.

As I previously owned and used a Takahashi FS-128 for many years and a CN-212 I was able to try the best of both worlds, for myself personally the refractor was the most enjoyable and versatile bearing in mind my setup is portable.

Assuming you are considering the 8" Edge because you prefer refractor like stars rather than having diffraction spikes then a cassegrain would seem the most likely option beyond a refractor provided little or no distortion/mirror shift during long subs.

I'm a bit out of touch with what makes and models are out there but if I had a home observatory I would probably consider something with a little more aperture than 8" to be a significant enough step up from your TEC-140 probably 10" to 12" with an f-ratio not more than f10.

Saying that it's hard to get past the versatility of a refractor, can be reduced, can be used at native, can be barlowed/extended and usually doesn't need collimated and can produced nice stars in flat fields.

Mike

At long focal lengths I can live with diff spikes but I'm wedded to the idea of closed tubes only. You're quite right about the refractor positives. The focal length only projects a small image onto the chip but if the chip has small pixels you get better resolution at capture and a bigger image on the screen at the end. Hmmm...

Olly

[philj]

Was this a carbon or a metal tube, Phil?

Olly

Metal tube Olly, couldn't afford the carbon  Sorry for the late reply.

It behaved OK provided the temperature drop over the imaging run wasn't too great but in the depths of winter with temperatures dropping rapidly I had to keep a check on the focus regularly. The 2 hour run on M1, recovering only 30 minutes of useable subs  incident swung it but I had several galaxy shots messed up as well. Yes there are thermo compensating focusers but to an old IT luddite like me theres  too many messy computer type thingies in this imaging lark for me to forget, get wrong, forget to switch on, adjust or faff about with lost comms, so I switched to a well behaved refractor.

[nytecam]

Suggest, if interested in what a large SCT can do, you check Rick Johnson's galaxy work on CN ;-) 

Nytecam