Which 4" refractor for astrophotography

[Leebert]

I am considering a 4" or so refractor to set up an imaging rig on an HEQ5 Pro with a William Optics 66SD for a guide scope. I have had a look around and have a list of OTAs that look like they might suit the task.

They are...

Altair Wave Series 102mm F7 Super ED Triplet APO (102/715);

TS PHOTOLIN'E 102mm f/5,2 Flatfield Super APO telescope;

APM 107/700 F6.5 Super ED Triplet APO w 3" APM focuser;

William Optics FLT 98 APO Triplet;

Skywatcher Esprit ED 100 PRO 5-element.

These vary from about £1400 to £2300.

My budget is around £1700 but could increase for the right OTA (and flattener/reducer if required).

I only want to buy once and am prepared to wait. I am a relative newbie where Astrophotography and refractors are concerned. Looking at the short list, i have a few concerns from looking around the internet...

Altair - a bit slow at f7 and no dedicated reducer yet?

TS 102 - not found any reviews etc on them?

WO - Recent quality / build issues concern me.

Esprit 100 - Really stretches my budget and they don't seem to be in stock anywhere.

I would be interested in peoples thoughts on these or any other scopes...

Many thanks,

Lee

[nightfisher]

a lot of people use the ED 80?

[Ben Ritchie]

What sensor are you using? I'm assuming DSLR?

I know you said 4" but i'm going to clutch at the 'or so' and plug the 80mm as a starting point.

I'd agree with nightfisher, the 80ED is a very good starting place - it's maybe not a dream 'scope and a touch slow at f/7.5, but it keeps most of your budget intact while you work your way up the learning curve and easy to sell on once you become limited by it. I've not owned one since it was the insanely good value gold tube (think it was £220 new at one point!), but the field is reasonably flat and most of a DSLR field of view is usable without a flattener. Crop the corners and you still have a ton of sky left to work with.

At higher cost, i've always liked the LOMO/TMB 80/480 with TRF-2008 and a pile of T-ring spacers. Once you've got the sensor spacing sorted out with the TRF-2008 (it needs a lot of trial and error) it's a great imaging setup, fast, flat, and well corrected.

[LukeSkywatcher]

a lot of people use the ED 80?

ED 80 is a scope i see mentioned here soooooooo often by seasoned imagers, that it must be doing something right.

[steppenwolf]

The FLT 98 is a pretty well kept 'secret'. Possibly one of William Optics finest creations especially with its new rack and pinion focuser.

That said, I loved my old blue tube SW 80 ED ........

[ollypenrice]

Really I'd put the whole setup into the melting pot. An ED80 with flattener and good CCD will easily beat any optics you care to name with a DSLR.

Ollly

[robin-m]

I'm not a seasoned imager (yet anyway) bit I'd second Olly's comment. Any images you get are limited by the weakest link in the chain and a dslr as opposed to a ccd is definitely the weakest link. Get an ED80 and the best ccd you can afford. If, later you decide to upgrade the scope at least the ccd will go with you.

[Leebert]

I agree that the 80 seems to be the OTA to start with and many fantastic images are taken with them. I already have an WO 66SD and, part of my thought process was that the 80 would be a bit too similar. I do enjoy a bit of visual observing too occasionally so my thoughts turned towards something in the order of 100mm.

I looked at 115 and upward but discounted that due to mount capacity (and budget).

The budget is also another reason for going with an OTA first. The CCDs that I am considering are out of my budget at the moment so I'll be using a DSLR (APS-C). The plans there are to use what I have for the time being and, when funds allow, upgrade it with filter removal / cooling. That way, I get used to what I have over a period of time and the equipment grows with my knowledge. I guess that's a similar approach as mentioned by Olly and Robin but coming from the other direction. I am open to all suggestions but I would rather not start to assemble an imaging rig and then feel the need to change / upgrade OTAs. Is a 4" OTA not a good imaging option (more aperture = more light gathering)?

Many thanks,

Lee

[Ben Ritchie]

It's a bit unintuitive, but the OTA is arguably the least important bit of the imaging chain - as long as the focuser can cope with your camera without slipping, the field is reasonably flat (or a suitable flattener is available) and colour correction is good enough to avoid bloated stars from unfocused light in the far blues or reds then you're pretty much ok. These days most refractors meet those criteria, other than maybe some of the very fast doublets that are apochromatic only in the mind of the marketing department. The benefits of much more expensive OTAs are largely incremental, and are mostly only needed for particular high-end applications (e.g. Olly's FSQ106 for his Art11k, when field flatness and illumination become a challenge).

The mount's the most important bit IMO, but it's a bit of a headache as there's a huge price difference from the sub-£1000 (N)EQ6s to the £4-5k plus high-end mounts that really start to deliver performance on a different level to the Skywatchers. There's never been a great middleground, e.g. the G11 that I had didn't really deliver a greatly improved performance over the EQ6 I have now (and left me spending hours fighting the infamous 76s error). The AP1200GTO was superbly engineered and delivered brilliant performance, but cost more than my car...

As for sensors, I do agree with the others about CCDs but a DSLR is still a nice entry before leaping into CCDs and all the assorted gubbins that go with them.

So I guess my take on it is that i'd keep as much of your budget in the bank as possible for now, and go for an entry level refractor - you wouldn't go wrong with an 80ED, but there are other reasonably-priced alternatives. Work first on getting guiding working to the extent that you can get effectively 100% usable subs of 300 or 600 seconds, and start getting the post-processing figured out. That way you'll rapidly learn what you find to be the weak spots in your setup, and have some money put aside to do something about it.

[ollypenrice]

Good post from Ben. Mount-camera-optics for me, too. On high end mounts there is, now, an upper middle ground in the form of the roller drive Mesu Mount 200 which offers vastly better performance than the Losmandy at a similar price.

In imaging it is misleading to think in terms of more aperture gaining more light. It does gain more light but if the focal ratio remains the same (ie the focal length goes up with aperture) the light intensity at the focal plane remains the same. The light intensity at the focal plane only goes up with aperture if the focal length remains the same (ie the F ratio lowers.) More counter intuitive ideas to assault the poor brain!

Olly

[Sp@ce_d]

I've got the ED80 & ZS66 which I use DSLR (1000D) & CCD (314l+) with. Now, for something like M31 the ZS66 + DSLR has the perfect FOV, with the ED80 it's just a bit too tight. Also, the ZS66 is a lot faster which is good with uncooled imaging too. You haven't said what targets you're interested in but if I had to choose a combination with DSLR, I'd go with the ZS66. Having said that, if it was the CCD I'd use the ED80. Why? Because there is a difference with stars. It's much more noticeable using the CCD but the ZS66 tends to show bloat in RGB ( Narrowband is excellent). So I'd say the optics are better on the ED80.

Given what you've said and your current kit list, I'd consider a (N)EQ6 with your DSLR & ZS66. As said, get used to guiding, processing etc with this first before spending too much of your budget. Do you have the WO FFII Red/Flattener?.. you'll need it. If not, before I got one I used a WO GT81 flattener that works well with it but doesn't reduce.

As far as modding the DSLR, I'd do the filter removal if you're comfortable with it but I wouldn't go down the cooler route. I spent too long before I got a CCD & once I had kicked myself for not doing it sooner. Just have a search on here for some of Gina's threads to bring tears to your eyes..

Another scope that might be worth looking at is this: http://www.firstligh...71-ed-2013.html

martin_h on here seems to be getting some nice results with it & it's one I'm considering for next winter when I reorganise my twin shooter setup ( or triple even.. )

[Ben Ritchie]

On high end mounts there is, now, an upper middle ground in the form of the roller drive Mesu Mount 200 which offers vastly better performance than the Losmandy at a similar price.

Ah, didn't know about that one - i'm a bit out of date on mounts, these days i'm a simple visual person and stopped worrying about mount performance a couple of years ago

Another thought about optical quality and why pretty much anything is ok as long as it doesn't have significant false colour. We're sitting under miles of atmosphere that is usually fairly turbulent. As a visual planetary observer, I sit for long periods of time waiting for the moments of still atmosphere that give me a sharper view. Planetary imagers use 'lucky imaging', taking long sequences of exposures and throwing away all those where the seeing is poor. However, as a long-exposure imager you have no choice, you integrate through all that turbulence and therefore stars, nebulosity, etc. are smoothed out to the local seeing - which may often be 2.5" or worse in the UK, especially in urban environments (and you effectively get the worst seeing during your integration time, not the mean seeing). So under most normal imaging conditions in the UK you're quite heavily seeing-limited, rather than optically-limited, and premium glass will perform just the same as mass-produced Chinese glass (which, to be fair, is pretty decent these days as long as you avoid the worst ebay junk). I reckon there are only a few nights a year where you could tell my AP130 apart from a SkyWatcher 120; worth it for me when they happen, but doesn't make much sense in value for money terms

However, CA is a killer, so that's the one thing to be careful of at the entry level. If your 'scope is starting to diverge in the far-blue of far-red you'll get very soft stars from unfocused UV or IR (cameras are far more sensitive to this than the human eye). In the entry level selection of telescopes this means that slower f-numbers are actually better for imaging use, as you'll get better colour correction and can just integrate a bit longer to make up for the signal (a benefit of getting guiding nailed down). Fast and well corrected = expensive, as any FSQ owner knows although I think that's corrected out to 1000nm? Which is great if you have a red-sensitive CCD, but a bit excessive otherwise

[kirkster501]

I love my ED80, it is a superb scope and I cannot see myself selling it any time soon. Olly, Steppenwolf, Gina and others all recommended this to me as a AP beginner and they were 100% bang on the money. Punches massively above its cost. I am going to put a CCD onto mine soon as well; as Olly says the ED80 + CCD is better than any scope + DSLR.

[Leebert]

Some very interesting reading there - thank you for taking the time to reply.

I do have another option that I can consider which is an second hand Orion ED80 and reducer/flattener. This would leave plenty budget spare for a modified Canon DSLR (photographically, I prefer Nikon but they are nowhere near the Canon DSLRs in terns of support for astrophotography).

Would the Orion be suitable? I have no experience regarding these scopes or how bad CA is in them etc.

[Ben Ritchie]

Is that the gunmetal grey-tube 80ED f/7.5? If so it's fundamentally the same telescope as the blue-tube or gold-tube SkyWatcher, depending on when it was made. Great telescope for the money, focuser can need a bit of work to stay solid under the weight of a DSLR but is fairly easy to fix up.

A modified DSLR is good, but you only really require it for imaging emission-line nebulae, as the H-alpha emission at 6563 Angstroms gets severely attenuated by the stock IR blocking filter otherwise. Everything else is fine with a standard DSLR (e.g. galaxies, reflection nebulae, open clusters, globular clusters). You could image for a very long time before you ran out of suitable targets for a standard DSLR. Personally i'd start on clusters (splitting the diffuse broadband emission of galaxies from the background is harder), and put the cash towards a first CCD when you're ready to upgrade.

[laser_jock99]

Not much to add to the above other than that if you already own a DSLR it might be worth sticking with that until you feel you have reached the limits of it's/your ability. Then step up to a cooled CCD? The venerated ED80 is cheap enough- but optically quite slow, so I'd be tempted to spend the extra on a fast APO triplet (if only they came at F2.8...........!!)

[Leebert]

Is that the gunmetal grey-tube 80ED f/7.5? If so it's fundamentally the same telescope as the blue-tube or gold-tube SkyWatcher, depending on when it was made. Great telescope for the money, focuser can need a bit of work to stay solid under the weight of a DSLR but is fairly easy to fix up.

A modified DSLR is good, but you only really require it for imaging emission-line nebulae, as the H-alpha emission at 6563 Angstroms gets severely attenuated by the stock IR blocking filter otherwise. Everything else is fine with a standard DSLR (e.g. galaxies, reflection nebulae, open clusters, globular clusters). You could image for a very long time before you ran out of suitable targets for a standard DSLR. Personally i'd start on clusters (splitting the diffuse broadband emission of galaxies from the background is harder), and put the cash towards a first CCD when you're ready to upgrade.

Yes, it is the f7.5 grey one. I see what you mean regarding the focusser. There is a few fix tutorials around the web too.

the plan for a modified DSLR is two fold. One is to get the extra red sensitivity and the second is so that I can use a clip light pollution filter. With the Nikon, I have had issues using a 2" screw LP filter in conjunction with a fr/ff on my WO 66SD. I can either use the fr/ff or the LP filter but not both together. The idea being that I can use a clip filter with a DSLR and not have any issues attaching it to ff/fr or directly to a focusser.

[Leebert]

...duplicate post...

[earth titan]

I can use a clip light pollution filter. With the Nikon, I have had issues using a 2" screw LP filter in conjunction with a fr/ff on my WO 66SD. I can either use the fr/ff or the LP filter but not both together. The idea being that I can use a clip filter with a DSLR and not have any issues attaching it to ff/fr or directly to a focusser.

If you want simplicity with an LP filter then the clip filter and Canon is the way forward. I fitted mine and now forget it's there.

A modified DSLR does improve matters. Still too much noise for my liking though, when compared with a CCD and the CCD if using mono, gives a much superior final image.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[PARTY MARTY]

I must admit that the TS Photoline 102mm Triplet APO, is something else, I bought the TS Photoline 115mm Triplet , and by god the images are superb!! The sheer german engineering quality is something else, That's why most of my cars are all german including my wife!!

[ronin]

WO have a GTF-102 out that has a built in flattener. Since it is not for sale here, or it seems Europe in general, you would have to order from WO or a US supplier, Astronomics and Agena come to mind.

Price is around $1800 and if you added 30% for shipping/duty/VAT that is around $2400, say $2500, which comes out to about £1700.

Mention it as it already has the flattener and the reports on it seem to be very good indeed from the US people.

All you have is the decision to go through all the necessary bits to get one.

If you did then specify getting it sent by someone like FedEx that at least seem well practised in customs etc.