7" (180mm) Refractor

[Commanderfish]

Hi there,

 

Does anyone know of an achro/ED short tube refractor in the region of 7"/180mm aperture?  I'm looking for a deepsky scope around an F6, to give a reasonably wide field and keep the weight and tube length more manageable.  I already have a 5" Apo, so whilst a 6" achro would be nice, it's not quite as much  of a step up as I'd like, preferrably as I'll probably sell my 9.25" SCT.

I've found lots of 6" class fracs, my favourite of which is the 152 F5.9 Altair Starwave.  At the other end there's the 8" 204mm iStar Phoenx, though this comes in at £4000 and the budget is around £2,500.

I've found some 7" Apos but obviously they are very pricey. I feel there's a gap in the market to be exploited here for 7" Achros.

Cheers

[DirkSteele]

I do not know of any in current production but I believe there was a 180mm TMB achromatic refractor produced several years back.  APM currently offer a 203mm f/9 achromatic refractor but at around €8000 it is above budget.  Possible either of these models could be floating around in the second hand market and be in budget.

[michael.h.f.wilkinson]

F/6 at 7" for a Fraunhofer achromat would be pushing the limits, I feel. Chromatic aberration rises quickly with aperture for a given focal ratio: The ST80 has much less CA than the ST120 or ST150. This might be the reason no 7" F/6 achromat is available

[Commanderfish]

18 minutes ago, DirkSteele said:

I do not know of any in current production but I believe there was a 180mm TMB achromatic refractor produced several years back.  APM currently offer a 203mm f/9 achromatic refractor but at around €8000 it is above budget.  Possible either of these models could be floating around in the second hand market and be in budget.

Yep - there's a TMB 180 available second hand in the US, though the asking price is $4000 and I don't know if the seller would ship to the UK.  Might have to consider this.  However, for $4,300 plus import duties I could get the iStar 204mm Phoenix.

[Commanderfish]

10 minutes ago, michael.h.f.wilkinson said:

F/6 at 7" for a Fraunhofer achromat would be pushing the limits, I feel. Chromatic aberration rises quickly with aperture for a given focal ratio: The ST80 has much less CA than the ST120 or ST150. This might be the reason no 7" F/6 achromat is available

Yes, that makes perfect sense.  However iStar do an 8".  At these apertures I'd expect a short F ratio scope to be good only for faint objects (DSOs) anyway, but perhaps CA is why there aren't many (any?) in manufacture.  Still seems like a a gap in the market, after all Achro CA can be greatly improved with a fringe killer; in this price range manufacurers could bundle one in with the scope. 

I can't really spring 5 figures for an Apo, not to mention the weight...

[DirkSteele]

This might sound funny coming from a guy who owns six telescopes based on lenses and only one scope with a mirror in it (and might have been heard saying "mirrors are for shaving") but why not just get a high quality 8" newtonian?  Colour free, still wide-field and you would have a lot of spare change left over other accessories you might want.  The Orion (UK) VX8L is an 8" f/6.

[Stu]

I would be wary of going above 6" for this type of scope, you get into serious mounting issues and they are quite single minded in their target ie low power deep sky. If that's what you are after then that's fine!

Interesting and frank views of the Istar Phoenix here:

http://www.cloudynights.com/topic/471206-istar-8-f6/

[michael.h.f.wilkinson]

23 minutes ago, Commanderfish said:

Yes, that makes perfect sense.  However iStar do an 8".  At these apertures I'd expect a short F ratio scope to be good only for faint objects (DSOs) anyway, but perhaps CA is why there aren't many (any?) in manufacture.  Still seems like a a gap in the market, after all Achro CA can be greatly improved with a fringe killer; in this price range manufacurers could bundle one in with the scope. 

I can't really spring 5 figures for an Apo, not to mention the weight...

 

It is a myth that fast scopes are needed for faint DSOs for visual. My F/10 C8 with a Nagler 22T4 is quite the faint object killer (600+ galaxies to date), and produces the same image as an 8" F/5 would with an 11mm Nagler, assuming central obstruction is the same. Same holds for fracs: an F/12 achromat with the 22T4 yields the same image as an F/6 of the same aperture with an 11mm Nagler. Same exit pupil, same image brightness, same FOV. In fact, the lower CA of the F/12 may well show more stars due to reduced aberrations. What slow scopes don't do well is show a really wide true FOV. That's where my little 80mm F/6 and the binoculars come in. An 8" F/6 Newtonian will most likely outperform a 7" F/6 achromat on DSOs with ease.

[Commanderfish]

1 hour ago, DirkSteele said:

This might sound funny coming from a guy who owns six telescopes based on lenses and only one scope with a mirror in it (and might have been heard saying "mirrors are for shaving") but why not just get a high quality 8" newtonian?  Colour free, still wide-field and you would have a lot of spare change left over other accessories you might want.  The Orion (UK) VX8L is an 8" f/6.

I'm after a nice, sharp contrasty refractor view.  I have a C9.25 SCT but its not quite as sharp or contrasty as the fracs ,and the field is narrow.  A Newt would beat the c9.25 on FOV but probably not on sharpness and contrast.

[Commanderfish]

1 hour ago, Stu said:

I would be wary of going above 6" for this type of scope, you get into serious mounting issues and they are quite single minded in their target ie low power deep sky. If that's what you are after then that's fine!

Interesting and frank views of the Istar Phoenix here:

http://www.cloudynights.com/topic/471206-istar-8-f6/

Yep, this would be just for DSOs, which are my favourite targets.  It would be mounted alongside my ED127 which is very good at Planetary/Lunar.

The thing about that Istar 204 is it weighs in at 14Kg including Tube rings.  Yes that's heavy, but it should be within the capabilities of the Ercole (18kg) I beleve?

[Commanderfish]

1 hour ago, michael.h.f.wilkinson said:

 

It is a myth that fast scopes are needed for faint DSOs for visual. My F/10 C8 with a Nagler 22T4 is quite the faint object killer (600+ galaxies to date), and produces the same image as an 8" F/5 would with an 11mm Nagler, assuming central obstruction is the same. Same holds for fracs: an F/12 achromat with the 22T4 yields the same image as an F/6 of the same aperture with an 11mm Nagler. Same exit pupil, same image brightness, same FOV. In fact, the lower CA of the F/12 may well show more stars due to reduced aberrations. What slow scopes don't do well is show a really wide true FOV. That's where my little 80mm F/6 and the binoculars come in. An 8" F/6 Newtonian will most likely outperform a 7" F/6 achromat on DSOs with ease.

Well I'm not saying you need a short F ratio for DSOs, but if I want a large aperture frac, then for it to be small enough for one person to handle and mount it, a short F ratio is a big help. For instance, the iStar Phoenix 204mm is an F5.9, resulting in an FL of 1200mm and a weight of 14Kg.  If it were an F9, it would be 1.8 metres long and over 20 Kg - too heavy and unwieldy for one person to mount safely (and beyond the 18kg limit of my Ercole mount).

[DirkSteele]

3 minutes ago, Commanderfish said:

I'm after a nice, sharp contrasty refractor view.  I have a C9.25 SCT but its not as shapr or contrasty as the fracs and the field is narrow.  

I would read the link that Stu posted.  I just skimmed through several pages and there is certainly a contrasting (no pun intended!) view on its performance as anything other than a very low power, wide-field scope.  It is just one persons view however, and should just be used as one data point among many.  Others have had different views of that scope.  I agree nothing can compete with the view of a high quality refractor, and this is just my own musing with no basis as I have not viewed through that scope, but all that chromatic aberration will be spread over the entire field and might diminish that desired contrast abit, compared to your own 5" Apo.  I think the longer focus Newtonian scopes with smaller secondary obstructions sit somewhere between the SCT view and refractor view when it comes to contrast and overall quality of view.  If you can take the opportunity to look through another example first just to make sure it delivers all you are expecting before you hand over the cash.

1 minute ago, Commanderfish said:

Yep, this would be just for DSOs, which are my favourite targets.  It would be mounted alongside my ED127 which is very good at Planetary/Lunar.

The thing about that Istar 204 is it weighs in at 14Kg including Tube rings.  Yes that's heavy, but it should be within the capabilities of the Ercole (18kg) I beleve?

I think the Ercole is excellent and while you correctly state that its max load capacity is given as 18kg, moment of inertia needs to be considered.  That is a fairly long scope and might test the Ercole a bit.  You will certainly need a very solid tripod or pier otherwise the vibrations could prove frustrating.

[michael.h.f.wilkinson]

1 minute ago, Commanderfish said:

Well I'm not saying you need a short F ratio for DSOs, but if I want a large aperture frac, then for it to be small enough for one person to handle and mount it. a short F ratio is a big help. For instance, the iStar Phoenix 204mm is an F5.9, resulting in an FL of 1200mm and a weight of 14Kg.  If that were an F9, it would be  1.8 metres long and over 20 Kg - too heavy for one person to mount safely (and beyond the 18kg limit of my Ercole mount.

True, but an 8" F/6 Newtonian would be easier still (around 8kg ). At 14kg the Phoenix is a heavyweight compared to my little C8 OTA (45cm long or so and just 5 kg or thereabouts)

[Commanderfish]

4 minutes ago, michael.h.f.wilkinson said:

True, but an 8" F/6 Newtonian would be easier still (around 8kg ). At 14kg the Phoenix is a heavyweight compared to my little C8 OTA (45cm long or so and just 5 kg or thereabouts)

Good point about weight.  However for the light gathering to be equivalent to an 8" frac, because of light losses via mirrors and secondary obstructions, a newt would need to be around 10", and I believe that puts us into dob territory, which I don't have the storage for (and I'm not certain I could transport to dark sites either).  I've viewed  a few times through a 10" dob at a dark site and it was pretty good but I preferred the view in my 5" Apo.  That said we were mostly looking at planets and clusters.

[Stu]

38 minutes ago, Commanderfish said:

Good point about weight.  However for the light gathering to be equivalent to an 8" frac, because of light losses via mirrors and secondary obstructions, a newt would need to be around 10", and I believe that puts us into dob territory, which I don't have the storage for (and I'm not certain I could transport to dark sites either).  I've viewed  a few times through a 10" dob at a dark site and it was pretty good but I preferred the view in my 5" Apo.  That said we were mostly looking at planets and clusters.

Perhaps the middle ground would be to try a 152 f5.9 and see how that sort of scope suits you? Much easier to handle but with similar characteristics.

[t0nedude]

While I'm the first to want a big achro FL=1 () for DSO's (refractor nut too) there comes a point where past a certain size a DOB will come into its own..it's not just weight but lenght, I feel the current selection of 152 F6 instruments are great value, and the biggest practical. Only fifteen years ago there were hardly any around..and they're all the rage now. I have to agree with Stu and suggest you try one and see what you think, a 152 gives 43% extra light grasp over 127. The only 7" I know of is an APO which will need a mortgage, or a long FL achro which will need a mortgage for an observatory to house it.

Tony.

[nightfisher]

1 hour ago, Commanderfish said:

Good point about weight.  However for the light gathering to be equivalent to an 8" frac, because of light losses via mirrors and secondary obstructions, a newt would need to be around 10", and I believe that puts us into dob territory, which I don't have the storage for (and I'm not certain I could transport to dark sites either).  I've viewed  a few times through a 10" dob at a dark site and it was pretty good but I preferred the view in my 5" Apo.  That said we were mostly looking at planets and clusters.

An OO VX 8 F4.5 with 1/10 grade mirrors would more than likely amaze you for deep sky

[michael.h.f.wilkinson]

58 minutes ago, Commanderfish said:

Good point about weight.  However for the light gathering to be equivalent to an 8" frac, because of light losses via mirrors and secondary obstructions, a newt would need to be around 10", and I believe that puts us into dob territory, which I don't have the storage for (and I'm not certain I could transport to dark sites either).  I've viewed  a few times through a 10" dob at a dark site and it was pretty good but I preferred the view in my 5" Apo.  That said we were mostly looking at planets and clusters.

I think that statement needs some qualification.

Let's take an 8" scope with Hilux (97%) reflective coating and 25% secondary obstruction (by diameter, this can be made smaller). This yields 88% transmission. This corresponds to the light transmission of an unobstructed 7.51" refractor with perfect transmission. In reality, an air-spaced doublet has 4 surfaces with (ideally) 0.2% loss, or 0.8% total. Modern optical glasses have very low transmission losses, so these losses can be ignored. Older glasses can have losses of up to 5% per 10mm thickness. 

Thus an 8" F/6 Newtonian should beat a 7" achromat for light grasp. Even my C8 with its rather large secondary beats it.

[John]

I don't know of currently made achromats of that spec. Here is a review of an 11" F/5 (!!!) though so such things obviously are made from time to time:

http://www.astromart.com/reviews/article.asp?article_id=342

Maybe buying a fast objective lens from someone like Istar or Intane then getting it mounted in a tube is the way to get what you want ?

[Commanderfish]

Hmm lots to think about. I need to spend more time looking through people's dobs, though my instinct is that I'll be pleased with a 152 F5.9 starwave and get a lot of use out of it given the short length and comparatively low weight. 

[Commanderfish]

Someone still needs to commercialise a 7" though, possibly with ED glass.  It can be done with an 8"...