Which is better for DSOs? C9.25 or Altair Starwave 152mm F5.9 Refractor?

[John]

Refractors tend to "punch above their weight" when the seeing conditions are so-so.

 

[michael.h.f.wilkinson]

On 5/12/2016 at 17:44, Commanderfish said:

Re the Altair 152 having the same light gathering as an 8" Newt, I did some maths on this.  

Actually I did the maths based on an 8" SCT.  I used the aperture diameter of the scope in millimetres to calculate the apreture area by applying pi r squared (diameter = 2r).  I calculated this for both scopes.  I then subtracted the central obstruction from the sct.  Finally I adjusted for the mirror losses of the SCT, assuming that the SCT had the top of the line Starbright XLT coatings for both mirrors, so that accumulated losses from both mirrors and corrector plate are only 16.5%.  The results were as follows:

152 frac: 18,144mm squared

8" SCT: 22,825 mm squared

9.25" SCT: 29,024 mm squared

For a newt with less meticulous coatings, losses would likely be higher than 16.5%, so saying the 152 frac has equivalent light gathering to an 8" newt seems pretty accurate.  In addition, the frac doesn't have diffraction from spider vanes, secondary obstruction etc.

Not really: the Newtonian will have a significantly smaller central obstruction than the equivalent diameter SCT. This has a much bigger impact than alleged differences in coating. The 35% central obstruction (by diameter)  of the SCT yields 12.25% loss of light, the rest comes from the mirrors. A typical 8" Newtonian has 25% CO by diameter or only 6.25% loss. With Hilux coatings this yields a total transmission of 88.2 %, better than the SCT (just 11.8% loss). With standard GSO coatings we get 82.8% transmission or 17.1% loss. If the achromat is air spaced, and has top multi-coatings you arrive at a 0.8% loss, ignoring transmission losses in the glass (which is fair enough for modern optical glass). For more standard multi-coatings you get between 2 and 4% loss. Not much, but it should be factored in.

Last niggle: Both the SCT and the frac need a diagonal, the Newtonian doesn't, that is another 1% loss.

 

[michael.h.f.wilkinson]

I personally use a combination of an 80mm F/6 triplet for wide field, and my trusty C8 for the vast majority of DSOs. Aperture is king for finding faint fuzzies (dark, transparent sky is emperor ). My C8 has let me see over 900 DSOs, amongst them 600 galaxies. I have looked through several refractors (including Olly's superb TEC140), but larger aperture wins on DSOs, I find. The TEC140 gave very nice views of M13 indeed, but my C8 showed some more stars, if perhaps not as pinpoint, and Olly's 20" Dob blew both out of the water.

 

[Commanderfish]

I enjoy my 80mm F6 Apo for wide filed views very much too, though 80mm only pulls in so many stars, so things like M37 are a struggle depending on the sky conditions.

To me, DSOs here mean a range of objects.

It makes sense that C8s and C9.25s can detect more faint galaxies, given a dark sky.  However, for open clusters and big nebs the C9.25 doesn't have the field of view -  and a C8 can't be that much wider.  Nothing else seems to quite produce refractor contrast either.  Combine that with local light poillution and I find I enjoy the view of galaxies more in the 127 Apo - but yes under good dark sky the C9.25 or a big newt can likely see galaxies which the 127 Apo or even 152 Achro cannot.  

That said, I'm focusing on a good garden scope set up for use in moderate LP, and galaxies are only one of my DSO targets along with open clusters, globs and nebs.  In my garden, I suspect the C9.25 might win on faint galaxies, and may win on globs - but the Achro will win on clusters and big nebs, and is likely to produce more contrasty views of brighter galaxies.  It will be interesting to see how well the 152 achro fares on globs compared to the C9.25.  In theory the C9.25 *should* win...

[John]

In my experience, having owned 30+ scopes from 60mm to 300mm in aperture, additional aperture always produces more resolution in globular clusters, the ability to see fainter galaxies and more extent and detail in brighter galaxies and nebulae.

Thats from someone who still regards themselves as a huge refractor fan !

 

 

 

[Telescope40]

Hello all. Did anyone get as far as doing this shootout with any definate winner ?? I have a C9.25 but keep seeing these 152 achros getting good press.

i do own  Meade 127 apo. Going to 4 scope is not an option. Maybe the C9.25 is switched for a 152. 

Thinking the C9.25 is ahead on the weight front, as a starting point ?? 

 

 

Thx. John 

[Moonshane]

Bear In mind the need to use the same magnification when comparing things like 'contrast' and 'brightness'.  I have never really been happy with less than ED quality but I like to use my scopes for all objects.