Help with new scope selection

[Northernlight]

Hi all,

Looking for some feedback on a new scope selection. The scope will be permanently mounted in obs on a EQ8, so weight shouldn't be too much of an issue. I realized that spending 1k on a scope for imaging at the 1m mark just isn't going to cut it, so upping my budget to 3k

Would appreciate feedback on pro's and cons. The scope selection criteria is as follows :

● Scope to be used primarily for imaging with a Kaf8300 chip. If it can do visual as well its a bonus.

● Looking for something fast F4-F5

● Focal length around 1000-1200mm

 

Options i'm considering are :

Altair 10" F4 Truss Imaging Newt

10" F4 or F4.7 Teleskop Express ONTC Newtonian

10" Meade ACF with a reducer

APM 152 (1200mm) F7.9 Doublet with reducer

 

Cheers,

Rich.

 

[Northernlight]

Anyone ?

[ollypenrice]

The thing is, I personally wouldn't go down the 'fast' road to get a metre of FL. I use an F7 refractor and put up with the exposure speed in exchange for having a scope which just works and doesn't eat up imaging time by needing attention. I also like the quality of image I get that way.

Olly

[DaveS]

Have you considered the 190 MN?

[Northernlight]

Hi Dave,  I did try a MN190, got one new but it's build quality was exceptionally bad.  In my case the biggest problem was that the secondary wasn't marked correctly.   The center dot on the secondary was infact a U shape and was drawn on in pen by someone with a very shaky hand. I would love a MN190 if it wasn't a project in itself just to getit setup working right.

Olly, I could be wrong here but i thought i saw a post somewhere where you mentioned a Meade F8 ACF tube.  Have you got any experience of them ?

In regards to using a refractor at F7, i agree it would be simpler but i dont live in the darkest of area's, so i dont want have to jump up to 15-20min subs to get some good data.   I normally do my Lum & Narrow band @ 600s subs at F5.6ish,  and i'm guessing i would need to jump to 900 or 1200s if i was imaging at F7.

[ollypenrice]

4 hours ago, Northernlight said:

Hi Dave,  I did try a MN190, got one new but it's build quality was exceptionally bad.  In my case the biggest problem was that the secondary wasn't marked correctly.   The center dot on the secondary was infact a U shape and was drawn on in pen by someone with a very shaky hand. I would love a MN190 if it wasn't a project in itself just to getit setup working right.

Olly, I could be wrong here but i thought i saw a post somewhere where you mentioned a Meade F8 ACF tube.  Have you got any experience of them ?

In regards to using a refractor at F7, i agree it would be simpler but i dont live in the darkest of area's, so i dont want have to jump up to 15-20min subs to get some good data.   I normally do my Lum & Narrow band @ 600s subs at F5.6ish,  and i'm guessing i would need to jump to 900 or 1200s if i was imaging at F7.

No, you're quite right.* I bought a 10 inch ACF from one of our regular guests some time ago but have yet to try it properly. (Long story, it was going to go on one of our robotic guests' mounts as a joint venture but the mount in question had to be returned for a refund and its replacement is too small for the ACF.) My aim is to compare the performance of the ACF/large pixel Atik 11meg with that of the TEC140/small pixel Atik 460.

I will get to try the ACF once I've given the TEC/460 a thorough workout. Although I have the Astro Physics reducer for the Meade I don't see myself using it. My targets will be mostly small galaxies so the F ratio myth applies. (Exposure time will be dominated by aperture.) 

Olly

*Actually mine is F10 and can be reduced to about F.6.5. The way to get an F8 ACF is to go for the 12 inch with the same FL as the 10 inch. Here the reduction from F10 to F8 is myth-free because the drop in F ratio comes from an increase in aperture, not a reduction of focal length.

 

[Northernlight]

So in my simplified mind, you are saying that there wont be much difference in the amount of data collected between a 102mm refractor @ F5.6 vs a 152mm refractor at F7 if both are doing 600s subs

 

[ollypenrice]

32 minutes ago, Northernlight said:

So in my simplified mind, you are saying that there wont be much difference in the amount of data collected between a 102mm refractor @ F5.6 vs a 152mm refractor at F7 if both are doing 600s subs

 

Let's take the case of an object which is comfortably framed in both instruments. How on earth can the smaller one collect more object photons than the larger one? Very obviously it can't. F ratio has nothing to do with it. The area of a circle of diameter 102mm is 8171.2 square mm. The area of a circle of diameter 152mm is 18145.8 square mm. This defines the light grasp. That means that there will be 2.2 times as many object photons coming in from the larger scope. So far, surely, this is beyond controversy?

Now, where it gets more difficult is what you do with those photons in terms of putting them on your pixels. Put them on more pixels and each pixel will get less light, meaning that you may not get above the noise floor. But if the faint signal does get above the noise floor and you seek to present an image of the same small object at the same size in both scopes, how can having fewer than half as many object photons in the small scope make for a better image?

This is my perrenial argument with F ratio worshippers. Hey, I worship F ratio like any sane astronomer - but I worship it at a fixed focal length in any conversation, which means that what I really worship is... aperture!

Oh dear, but I also worship simplicity, reliabilty and predictability which leads me to worship refractors...

Sorry, I'm doing my best!!

Olly

[Northernlight]

Thank you Olly, that was a great explanation. So essentially the larger aperture F7 scope will give a brighter image due to larger aperture regardless of focal ratio ?

So does the focal ratio just impact the object size/image scale ?

[ollypenrice]

13 hours ago, Northernlight said:

Thank you Olly, that was a great explanation. So essentially the larger aperture F7 scope will give a brighter image due to larger aperture regardless of focal ratio ?

So does the focal ratio just impact the object size/image scale ?

My policy is to ignore focal ratio in any comparison in which the focal length is not the same.

'A brighter image' could mean different things. A focal reducer will make a given object 'brighter but over a smaller area.' But the only way to collect more object photons is to go for a larger aperture.

I must do this as a test and post the results: take the same object in 106mm F5 and 140mm F7 telescopes and resize the 140mm image down to the size of the 106 then compare the SN ratio. I cannot see how the larger aperture can fail to produce a better S/N ratio but I'll get round to trying this out... What I have done, though, and many times, is shoot a region of interest from a widefield Tak image in the TEC in order to enhance resolution of the region in question. What is interesting, and supports my claim, is that you need only a very small amount of data to enhance a region. The data, seen full size, looks terrible but resampled down to a quarter of its original size it is better than the widefield data. My explanation is that that it contained twice as many object photons.

Olly

[tony210]

Hi - I think this is accurate - aperture and resolution are inextricably linked and focal length is the fine detail-I am sure this is due to the number of photons captured by larger apertures-still the problem is that seeing limits aperture size a lot- we don't want  to resolve the turbulence in our atmosphere too much especially in Sussex! - Tony.

[Northernlight]

Just a shame skywatcher or someone else haven't made a sub <£2,000 deluxe version of MN190. I would happily pay double if they just did a version with an upgraded focuser & mirror cell to get rid of the terrible mirror slop, not to mention the paying attention to Quality control. Even at double the price it would still be worth it as it Just seems to have a nice balance of aperture and speed.

I think the reality is that due to my budget being limited to £3,000 I'm looking at either a 140/150mm doublet refractor if i want simplicity and reliability.