Fast or slow? Light or heavy?

[GordonCopestake]

I'm considering ordering a new newt. Im looking at 8" scopes and am trying to decide which focal length to go for. My main targets are planetary as i live under light polluted skies so i was thinking about 8"/f8. However the length of the tube puts me off. Instead i could go for f6, or even f4.5 to give shorter focal lengths and therefore a shorter more managable tube size. Using a televue powermate i could reclaim my lost focal length.

What are the disadvantages of going down this route? Using a 8" f4.5 OTA with a powermate over a straight 8" f8 OTA?

So far my negative points include a need to invest in premium quality eyepieces as faster focal ratios are less forgiving on eyepiece optics. Plus a need to check collimation more often as faster scopes are more sensitive to collimation.

And on the plus side, i only have a physically more managable OTA and a lighter OTA so my mount is less stessed.

Can anyone add to this and give comment? For reference the OTA's im mostly considering are the Orion Optics (UK) newts (or dobs as they have a offer at the moment)and my spec so far is:

8" OTA (anything larger and i'll struggle)

1/4 pv wave (dont need much more as the seeing rarely permits more, plus OO has 1/6th wave upgrade for free at the moment)

Hilux coatings (more light is always good, plus it's free at the moment)

Curved spider (difraction spikes are a personal preference and i prefer mine to be smeared out of existence)

Smallest practical secondary obstruction (contrast please)

2" Crayford focuser (gotta stick my DSLR somewhere)

Tube rings/vixen dovetail (so i can mount it on my CG5 sometimes. Although im starting to like the idea of the dob when i dont need tracking)

Comments are appreciated

[FLO]

What are the disadvantages of going down this route? Using a 8" f4.5 OTA with a powermate over a straight 8" f8 OTA?

So far my negative points include a need to invest in premium quality eyepieces as faster focal ratios are less forgiving on eyepiece optics. Plus a need to check collimation more often as faster scopes are more sensitive to collimation.

Consider also that the shorter/faster tubes produce wider light cones which need bigger secondary mirrors.

Are you sure you can't manage an f8? The advantages for planetary viewing are numerous.

[GordonCopestake]

The problem with f8 is the tube is approx 1.6 meters long. The f4.5 is only 900mm long. A significant difference! Obviously this adds to the weight and bulk when moving the OTA but it also reduces the central obstruction (assuming the manufacturer bothers to use a different secondary!)

[GazOC]

You'll lose contrast on the f4.5 model due to the bigger secondary. Even with a powermate you've compromised it's planetary ability straight away.

Have you thought of a 6" f8? they used to be very popular for planets (but not so much anymore it seems) or a 6" Mak-Newt if your budget will run thay far.

[FLO]

The problem with f8 is the tube is approx 1.6 meters long. The f4.5 is only 900mm long.

Use a milk crate

... assuming the manufacturer bothers to use a different secondary!

Orion Optics UK are reputable.

[GordonCopestake]

I have considered the secondary obstruction, and obviously i want it as small as possible. 20% or less ideally. I assume that Orion Optics and other manufacturers DO use smaller secondaries for slower scopes, but it wouldn't suprise me if they just used one secondary for all scopes regardless of f ratio.

I have looked at the 6" f8, but i would like as much aperture as possible, and as i wont have much chance to chance my scope again, i may as well get as much as i can now rather than have to change again later. The mak-newts are nice, and theres a meade 8" f4 for sale from pulsar optical for cheap at the moment. Not as good as the inters micro ones though. I only want to spend about £500 and for that i want as much aperture as i can get my hands on, and that means a newt. Although in reality i'de prefer a 8" APO but then i'de need an AP mount and a divorce lawyer.

Oh, and it's not the milk crate thats the problem, it's my back!

[GazOC]

Gordon,

The Meades are Schmitt-Newts not Mak-Newts. They are more for DSO imaging than anything else.

Gaz

[GordonCopestake]

Ah yes, you're right of course. I've looked at so many scopes tonight my head is getting mixed up!

I've fired off an email to Orion Optics regarding there central obstructions on the different f ratio scopes they sell. So that will give me more infomation to go by. At the moment im leaning towards the 8" f6 dob i think. The f8 will be too large to mount on my EQ i feel and the f4.5 will be too hard to keep good colimation. The f6 is a compromise but the best one perhaps. If Orion Optics come back and say there f6 secondary is 20% or less then thats probably my mind made up. Is this figure a standard for all 8" f6's? Or is it based on manufacturers?

[FLO]

Ah yes, you're right of course. I've looked at so many scopes tonight my head is getting mixed up!

Careful now

http://stargazerslounge.co.uk/index.php?topic=1271.0

The f6 is a compromise but the best one perhaps.

Heck! I'd be happy

If Orion Optics come back and say there f6 secondary is 20% or less then thats probably my mind made up. Is this figure a standard for all 8" f6's? Or is it based on manufacturers?

It is based on the size of the light cone. An f6 light cone is an f6 light cone ... whoever makes it. TBH, it never occurred to me that some manufacturers might take a one-size-fits-all attitude until you suggested it :?

[GazOC]

I've looked at so many scopes tonight my head is getting mixed up!

I've been there mate!

One thing Orion Optics used to do was let you select a smaller secondary than the scope really needed, you lose some of the FOV but gain in contrast. It was called a "planetbuster" is something like that.

It was pretty much (if memory serves) a 8" f6 newt but with the secondary from a 8" f8, you could even go the whole hog and ask for a secondary of a 6"f8 if planetary perfomance matters that much.

[GordonCopestake]

I didn't know that Gaz. That would give me confidence in the fact they do change the secondary for different f ratios and not use a one size fits all. As long as the secondary is at or below that magic 20% that im reliably informed is no longer noticable then im happy. If i loose some of my FOV then so be it. My main targets are solar system although i would still like to dabble in DSO's but my LP stops me doing anything serious.

[GazOC]

The scope was availiable about 5-6 years ago Gordon.

Dug this article out for you....

CENTRAL OBSTRUCTIONS AND THEIR EFFECTS

by David Knisely

In most reflecting or catadioptric telescope designs, a secondary

mirror is required in the optical path to send the light from the main

mirror to the correct position for viewing. This mirror and its holder

obstructs some of the light entering the telescope, and for larger

secondaries, this has the potential to cause some problems.

A central obstruction affects the image in two ways: 1. Causes a

light loss due to the blocking of light entering the telescope. 2.

Introduces diffraction effects which can cause a slight loss of both

light and contrast for high power images *if the secondary is too

large*. As far as light loss is concerned, the best human eye can just

detect a difference of 0.1 magnitudes, so with obstructions less than 29

percent of the aperture diameter (8.4 percent of the area), the actual

light loss caused by just the blockage is essentually undetectable

visually. Indeed, many people have trouble seeing a magnitude

difference of 0.2 magnitudes, so for obstructions of less than 41

percent of the aperture (16.8 percent of the area), the light loss due

to the obstruction is not all that noticable.

Of these two effects, the second (diffraction) is more

significant. The obstruction from the secondary and its cell slightly

alters the disk and ring diffraction pattern of stars, taking a little

light out of the central Airy disk and putting it into the rings (mostly

the first ring out from the Airy disk for common-sized obstructions).

If the secondary is large enough, this energy redistribution can result

in a slight reduction in the contrast of fine detail for high power

images of the moon and planets. How much of a problem this is for the

observer depends on how big the obstruction caused by the secondary is.

In practice, if the secondary obstruction is *less* than 20 percent of

the main mirror's diameter (less than 1/5th of the mirror is

obstructed), the overall effect on the image is negligable (Sidgwick:

1971, Suiter: 1995). For example, a six inch f/10 Newtonian with a 1

inch secondary mirror (16.7% obstruction) and good optical quality would

perform about as well as if the obstruction wasn't there at all. The

contrast reduction effect of the central obstruction is quite gradual

with increasing secondary size, so it doesn't suddenly become quite bad

beyond a certain point. Indeed, the secondary's obstruction can be

somewhat larger than 20% without greatly impacting the visiblity of

high-power detail.

As you use larger and larger secondary sizes, eventually, high

power images will tend to gradually aquire a slight "softness" to them,

which may make fine low-contrast detail a bit harder to see. A certain

limit comes when the secondary obstruction does become somewhat more

noticable, and that point is when the obstruction reaches about 25

percent of the main mirror's diameter. This amounts to over a six

percent light loss, and becomes significant, especially for daytime use,

when the shadow of the secondary mirror may become visible in the

eyepiece at low power. Only Rich-field instruments, Cassegrain systems,

or those instruments requiring large fully

illuminated fields should have secondaries which obstruct significantly

more than 25 percent of the aperture.

One frequent statement by some authors is that a larger secondary

can help increase the apparent resolving power of a telescope. This is

somewhat of an exageration. While the diffraction caused by the

secondary obstruction does cause a reduction in the diameter of the Airy

disk of a star, the actual amount of reduction for common central

obstruction sizes is slight, and would not help with detail in extended

objects. It may slightly improve the ability of the telescope to

resolve close double stars but only when the obstruction reaches a

somewhat large size. Indeed, the diffraction disk of a telescope with a

20 percent central obstruction is only about four percent smaller than

that of an unobstructed instrument. Even a 33 percent central

obstruction would only yield a 10 percent reduction in the Airy disk

size, so for common central obstruction sizes, the "improvement" in

effective resolution is minimal. The amount of energy put into the

first ring by the obstruction would negate any alleged resolution

increase on extended objects, reducing the contrast on high power

planetary images. It is still better to keep the secondary obstruction

under 25 percent if possible.

Keeping the obstruction under 25% is a good goal. However, even at

a 25 percent level, the image degredation is far from fatal, and the

telescope will still function. Often in Newtonian designs intended for

general purpose use, an 18% to 25% central obstruction range is a good

guidline to follow. As for resorting to ultra-small secondaries to

improve the telescope's high-power performance, this can backfire. The

proper size for Newtonian's secondary mirror will depend on many things,

but it is important to make certain that it big enough to catch all the

light from the primary mirror which is headed to at least a small area

around the center of the field. You don't really get a tremendous image

quality improvement by using a secondary size significantly less than 20

percent of the main mirror's diameter, and you may actually lose light

with a secondary mirror which is too small to catch all the light from

the primary mirror. In addition, low profile focusers used with such

small secondaries may allow some external scattered

light to get to the eyepiece without being blocked by the diagonal.

Small secondaries of high optical quality can also be harder to find or

to make, since small area defects can occupy a larger percentage of the

secondary mirror's surface than they would for a larger mirror. You

need to balance the desire for high power contrast with the need for

proper overall design.

Overall optical quality is more important in the long run than is

how small a secondary mirror your telescope uses. A quarter wave of

spherical aberration has about as much effect on the overall energy in

the Airy disk of an imaged star as a nearly 33 percent central

obstruction does. The effects of a quarter-wave (peak-to-valley

wavefront error) mirror, while just meeting

the Rayleigh Limit, would basically overshadow the benefits gained by

the use of a small secondary mirror. Resorting to a tiny secondary

mirror while tolerating a lower-quality mirror makes little sense to

those who are trying to get the best planetary images from their

scopes. Indeed, many of the often talked about performance differences

between various telescope designs are due more to lack of mirror quality

than to the problems caused by the presence of a secondary mirror.

The "modified" Schmidt-Cassegrain telescope needs a much larger

secondary, often obstructing 33 to 36 percent of the primary mirror's

diameter. This does cause a visible loss in contrast for high power

images and a slight reduction in limiting magnitudes for the fainter

stars, but overall, the telescope still performs adequately. Indeed,

many planetary observers and imagers do successfully use SCTs for their

work. The tradeoff is in contrast verses telescope compactness. The

SCT does offer a very convenient package for people who want portability

or ease of use for photography, so the secondary obstruction isn't the

only factor to consider. Once again, optical

quality is the most important thing to have when it comes to an

astronomical telescope.

David Knisely, Prairie Astronomy Club, Inc.

[GordonCopestake]

An excellent Article Gaz, very interesting! Many thanks for digging that one out!

[FLO]

Good find Gaz - reminds me never to get into a 'secondary obstruction' debate

[GordonCopestake]

I've received a reply from Orion Optics concerning their flats and they state:

weight of 150mm f5,f8 and f11....5, 7, 8 kilos flat size are 50, 35 and 25

weight of 200mm f6 and f8.... 7 and 9 kilos flat size are 50 and 36mm

weight of 250mm f4.8 and f6.3....8 and 9 kilos flat size are 63 and 50mm

This means the 8" f6 has a 25% CO, which is a little on the large side for my liking. The 8" f8 is a pleasant 18%, but then i have to drag 1.6meters and 9kg of OTA around with me. Plus 9kg is awful close to the 10kg rating of my CG5 mount.

Decisions decisions.

[OXO]

Have you considered a SCT? you could get a F10 SCT on your Mount and would be a killer on the Planets?

James

[GordonCopestake]

The SCT's have about a 35% CO, and contrast is almost non-existent with them. I had Russ's C8, and whilst a fine scope the lack of contrast pushed me to sell it in favour of the 4" APO. I prefer the views through the 4" to the C8 believe it or not. However im looking for a little more resolution and a 20% CO is my target. The 25% on the f6 would be a compromise. Im trying to decide if i could cope with the f8 as thats the best solution. However the OTA weight and length is putting me off.

I would keep the ED100 whatever i choose, so perhaps i should just go for the f8 and only mount it dob style... That does have some appeal to me as the lack of setup time would be good, however i've been spoilt by good tracking and GOTO and now i have it i dont want to loose it.

[russ]

Hi Gordon,

Been down this road soooo many times. And yes people find it hard to believe that 4" Apo gives a better planetary view than the 8" SCT but it does. Glad you came to the same conclusion.

The Planetary imaging is a [removed word] and the sole reason for me changing so many scopes in the last 4 years. For me the Intes-Micro M603 hit the sweet spot. F10 focal ratio, jaw dropping contrast, superb definition. To say it beats an 8" SCT would be understating it slightly. My Orion Optics 10" f4.8 Newt was also no match. I imagine the 8" f8 though with it's 18% CO will be a stunner. But I understand your hesitation. 1600mm focal length in a Newt will not be much fun.

I agree with Steve, the 8" f6 would be a better bet. A good balance.

Or go with Gaz's suggestion of the 6" mak.

Or checkout the reviews of the 8" LXD Meade. People love that OTA and not just for deepsky, planets too.

Regards

Russ

[Guest MoJo]

Hmmm MN 68??

for all of the reasons given??

Dunno, trying to work it out for myself too!!

MoJo

[GordonCopestake]

Hi Rus,

i have looked at the Intes-Micro M603 but apart from only being 6" it is also £1000 new. The Orion Optics Dob is only £500 and is 8". But i do like the M603 as it's a MUCH smaller package. The C8 was ideal, in both focal length and aperture. If it had more contrast i would still have it now and love it.

I have also looked at the c9.25, which apparently is a lot better than the other SCT's, but even so it must have a larger central obstruction than the f8 newt due to it's very short tube.

[GordonCopestake]

Hmmm MN 68??

for all of the reasons given??

Dunno, trying to work it out for myself too!!

MoJo

I do like the MN68, as it only has a 15% CO, however it's only 6" aperture, over 10kg heavy and costs £1000...

[GazOC]

The 8" f6 with the 6" f8 secondary works out about 17.5%, I'm sure Orion will do it if you ask.... it's probably going to be the cheapest way to achieve the specs you want (8" mirror, <20% CA).

[GordonCopestake]

The 8" f6 with the 6" f8 secondary works out about 17.5%, I'm sure Orion will do it if you ask.... it's probably going to be the cheapest way to achieve the specs you want (8" mirror, <20% CA).

I could consider asking Orion Optics to make me a f6 scope with a smaller secondary, this would loose me some FOV but i would gain contrast. Given my main targets are small im wondering if i could sacrifice FOV to get the best of all worlds? A 20% CO and an f6 7kg short tube...

I'm not sure how much FOV i'll loose. I have no idea how to calculate it... Or even visualise it. Anyone?

[FLO]

I could consider asking Orion Optics to make me a f6 scope with a smaller secondary, this would loose me some FOV but i would gain contrast.

You would also lose brightness...

[GazOC]

If the secondary is slightly undersized you don't lose FOV per se, what happens is that the FOV is not fully illuminated across the entire FOV. In effect the edge is darker than the centre because the secondary is not recieving light from all of the primary.

More undersized secondaries do cause vignetting.

Something to play with...

http://aberrator.astronomy.net/

You'll' need your own planetary images to test on though, there are other images to use as part of the package (double stars, waves, etc).