Telescope size vs aperture size in refractors

[vertigo262]

In looking around I noticed that the smaller refractors have a faster Focal Ratio. 

For example, a 80mm might have an f/5 whereas a 102mm might have an f/7

for photography and light you want a faster Focal Ratio to let more light in. But then, a larger tube also lets me light it. So I'm curious if, even with the slower focal ratio on a larger tube, does it come out even? or what are the tradeoff, or pros and cons?

aside from mobility, which is obvious, I'm curious to hear what people have to say.

[Christopher Davenport]

Subjects like these can get people worked up, because f ratio are very important in terrestrial photography but become less important in astrophotography as it is all about the size of the object, the focal length and if the object fits onto the sensor. Even at high f ratio of f10 people produce amazing images with Super sensitive mono CCD.

Bearing this in mind I'll caveat your question with "using a DSLR"

So the North America neb fits nicely onto a 300 to 400mm focal length with a DSLR.

Any lens that can get to this focal length and stay under about f7, will work.

The problem with f7 lens versus a f5 is that on slow lens you need treble the amount of data.

For this reason I use a FR/corrector,

I can reduce my 80ed to f6.5 or f4.5 depending on the object.

Having said that, I have never used a top end frac, so I am interested to hear others opinions.

[vertigo262]

I would think that as far as focal length and fitting things into frame, each thing is a different size, therefore, cropping, or using an EP in front of the lens if the only way to allow for framing. The larger scopes seem to have a longer focal length, but I'm assuming the distances your going

are so vast anyways, that something like 500mm to 2000mm, doesn't make much difference, and it's more important to get more light in the scope.

Although I could be wrong.

[Carina Lass]

Omega was done without a fr.

carina was done with a fr.

both are single exposures with no darks.

f10 8 inch cat. Wedge and alt az on a pier.

f10 is ok.

[vertigo262]

those are really impressive. Although it still doesn't answer the question of size vs aperture.

What was your exposure on these?

[Christopher Davenport]

I would think that as far as focal length and fitting things into frame, each thing is a different size, therefore, cropping, or using an EP in front of the lens if the only way to allow for framing. The larger scopes seem to have a longer focal length, but I'm assuming the distances your going

are so vast anyways, that something like 500mm to 2000mm, doesn't make much difference, and it's more important to get more light in the scope.

Although I could be wrong.

Distance through space don't affect anything as such, it's more about how bright objects are. 

For doing long exposures, it is best to work at prime focus, not through EP.

If you plan on buying a frac for imaging, best to do some research and read something like "make every photon count", written by stepperwolf on this forum.

[tekkydave]

Its not always obvious what is best. In terrestrial photography an f2.8 lens is still no use if your subject is 4 miles away and only a foot wide. There will always be a best combination for each subject i think.

[ollypenrice]

I think the key terms are getting muddled here. You need to keep them separate.

The amount of light collected depends on aperture and doubling the aperture quadruples the light grasp. That is absolute and non-controvertial. F ratio has nothing to do with light grasp. (Light entering your scope cannot know the F ratio it is about to participate in!)

Focal length determines what will fit on the chip. That's also absolute and non-controvertial.

Focal ratio is a minefield. Here goes;

- at a given focal length you will reduce exposure time at a fast F ratio because the only way to reduce the F ratio is to add aperture and, therefore, light. This, too, is non controvertial.

- BUT what happens if you keep the aperture the same and add a focal reducer to reduce the focal length?  The easy bit is that you get a wider field of view. But what about speed? If the object, say M51, will fit on the chip with and without reducer you do not gain time. The number of photons from M51 does not change. They just get poured onto fewer pixels, losing resolution but filling the pixels faster. However, resize M51 to the unreduced image size and you'll see you haven't really gained anything. However, if you want M51 and its surroundings in the image (you want all of what the chip covers) then you'll get a good image faster.

Fast apos get more expensive to make as aperture increases and eventually it becomes impossible to make them fast at all.

Olly

[Moonshane]

as I understand it (and I am a visual observer / photographer (of non astro) not an astro-imager) you need a fast focal ratio to collect the data as quickly as possible and a focal length to allow the whole of the object to fit accurately on the chip with no cropping. this allows the full resolution of the chip to be used. everything else is compromise as with all things in astronomy. fast focal ratios create issues like field curvature/coma which need to be corrected.

[Macavity]

The matter is reasonably complex for point-like sources:

http://www.astrometrica.at/papers/PointSources.pdf (interesting read)

And even before you get to extended objects.

Probably room for "subjectivity" too.

[ronin]

Agree with Olly the terms are getting mixed up.

The f number is a value that indicates how big and so how bright the prime image formed will be on a lens of a given focal length. The amount of light collected is only a function of the objective aperture. An 102 f/7 will collect the same amount of light as an 102 f/5.

The image brightness produced really is fairly simple geometry.

This was all easy when cameras didn't do everything for you, you had to have an idea of waht was going on.

The reason I suspect that a 102mm is f/7 and not f/5 is that an f/5 102 will give horrendous CA and SA. So to make an "acceptable" lens that people will buy they made it f/7, it also saves on the amount of grinding and so time that has to take place = less production cost. I would prefer 80mm ones to be f/7 not f/5, they would be a lot better. However people have the idea that faster is better so they manage to sell f/5 (achro) scopes and generally people are disappointed.

[ollypenrice]

Yes, for point sources the rule isn't the same but, in practice for amateur images, this won't be important I don't think.

It is possible to make F5 four inch scopes with outstanding colour correction. The Takahashi FSQ106, TeleVue NP101 and quite a few others qualify - but at a price!

Olly

[Christopher Davenport]

For somebody who is starting out and wants to image the large objects with a DSLR a f5 is going to be much easier.

The focal distance is much more forgiving on guiding, the weight less on the mount, the total integration time will be less and the noise on the DSLR sensor will be less as image can be run at lower ISO and sub lengths can be shorter.

This is the reason why the 66 72 80 have such a following. Correctors are a must with ED optics and are a lot cheaper than a triplet.

These are my findings I don't have funds for triplets or optics in the 100+ zone.