80mm Refractors

[M L W]

Why are 80mm refractors so popular for AP? It seems even more so than 100-120mm scopes!

[Daddystu]

I noticed that too. I'm actually looking for a new scope - maybe a 100mm Apo refractor but I notice quite a few members with the same brand 80 ED in their sig. The only reason I can think is the f-ratio being 6 instead of 9.

[fredster]

Small and lightweight too. Coupled with an ST80 for guiding it won't tax the mount.

[M L W]

Hi mate, i see your near Newport... Not too far from me in llanelli. I have also looked at the 100/120+ but all suggest the 80mm is best. I instinctively wanted to buy the biggest refractor I could afford but have learnt that biggest isn't always best!

[ronin]

Like much in this it is a compromise.

A 100ED is a fair bit more costly, the additional focal length and weight means that the tracking has to be better, the mount has to be more solid therefore and on a refractor as the diameter increases then you are more prone to spherical aberrations. You will find that most are still doublets so the SA and CA are a factor, an apo triplet is plain very expensive. I would guess that a decent flattener for a 100 is more then for an 80.

If you want to collect a bit more light then a bigger aperture is one way, or a bit more time on the exposure is another.

[M L W]

I can certainly see the weight benefit here but optically how does the 80 out perform larger refractors. I understand it has something to do with the field of view! I can't wait for my book, making every atom count as I've been told its like the astronomy bible and will explain all.

[M L W]

Also I understand that the 80mm is a faster scope than the 100-120. I'm a looking at getting a heavy weight mount but will go on advice and get the 80. I just wanted to hear from people how the compare in picture resolution.

[Daddystu]

Yes not too far in terms of time taken but you're a fair few arc-minutes away

I like the look of the mount you've chosen (without go-to) but myself i'm thinking on the 100mm apo. I take the advice given above with regard to spherical aberration and the increased weight but I would like the scope to be a bit more of an all rounder I think. I'm not settled yet though as i've still got to sell my ETX, DSI and full-suss. mountain bike to free up some funds. Have to keep my darling wife happy!

[johnrt]

Aperture means nothing in AP, unlike observing. What is important is the f/ratio of your scope. 80mm scopes are popular as they can be reduced to around f/4 with a decent x0.8 reducer and they are light while offering a useable field of view for a great many targets. A heavy scope with a longer focal length and slower f/ratio means longer more accurate tracking is required by the mount which is under more stress by the heavier payload.

They key is DSO imaging is light fast scopes and expose for as long as you can before tracking errors appear in your subs.

[Daddystu]

The 100 is just 800g heavier than the 80 but i appreciate the extra instability may well come from its increased length. I have a fl5/fl3 reducer which may be suitable and would help with the exposure times.

[johnrt]

The 100 is just 800g heavier than the 80 but i appreciate the extra instability may well come from its increased length. I have a fl5/fl3 reducer which may be suitable and would help with the exposure times.

I'm not sure what that reducer is, but reducers suitable for refractors are all about x0.8 which would bring the 100ed down to f/7.2 which is woefully slow for DSO imaging. A reduced 80mm at f/4 would be x4 faster, meaning what you can capture in 20 minutes on your 100ed I can do in 5 on my 80ed.

[Daddystu]

Thanks for that John. The reducer i have is SAC ( http://www.sac-imaging-europe.com/SAC/SAC%20accessories.htm ). Its for 1.25 eyepieces. If it isn't suitable for the 100 then based on your info I would definitely go for the 80.

[johnrt]

Thanks for that John. The reducer i have is SAC ( http://www.sac-imagi...accessories.htm ). Its for 1.25 eyepieces. If it isn't suitable for the 100 then based on your info I would definitely go for the 80.

I doubt that would be suitable, you will need something like this http://www.firstlightoptics.com/reducersflatteners/skywatcher-focal-reducers.html

[nephilim]

As mentioned above, at f/9 the 100 is way too slow,although I'm new to this AP business myself i think you need much longer exposures with a slow scope and 'only' 800g heavier is a whole world of weight as far as AP is concerned.

Steve

[nephilim]

Also take into account that if you bought the ED80 (perfect for AP) with a view for imaging, I assume you'll be buying a mount to match (HEQ5 or upwards) in which case the difference you'll save in buying the 80 over the 100 will more or less buy u this http://www.firstligh...r-150p-ota.html which is an excellent OTA & should suit your observing needs very well (plus you'll have a mount for both & altho I dont want to confuse issues, it is possible to guide with the reflector & image with the frac, piggybacked or side by side, but thats a whole different thread .......Just a thought).

Steve

[lensman57]

Why are 80mm refractors so popular for AP? It seems even more so than 100-120mm scopes!

Hi,

It is relatively cheap to produce an 80 mm ED ( Apo) short focal length , fast "F" scope of doublet design with a half decent glass ( mostly FPL51 or Equivalent ), in another word a fast scope of F5 or F6 compared to a 100 mm or 120 mm Apo of triplet design using FPL 53 glass or better of smae F no. Most 80 mm ED fast scopes are totaly useless for astronomical observation because of the relatively small aperture, the human eye needs so much light to register form and detail, that is before you factor in the local light pollution and other detremental facors that affect your local limiting magnitude. As far as the AP is concerned there is no need for a very large aperture but what you need is a decent compromise between aperture and a relatively fast F no to help keep the exposure times down, in fact the exposurre time for a given sensitivity ( either film or digital ) is only dependant on the F ratio. The larger aperture though will give you a larger magnification for the same F no, compared to a smaller aperture which is at times quite desirable.

This by no means excludes the slower F no scopes for DSO imaging provided that either your ccd camera has a very good high sensitivity or your mount can track the object very accurately over a long period of time, the 80 mm F4 Eds of today provide a very fast way of imaging given the fact that most of them are mounted on relatively cheap mounts, such as EQ-5 or HEQ-5 or equivalents ( I have the cheap one BTW ). I think that by comparison a good and fast 120 mm triplet Apo design will cost an arm and a leg compared to an 80 mm doublet apo scope. I hope that this made sense to you.

BTW I have been told that the supply of ED "apo" scopes might be in for a rough ride as the Chinese manufacturers are asking for a very large minimum order to keep the cost down, there is just not enough demand for so much Apo glass at the moment.

Regards,

A.G

[M L W]

Hi AG, I had to read your post a couple times for it to sink in but yes, all was understood and thanks for you input. As said earlier, I did consider a larger fractor but listened to advice as you have also pointed out above. I have seen some great images from both smaller and larger refractors and despite the itch to spend I will stick with the 80ed and then get a good tube of 200-250p for visual however, I would also like to try taking photos with the tube and for the same reason ad with the refractory, I would like to hear people experience if using sw 200-250p for pics.

I intend on getting a Heq6 to ensure a sturdy base and also believe in spending the right money on the right kit. Learning how to get good quality photographs and the most from your kit Is also part of the excitement / learning curve of the hoby I think. Needless to say I think were meant to make a few mistakes in order to learn from them and progress. I leant that when I started kitedurfing.

Mario

[lensman57]

Hi AG, I had to read your post a couple times for it to sink in but yes, all was understood and thanks for you input. As said earlier, I did consider a larger fractor but listened to advice as you have also pointed out above. I have seen some great images from both smaller and larger refractors and despite the itch to spend I will stick with the 80ed and then get a good tube of 200-250p for visual however, I would also like to try taking photos with the tube and for the same reason ad with the refractory, I would like to hear people experience if using sw 200-250p for pics.

I intend on getting a Heq6 to ensure a sturdy base and also believe in spending the right money on the right kit. Learning how to get good quality photographs and the most from your kit Is also part of the excitement / learning curve of the hoby I think. Needless to say I think were meant to make a few mistakes in order to learn from them and progress. I leant that when I started kitedurfing.

Mario

Hi Mario,

I am glad that I was of some help, the ED 80 will do nicely for imaging but you will also need a guide scope and a guide camera so you will need to budget for all these, the Heq6 is nice mount and should be able to carry all the weight of the above equipment and some to spare. Good luck and let us know how you get on please.

Regards,

A.G

[M L W]

Cheers AG, will do.

Mario

[laser_jock99]

Aperture means nothing in AP, unlike observing. What is important is the f/ratio of your scope. 80mm scopes are popular as they can be reduced to around f/4 with a decent x0.8 reducer and they are light while offering a useable field of view for a great many targets. A heavy scope with a longer focal length and slower f/ratio means longer more accurate tracking is required by the mount which is under more stress by the heavier payload.

They key is DSO imaging is light fast scopes and expose for as long as you can before tracking errors appear in your subs.

Aperture AND focal focal ratio are both important. Big aperture means more photons. Try out a 12" F4 scope and then compare it to an 80mm F4 scope.......

[johnrt]

Aperture AND focal focal ratio are both important. Big aperture means more photons. Try out a 12" F4 scope and then compare it to an 80mm F4 scope.......

Your 12 inch scope will produce the same brightness of an extended object as a 3 inch scope at the same f/ratio.

Example - I have an 80mm f/4 scope - focal length 320mm. Hypothetically a pixel on my camera receives 25 photons per second when imaging. Along comes someone else with a 160mm f/4 scope - focal length 640mm. The larger scope captures 4 times as many photons - 100 per second BUT due to the doubled focal length the 100 photons are now spread out over 4 pixels instead of just 1, so each pixel still receives 25 photons per second resulting in the same brightness and therefore the same exposure time is required.

If you fix the focal length to maintain the same field of view and double the aperture then you will collect 4 times as many photons and decrease your exposure time by a factor of 4, but to achieve this you have halved the f/ratio to f/2.

The exception to this are point sources such as stars too small to be spread out over multiple pixels when the focal length is increased, they will indeed by x4 brighter and quicker to capture.

[RikM]

Aperture AND focal focal ratio are both important. Big aperture means more photons. Try out a 12" F4 scope and then compare it to an 80mm F4 scope.......

They will be the same image brightness, just the 12" will show a higher magnification vs. the wider field of view in the 80mm. The 12" does gather more photons, but they are spread over a bigger area because of the longer focal length. The f/4 is the important bit, the focal length sets the field of view (aperture is simply a byproduct of the other two).

The only time aperture is important in imaging is with planetary imaging where you need the increased resolution.

[lensman57]

Aperture AND focal focal ratio are both important. Big aperture means more photons. Try out a 12" F4 scope and then compare it to an 80mm F4 scope.......

Hi,

To make this simple to understand from a practical point of view, take your DSLR or 35 mm film camera and mount a 50 mm standard lens on it, set the iso ( sesitivity ) to what ever you wish but from here on do not alter it. Now turn or set the aperture lets say to f 5.6 and record the exposure for given constant source of light, change the 50 mm lens to something else, what ever you have lets say a 200 mm , you will notice that for the same aperture setting the camera will give you the same exposure mark, regardless of the focal length. The strength of light falling on the sensor is only dependant on the F raio ie: the F number and not the lens aperture or focal length, the magnification and therfore the field of view will however vary , according to the focal length. For visual observation the aperture is important and the bigger then at least theoretically the better, for photography the two main factors are the F number and the sensor's or the films sensitivity.

Regards,

A.G

[boris sputnik]

Hi all !

I just bought a 80mm triplet Apo for astrophoto. The reason is because I want a wider field of view... My other scope is a STC 8" so the focal length is 2032mm at f/10. With a focal reducer/corrector it goes to 1260mm at f/6.3. My 80mm is 480mm focal length at f/6.

With the STC I can take only the core of M31 ,I can't get the whole Pleiades ,the witch head nebula ,the rosette ,north america nebula and other wonderful objects...With the 80mm I can photograph all that.

greetings from Québec !

Maxx

[johnrt]

Well, I think there we have 3 very good explanations of why needing the bigest scope possible for astrophotography is a misconception. It is for the reasons above that an 80ED scope is the most popular imaging scope going (along with being light, cheap and good quality).

Of course once you have some exposure time under your belt with an 80mm you'll want to complement with something else, which is why I bought a 6" RC which when reduced gives me a 900mm f/6 scope just right for picking out tiny planetary nebula and galaxies which are just too small in the fov in my 80mm refractor.