# 30 Jan, 2020 19:20
I'm new and my only experience in astrophotography consits in taking pictures with my 14mm wide angle and a few tries with the SW Star adventurer. I'm looking forward to buying my first telescope and because my principal aim is to take crisp esthetic pictures of the night sky. I have read a lot about telescopes and I'm a bit lost. I believe an APO would be fine for me, but I don't quite understand how I have to understand the focal length of an apochromatic telescope. In comparison to my Sony photographic teleobjective (400mm) do I get the same magnification with a 400mm APO if the camera is directly attached to it? Or can I attach the camera on an eyepiece? Sorry for this maybe stupid question. I just don't understand how one can get a fine picture of Andromeda with a 61/360. Thanks for your help!
# 31 Jan, 2020 02:43
|You can get a great picture of Andromeda, with a 60 mm Apo refractor at 360 mm f.length because Andromeda is a big galaxy! It needs a shorter fccal length to fit in the field of most cameras, but not all, if you are just starting out I highly recommend you start with a short focal length Apo refractor in the 60 to 80 mm size, that is the size of the objective they usually have a focal length of 360 to 500 mm. Great for wide field objects, and a perfect size to learn on, till you build up your skill level, you can go 2 ways on cameras a DSLR type or a dedicated astronomy camera, but I would read up on the basics of astrophotography first! Good luck and clear skies. Shawn|
# 31 Jan, 2020 08:41
Many thanks for your precious answer and advice! I will consider it also because the expert of astroshop.de told me the same. Now, as a beginner and total novice, I still do not understand how an apochromatic telescope works. Let me say it with easy words: if I point at Andromeda with my Sony photographic 400mm telelens and with a 400mm APO telescope, will both fill the same amount of my picture? Or does the telescope with the same focal length work like binoculars and approach Andromeda? I'm clear with that for ather types of telescopes like Newtons, but I don't understand the magnification (if so) of a refractor. Hmmm, you see that I'm a novice… Kind regards, Markus.
# 31 Jan, 2020 12:58
Yes, a 400mm APO and a 400mm camera lens with give you identical images on the camera sensor at prime focus. All Optical Tube Assemblies, whether lenses, refractors, reflectors, etc. behave in this way. The focal length gives you the field of view, the aperture (in millimeters) determines the total amount of light let in. Given two OTAs of the same focal length, you will get the same image on the sensor; however, the faster (lower f number) OTA will let in more light and have a "brighter" image and allow you to collect more light in a given time.|
In buying a telescope, make sure also to look at the corrected image circle in the specification. It needs to be large enough to cover your sensor. Many telescopes are made to cover APS-C (crop) or even micro 4/3 sensors, which would be too small an image circle if you are using a full frame DSLR. Also check whether the total weight of the scope and lens is within the weight limits of the Star Adventurer and your current mount; the tracking accuracy and mount stability needed at 400mm is vastly greater than at 14mm. More than likely, you will want to upgrade your mount as well.
# 31 Jan, 2020 13:49
I agree! either that or invest in some form of guiding. For M31, the Star Adventurer will give you 2min-subs with minor star elongation with a 432 mm APO (by my experience) And more than 3kg imaging load is too much for it. Here's an image of M42 made on a guided star adventurer with said 432mm Apo, a modified APS-C DSLR and a Flattener, weighing 2,8 kg altogether: https://www.astrobin.com/nfhweu/?nc=user But to achieve this kind of accuracy on a star Adventurer is tricky, even when guiding, I needed one year to get it working that well.
Also, for most APOs you will have to buy a seperate Flattener or Flattener/reducer for photographic purposes, keep that in mind! Otherwise the stars will be all wonky toward the edges of the image.
Clear Skies, Marc
# 31 Jan, 2020 14:18
|Yes, everyone's advice is excellent, but to your question of how an apo refractor works, it is the same as any telescope, only it incorporates special glass, and sometimes more elements in the objective lens assembly, this is to improve color correction, a non Apo refractor does not bring all colors of light to the same point of focus, so you can see fringing around bright objects and stars, an apo just corrects for this problem. C. S. Shawn|
# 31 Jan, 2020 15:31
Many thanks for your explanations! Now, if I don't get closer to Andromeda, why should I buy a 400mm APO? My 400mm Sony lens does a good job! I would like to show you the picture I took with it of Orion nebula, but I don't know how to do this. If I click the "insert image" button it asks me for an URL. My pictures are on my harddrive…
Maybe my question is too basic or even too stupid to be understood by the experts you are. Let me try again with a simple comparison: if I shoot a bird sitting on my appletree in the garden with my Sony 100-400mm set at 400mm and if I get a picture of the entire bird with a small space on the top and at the bottom, what would I see on the picture if I shoot the same bird from the same place with an APO 400mm? The same as before, the entire bird with the small space around it or just a part of its head?
# 31 Jan, 2020 20:33
|You , would see the same thing at 400 mm focal length.|
# 31 Jan, 2020 20:36
|Ok, thank you for this precious information! So I don't understand how one can take a picture of Andromeda with a 400mm APO…|
# 31 Jan, 2020 20:39
|Telescopes are no different than lenses , with the size of the image they will produce at the same focal length. 400 mm focal length is the same with either c. Skies.|
# 31 Jan, 2020 20:49
|Hi Marcus , you should be able to post an image you took, is it a picture already posted on atrobin? I'm no expert on posting pictures on a forum topic, but if it's a picture you already have on astronin you should be able to post it.|
# 31 Jan, 2020 20:52
|Hi Shawn! I already have registered… I'm trying to find out what I should by as my first telescope. Not easy!|
# 02 Feb, 2020 16:17
Well, one thing which helped me decide which scope to choose was the freeware Stellarium|
There, you can put in your camera's sensor and pixel size and the focal lenth of different scopes which you are considering to buy. The program will then show you how big your field of view is going to be and you can easily compare scopes in that regard. And later on, it's great for planning sessions.
There are two websites which allow you to do the same thing: blackwater skies and Telescopius. Simply choose a camera model and a telescope and you can see what the field of view is going to be.
# 02 Feb, 2020 16:33
Many thanks for this really helpful informations. I already know what I will do this evening! This is very kind of you. Indeed, I have difficulties to imagine what I would be able to see with an APO at a given focal length. Am I right, is a Barlow lense the only possibility for any magnification when using a camera? One can't mount a camera on an eyepiece?
# 12 Feb, 2020 06:07
Regarding afocal imaging (camera on eyepiece), I would dis-recommend this method and go for prime focus eventually with a barlow in between
telescope and camera. The less glass in the optical path the more optical fidelity you can expect while high magnification is not the non-plus-ultra.
Provided the only mount you are currently owning is the StarAdventurer (and not considering to upgrade), you will definitely need a light-weight setup. The SA is specified with 5kg payload max., BUT the 1kg counter weight is included while in the interest of tracking accuracy you should never load the maximum capacity. Sooner or later you will wish to autoguide adding another say 0.6kg. The SA is everything else but accurate at say 360mm focal length without autoguiding, even then requiring maximum attention to polar alignment. The same should apply to your Sony lens which is supposed to load less weight though.
Some number crunching,
payload = 5kg (this is optimistic maker spec)
"Why should I buy a 400mm APO while my lens is from 100 to 400mm?" is a justified question. As everybody confirmed, with the same camera you get the same field of view. The only difference being, a say 360mm APO with flattener can produce images of higher fidelity and resolution, especially in the edges of the photos. Also, a prime lens with fixed focal length such as an APO is expected to produce finer pictures than a zoom lens. I do not know the model and f-number of your Sony lens at 400mm, but a Ø60mm/FL=360mm APO is F6. The smaller the f-number the less exposure time you will need for about the same result while tracking is more forgiving at shorter exposures.
I can only recommend what I have tried myself, namely this one in combination with a Nikon D5300 DSLR:
It produces fully illuminated, great wide field DSO images, but sitting on a larger and more accurate mount, though still without autoguiding.
On the other hand…
If you judge that the quality and f-number of your Sony lens roughly matches that of an APO (doublet with FPL-53 glass) of similar focal length then I would stay with the lens and consider for later a more stable and accurate mount with a low cost Newtonian, such as the Sky-Watcher BKP130 (Ø130mm/FL=650mm) or BKP150 (Ø150mm/FL=750mm) which sport F5 focal ratio (which is fast) and are suitable also for lunar and planetary imaging with a CMOS camera, such as the one you may later choose for autoguiding (two birds with a single stone). Note, however, that Newtonians, like APOs, require an optional (coma) corrector lens for best results plus frequent adjustment (known as collimation).
Fortunately or unfortunately, the astronomical equipment market offers too many choices in that it is next to impossible to pinpoint exactly what would be just right for you, also considering that everybody has different views and targets for her/his hobby. Most importantly, you do not spend too much money at the beginning and stay away from achromatic refractors and wacky mounts. You can always sell quality equipment to help finance your next, but scrap won't sell.
You could upload a few images taken with your Sony lens and see if anybody in AstroBin can tell whether it would be worth spending money for a small APO.
Step by step, like in a mine field.
All the very best
# 12 Feb, 2020 07:01
I thank you very much for this detailed and helpful answer. The most accurate consideration is that the choice is not easy and very personal. I'm today glad to announce that I have made mine, after all these days, and your answer helped me to make it: I have ordered a TS-Optics APO Triplet 102/520 Imaging Star, a SKW HEQ-5 GoTo and 3 eyepieces with 3.2, 8 and 25mm, completed with a mirror star and a power station. I'm looking forward to getting this material and trying to find my way.
Many thanks to all for your prcious help! As soon as I have my first experiences, I will post my impressions. Bet I will need your help and advice?
# 12 Feb, 2020 10:36
Alongside a new mount this is a great choice for imaging DSOs while eliminating need for a flattener. Congratulations!
Because of the short back focus of 55mm, a standard 90° diagonal will not bring your eyepieces into focus for which reason you may need this part:
You may be aware of this, just for safety :-)
Bin gespannt auf Deine ersten Bilder.
# 12 Feb, 2020 10:41
Thanks for this advice. The very competent and nice people of Astroshop.de have informed me concerning the "Zentitspiegel" and I have ordered the special one for the Imaging Star.
Danke für Deine Hilfe! Nun, bis ich alle Teile erhalten habe, wird es noch eine Weile dauern. Zeit für Lektüre!
Grüessli aus der Schweiz
# 15 Feb, 2020 05:54
I have to be patient… I have changed my order and opted for a HEQ-5 instead of the Meade LX85, and now I have been told that there is a delay til april. Snifff…
I will use the time with reading…
# 15 Feb, 2020 14:10
The delivery narrowpass indicates that your choice was right, Markus.|
Happy reading :-)
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