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Thread: Astrophotography with Azimuth Mounts

  1. #31
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    Default Re: Astrophotography with Azimuth Mounts



    Hi Jigar,

    Those are some very nice photos you took with your 5SE. Have fun as you progress.

    Some things to keep in mind. Telescopes like the 4/5SE are not really suited for astrophotography. However, if a person already has one and wants to get started in astrophotography, you can do a lot of very nice work with it. I love using my 4SE mount for astrophotography because of its very light weight and its portability. I am willing to pay the price in decreased capabilities and increased issues incomparison to a traditional astrophotography setup. I am very happy with the 4SE mount that I purchased specifically for astrophotography ....

    However time marches on and technology advances. Today, if I were buying a lightweight and highly portable telescope for astrophotography, I would buy a Skywatcher-Startravel-80 short tube refractor (aka an Orion ST-80A) and an iOptron SmartEQ Pro mount (~$700 US total).

    Here are some things you may want to consider before you spend any money making your 5SE ready to photograph with a DSLR. As explained in the azimuth mount astrophotography sticky you read; with the 5SE mount you will be limited to very short exposures .... 30 seconds or less. This is not much time and your exposures will be dreadfully underexposed. This is especially true with the 5SE which is a Schmidt Cassegrain Telescope with a focal ratio of f/10. To do any decent work, you will need to purchase a 0.65 focal reducer for your 5SE which will cost between $100 and $150 here in the USA or ~100 to 150 euros if you are buying from Europe.

    For about the same price as a focal reducer you can purchase the SkyWatcher StarTravel 80 mm refractor; see http://www.teleskop-express.de/shop/...r-80-400mm.htm (equivalent in the USA is the Orion ST-80 A at $180). This telescope is known as a short tube refractor because of its short focal length. It has a focal ratio of f/5 that makes it ideal for very short exposure photography. The scope attaches directly to the 5SE mount, just like your 5SE OTA does. It has a built-in T adapter so all you need is a t ring for your camera to attach a DSLR to the scope. The downside is chromatic aberration. My research shows that chromatic aberration will not be a big factor but you never really know until you try out things yourself (a lot of miss information is on the internet).

    I have purchased the Orion ST-80A and will be evaluating its performance in February... weather permitting. Before spending any money on focal reducers, you may want to wait until I see how well the ST-80A performs as a camera telescope. Optically, I suspect it will be very good but not as good as a Schmidt Cassegrain Telescope (also known as a SCT) like the 5SE or the 4 inch or 6 inch SCTs that I use.

    If I suspect that the ST-80A's performance optically is not as good as the SCTs that I already have, you may wonder why I have spent money to buy one for astrophotography?

    The 4SE and 5SE use the exact same mount and tripod. It is rated for a 10 pound (4.54 kg) load. This is no real problem when using the mount in the azimuth mode with its 20 to 30 second exposure limitation.

    When you use the wedge on the 5SE to put the mount in the equatorial mode you can obtain exposures ranging from 45 to 90 seconds in duration. However, weight becomes a big issue. While in the Azimuth Mode of operation, mount payload weight is not critical and exceeding a mount's rated payload by 10 to 20% has little impact. This is not true for the equatorial mode where payload weight is very critical and even more so if you plan to do astrophotography. Add to this the fact that you can not balance the OTA on the SE mount in the equatorial mode and .....

    For an equatorial mount that is used for astrophotography, the mount's effective payload is cut by 40 to 50%. This means that the 4/5SE mount in the equatorial mode when used for astrophotography has an effective payload of around 5 to 6 pounds (2.3 to 2.7 kg). Your 5SE with a focal reducer and DSLR will probably weigh in at around 7 pounds (3.2kg). How well the 4/5SE mount will handle this load in the equatorial mode using the built-in wedge, I really don't know but I suspect that your exposure times will be between 30 and 45 seconds.

    The ST-80A setup with a DSLR and finder scope weighs about 2.3 kg (5 pounds) which is about 0.45 kg (1 pound) less than my 4 inch SCT setup While this weight difference may not sound like much, it is a significant percentage of the 4/5 SE mount's payload. Hopefully, the lighter weight ST-80A will allow me to extend the average length of time I can make exposures and reduce the number of exposures that are unsuitable due to mount tracking movements.

    Whether or not this will be the case, I don't know at the moment. One thing for sure, the lower focal ratio of the ST-80A (f/5 vs f/6.3 SCT with focal reducer) will provide a better signal to noise ratio for my exposures. The expected longer exposure times and lower focal ratio, I hope will provide a final result that makes-up for the chromatic aberration and field curveature issues introduced by the short tube refractor. As I said earlier, I really will not know the answer to this question until late February or early March.

    One other aspect to consider. An image using 30 second exposures taken with an equatorial mount is superior to an image using 30 second exposures taken with an azimuth mount (all other factors remaining the same). Why?

    1. An equatorial mount will correct for field rotation while an azimuth mount does not. This means that much less information will be lost in the stacking process.

    2. Since exposures in the equatorial mode do not have to be rotated by stacking programs as do exposures taken in the azimuth mode, the final alignment tends to be more precise thus providing a alightly sharper image.

    3. A polar aligned equatorial mount requires fewer tracking corrections than an azimuth mount requires to keep an object stationary in the field of view which results in fewer rejected exposures.

    4. In the azimuth mode field rotation is present...even though the very short exposure (20 to 30 seconds) makes it not noticable to the eye, it remains and actually all objects in the image are slightly blured.... decreasing image sharpness.

    To use the 4/5SE mount in the equatoral mode using the built in wedge, you will need to modify the tripod head. The process is not difficult and is 100% reversable. Here is a link showing you what is needed to be done.

    4SE Polar Alignment and Modified Latitude Adjustment Bar
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  3. #32
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    Default Re: Astrophotography with Azimuth Mounts

    Hi Joe,
    First of all, thank you for the compliments. I have just taken one picture of the crescent moon yet.

    However, before buying the telescope, I did a one week research on the equipment. Capabilities and limitations of each type;
    Refractors : Nice and simple, Good clear pictures in AO, very expensive as the aperture increases
    Reflectors : Short tubes, long focal lengths, just mirrors, not ideal for AP, cheaper compared to refractors
    MAKs : Somewhat similar to CSTs but not as good, not ideal for AP, got negative reviews from users
    CSTs : Mid range economic, cheaper and smaller than same aperture refractors, AP possible but some loss of light and compromise in contrast due to the mirror in the centre.

    Being a film maker, I always look for quality in my images. Contrast and Focus being very crucial elements in it. I saw that 4SE was a MAK, so I didn’t opt for it. 6SE was a bit larger but the wedge thing in the mount was not available ( or so I found ). I didn’t want to invest a lot in the mount yet on this stage of my learning curve. After this, I did a model/company research and found 5SE to be suitable to my budget and encouraging for a budding AP enthusiast. Although I didn’t know anything about AP, due to my research I knew that EQ mount is a must for long exposures. So I was getting an SCT, 1250mm, 125mm, f10, a wedge for a low cost EQ mount, and a computerized setup. So that was my obvious entry choice in the field of AP. I may be wrong here but it seemed right to me.
    I have some experience in long exposure photography ( not astronomical ). So I know the problems and the solutions involved as far as the camera is concerned. But telescope is a totally new instrument for me to learn and understand.
    I also checked the link where you have shared the images of custom made adjustable wedge for your 4SE. It is great, I like it very much. Really handy and neat. It will be useful design when I decide to make and use the wedge for AP in EQ mode. I already knew that the weight will pose a problem because I have experience using different cameras on different tripods in different situations. But as I didn’t have the instrument in hand, I left the issue for time being. But now that you have mentioned it in detail, when I go for the EQ setup, it will certainly help me.
    At present I am clicking images with a smartphone ( DSLR will be difficult without a T adaptor setup) through the EP (Afocal photography?). After I get a hang of it and understand other factors involved, and learn to use my instrument efficiently, the obvious next step is to get a piggy back and a T adaptor set up.
    Yes, I have read in the forums that a focal reducer (.63?) is needed for the T adaptor setup to compensate the slow focal ratio. But that stage is a little far for me at this point of time.
    The budget part is also a little complicated. I live in India and the equipment I get here is comparatively more expensive than in US or in Europe. So I have to balance my options wisely. Of course I will wait for your result with the ST-80A and will balance out my options.
    At present, my next target is to invest in a set of EPs one by one along with a decent enough barlow( I heard there are ones which give you 1.5 + 2x mags) to extend the use of my tube.
    I really appreciate your taking time and sharing your valuable experience with a novice like me. I am sure with friends like you around, helping me out so enthusiastically, if I have talent ( and money ) enough, I will make good progress in AP in relatively shorter period of time.

    Thank you my friend. I really mean it.
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  4. #33
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    Default Re: Astrophotography with Azimuth Mounts

    I have a question. The article says that you can take 200-400 short exposures, but wouldn't your individual images suffer from field rotation since the camera isn't equatorial? What I mean is, wouldn't the surrounding stars or even the DSO itself, begins to shift relative position to each other since the camera isn't moving with the field rotation and wouldn't that become an issue when you go to stack the images? (unless I suppose DSS has the ability to shift the images slightly to allow them to realign properly)

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  5. #34
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    Default Re: Astrophotography with Azimuth Mounts

    DCBrown,

    Field rotation is a problem with azimuth mount photography that will not go away. Each exposure is slightly rotated from the previous exposure. DSS rotates and aligns each image as it stacks. The final result resembles a deck of cards that have been twisted. This fanning effect causes data to be lost as only the common portion of all the stacked exposures is where the maximum noise reduction takes place. This is not a problem if the object is relatively small and centered in the exposures as the common area of the image can be cropped and the rest discarded. However, if the object is large and covers most of the frame or is not near the center, then a lot of data is lost. A properly aligned equatorial mount does not have this issue.


    Another issue with azimuth mount photography caused by field rotation is image sharpness. With an azimuth mount field rotation is countered by using very short exposures. The exposure time is short enough where the movement caused by field rotation is not visible to the eye unless the image is greatly magnified. In effect, each image is slightly blurred which has a negative impact upon image sharpness. Here again, a properly aligned equatorial mount, all other things being equal, will not have this attribute.

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    Default Re: Astrophotography with Azimuth Mounts

    Joe is spot on about DSS processing. Also, keep in mind that any object that fills the entire image is going to suffer from a certain amount of star trailing toward the outer edges. If you are able to capture an image as it is rising out of the East, field rotation is greatly reduced. I recently did this with the Horsehead and encountered very little outer edge rotation.

    Terry

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    Default Re: Astrophotography with Azimuth Mounts

    Hi terry, You obviously piggyback on your scope and must be happy with your setup.If that image is your usual fare it's no wonder you are happy with alt-az photography. Thats pretty damn good. As someone who is just starting ap and wishes to explore the minimalist boundaries prior to getting a eq mount, may I ask a question. Silly as it may seem,would it be beneficial to take a few images on consecutive nights at the same time, rather than all at once over a couple hours. Just a thought. Cheers.
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    Default Re: Astrophotography with Azimuth Mounts

    Johnny, thanks, and yes, I do piggyback my 80mm Refractor on my LX90. As for different nights, as long as you are using the exact setup I guess there's no reason why you couldn't capture on different nights. There are those that image in polar mode and therefore capture long exposure images. They then go back at a later time and capture using different filters. I prefer to do an image all at once since my exposures are short. My Horsehead was a total of 77 images at 30 seconds ea. I probably shot around 30 lums, and 15 each of R, G, and B. Doing so only takes about 45 -60 minutes (accounting for time to switch filters, recenter as needed due to slight drifting, and getting another beer). DSS does an excellent job of stacking and aligning.

    Terry

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  11. #38
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    Default Re: Astrophotography with Azimuth Mounts

    John,

    If you have not yet done so, visit Terry's web site (see his signature above). It shows what someone with good equipment and processing skills can achieve using an azimuth mount.

    My work, Flickr: Sxinias' Photostream was done using less sturdy mounts and tripods and much slower optics. I also do not have the photoprocessing skills that Terry has.

    I have tried doing what you suggested about imaging over several nights but with little success. At my house once an object passes the Meridian, it enters into the light glow from Athens and that is pretty much that for trying to photograph a faint object. I have attempted getting images over more than one night with the object occupying the same spot in the night sky. Strangely enough, the first object I attempted doing this was the Horsehead Nebula. I could see no improvement over exposures taken on one night (both efforts were extremely noisy). I tried this again with a few other objects but weather and clouds were a problerm so I dropped the idea.

    As Terry said, if an object is in the East or West and especially if it is low in the sky, you can make exposures of several minutes without field rotation being a problem. I have never been able to obain an exposure, even for objects low in the eastern sky, that was longer than about 30 seconds as my mount tracking and gearing mess up longer exposures.

    One thing I've noticed is that azimuth mounts, at least mine, are not as sensitive to weight as are equatorial mounts. I often run my SkyWatcher AZ goto mount at 50% overload with no difficulties. You may want to try piggybacking your camera on the 127 MAK that you have.

    I recently purchased a f/5 Orion ST-80A short tube refractor to see how it works as an inexpensive, relatively fast, telescope for astrophotography.The first objects I plan to image with the short tube refractor are the Horsehead Nebula and the Rosetta Nebula when I return home next month. Both objects have haunted me for a couple of years now. The main issue with the ST-80A will be chromatic aberration. Your 102mm refractor may do very well on your 127MAK's mount as a camera scope.
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    Default Re: Astrophotography with Azimuth Mounts

    Thanks, gives me hope a bit of encouragement for astrophotography with me nexstar 5se

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    Default Re: Astrophotography with Azimuth Mounts

    Great thread! Thanks for summarizing all this up!

    Just one question: What do you mean by "After that comes the darks which for 200 exposures can easily add another 1000 seconds of time" ?
    (I'm new to photography )

 

 
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