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Beginner at AP needs some polar alignment advice

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#1 mtenboer

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Posted 06 December 2019 - 01:17 AM

I am a pretty much an astronomy newbie but I am considering giving imaging a try.  I have a Celestron Evolution 8 which I have used visually for a several years.  I am aware it is poorly suited to AP so I will only use it to learn a few initial steps of AP before I buy a new OTA  ( I will buy a good EQ mount and put the SCT on it for initial practice).   I have spent the past 6 months reading many AP books recommended in these forums, watching YouTube videos, and reading hundreds of forum posts to answer my newbie questions, but before I take the plunge and buy equipment, I have to resolve a polar alignment problem I will have everyday .  I live in the Sierra Nevada foothills nearly surrounded by tall trees.  I can see pretty well from the east to southwest but I can not see Polaris or anything to the north.   

 

So here are my questions:

1.  Can I expect to get GOOD imaging results using alternative alignment techniques like plate solving, assuming I buy a decent equipment like:  a CEM60 mount, a triplet refractor,  a decent guide scope, and the software to run all of it ?  

2.  What limitations should I expect if I can't align to Polaris?    Will shorter exposures be necessary?   How short ?  I understand this also depends on the quality of the guiding, but assume I buy a decent guide scope and use PHD2.

 

Your advice is greatly appreciated.

 

 

 


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#2 mtenboer

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Posted 06 December 2019 - 01:21 AM

I forgot to mention - I will initially use a Canon Rebel SL3 since I already own it, then transition to an astro camera later.


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#3 rgsalinger

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Posted 06 December 2019 - 01:36 AM

You can use the PHD 2.6 polar alignment wizard which does not require that you can see Polaris and it's free with PHD. https://www.youtube....IGwbutr2z_yabD8 is where there are tutorials which show how to do it.

 

Rgrds-Ross


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#4 SteveInNZ

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Posted 06 December 2019 - 01:41 AM

There are other methods that don't rely on seeing Polaris and they are just as good as those that do. The most tried and true is "drift alignment" and there are various tools and implementations to assist with polar aligning that way. The main disadvantages are that it takes a bit longer to do and can take a while to get your head around it and learn the process. Once you have, it's not hard at all.

There are also polar alignment functions built into some mounts. I think the CEM60 has "Polar Iterate Align" which gets you to slew to stars and center them with either the hand control or the Alt-Az adjustments to get the mount polar aligned without using Polaris.

Both methods are every bit as good as a polar scope.

Steve.



#5 PirateMike

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Posted 06 December 2019 - 07:20 AM

I live at 18* north, so Polaris is a little useless for Polar Alignment. I use a drift alignment method, and as mentioned above, it takes a little bit of time to learn and it is not an "instant" proceedure but it is the most accurate and works excellent for me. I can take 20 minute subs without issue, maybe even longer but I haven't tried yet.

 

The real trick to excellent PA is to be able to make very tiny adjustment to the mount once the PA is close to perfect.

I would think a thorough person like yourself can handle doing a drift alignment. waytogo.gif

 

NGC-2244, 10mm telescope, 20 minute subs...https://www.astrobin.../full/384810/0/

 

NGC-2244-C2 px910.jpg

 

 

 

Miguel   8-)

 

.


Edited by PirateMike, 06 December 2019 - 07:27 AM.

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#6 kathyastro

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Posted 06 December 2019 - 08:23 AM

So here are my questions:

1.  Can I expect to get GOOD imaging results using alternative alignment techniques like plate solving, assuming I buy a decent equipment like:  a CEM60 mount, a triplet refractor,  a decent guide scope, and the software to run all of it ?  

2.  What limitations should I expect if I can't align to Polaris?    Will shorter exposures be necessary?   How short ?  I understand this also depends on the quality of the guiding, but assume I buy a decent guide scope and use PHD2.

 

Your advice is greatly appreciated.

Plate solving is not an alternative to polar alignment.  It is a good alternative to a goto alignment.  In fact I have never done a goto alignment on my CEM60!  But you have to do a polar alignment first.

 

As others have noted, there are alternatives to using Polaris for polar alignment.  Drift alignment is not difficult to learn and works without software.  Or you can use PHD2's built-in drift alignment routine.


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#7 fewayne

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Posted 06 December 2019 - 12:03 PM

Whatever method you use, if you run a drift-alignment check when you're done, you can be assured of the resulting accuracy. Drift alignment is a direct check on the alignment's accuracy because it actually measures the outcome. You can use one of the quick methods (e.g. iOptron 3-star polar alignment), and check that with a drift test, you've got the gold standard.

 

The good news is that by using a camera and your mount's controller, you can radically decrease the amount of time it takes. Squirrel these away for later -- the link is a textual description of how to quickly do a camera-enabled drift check. The hard part is remembering which knob to turn which way for a given result, and the image is a memory aid for that.

https://www.cloudyni...bert-vice-r2760

 

From The Astrophotography Manual: a practical and scientific approach to deep space imaging, Chris Woodhouse, 2016. Used without permission.

Drift_Mnemonic.jpg


Edited by fewayne, 06 December 2019 - 12:05 PM.

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#8 SonnyE

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Posted 06 December 2019 - 01:30 PM

Polar Alignment, to me, is your basic mechanically sound starting point. It is done to get your equipment aimed to begin with.

I can use Polaris for mine, so I guess I have that advantage.

Something I've done for a number of years now is once I decided on a where spot, I simply left my mount set there and it stayed pretty much polar aligned. I carried my equipment from the clamp up inside, and covered the mount over to keep it high and dry.

Pretty much Polar Aligned. But nothing stays exactly the same. So it required checking as a part of an evenings set up and alignment.

I use a Tasco Red Dot sight as my course aim, then PHD2 with my guide camera as my intermediate aim, and finally my Astro camera as my final aim for aligning. So my first course of action is making sure Polaris is in the crosshairs. That usually requires the minor tweaking of the Alt-Az, and Elevation adjustments to compensate for removing and replacing my telescope from night to night.

 

Point here being, if you can set up your tripod and mount head in a semi-permanent place, Polar alignment pretty much takes care of itself.

 

After checking my Polar Alignment, I simply ran my usual alignment (Celestron 2+4 alignment Stars) and was on my way.

 

Before I did my Spot Setting with my mount, I'd get globular stars. After I stopped moving my mount around I was able to get sharp stars.

I tend to figure my way around stumbling blocks, and come up with what works for me. I offer the suggestion to you.



#9 Alex McConahay

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Posted 06 December 2019 - 09:16 PM

>>>>> https://www.cloudyni...cant-see-north/

 

is a good place to look through

 

Alex



#10 vidrazor

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Posted 06 December 2019 - 11:34 PM

If you have a tracker like my iOptron SkyGuider Pro, the alt-az mount is so inaccurate that trying drift alignment would be an exercise in frustration. It would be impossible to fine adjust. Perhaps with the overpriced William Optics alt-az mount it would be feasible.



#11 hcf

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Posted 07 December 2019 - 12:12 AM

Plate solving is not an alternative to polar alignment.  It is a good alternative to a goto alignment.  In fact I have never done a goto alignment on my CEM60!  But you have to do a polar alignment first.

 

As others have noted, there are alternatives to using Polaris for polar alignment.  Drift alignment is not difficult to learn and works without software.  Or you can use PHD2's built-in drift alignment routine.

I actually do Polar Alignment with platesolving smile.gif

 

Like the OP I do not have any view to the North, but open skies to South and East.

What I do is follow the standard drift alignment method described in

http://astropixels.c...ralignment.html

 

except that instead of using a reticle eyepiece, I use the camera and platesolving. Instead of finding a star near the southern meridian I can point the scope/camera roughly near the meridian, start tracking and take 1-2 sec exposure pictures (low res jpgs) at a minute interval, platesolve them and report the RA/DEC of the center of the image (from the astrometry.net output). Then looking at how the Dec changes over time, I figure out from the above drift alignment rules, if an "imaginary star" at that location would be moving up or down, and adjust Azimuth accordingly. I try to get the changes in Dec to a very small amount (say 4th decimal place in degrees) which is the best I can do for my EQ2/3 mount.

Then point the scope/camera to the east, do the same thing and adjust Altitude of the mount.

 

Pros: Dont need a star to do drift alignment.

Cons: Takes time, and bad for the shuttercount and battery life of the DSLR that I use.

 

While I do this with the imaging camera, it could be done with a guide scope/camera too, if you can platesolve those images.

 

All of this is possible because I use command lines for solve-field from astrometry.net and gphoto2 for taking pics on a linux box (raspberry pi) and I can write simple shell scripts to loop over these two actions and report the Dec changes by grepping in the astrometry.net output of solve-field.


Edited by hcf, 07 December 2019 - 12:42 AM.

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#12 mtenboer

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Posted 07 December 2019 - 01:38 AM

Thanks everyone - this is immensely helpful.  Drift alignment looks like the preferred method to do after some sort of rough alignment first.  I read the links you provided  and watched the YouTube.  They are pretty clear, even to a beginner. 

 

Please comment if you agree on the following alignment steps:

 

1.  Set my latitude and use a compass to aim at North.

2.  Do one of the iOptron alignment methods ( 3-star or Iterative )

3   Do a drift alignment.

 

Do I need to do more than one drift alignment in different areas of the sky?



#13 the Elf

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Posted 07 December 2019 - 03:20 AM

Here is a video by Forest Tanaka. https://youtu.be/zQB6UnrTEEM?t=2039

As you can see you do not need any special software at all. He is using BYEOS but you could do the same thing on the camera screen using an intervalometer for the long exposure.

 

And no, no need for many alginment. The meaning of polar alignment is to put your RA axis exactly parallel to the earth's axis. This does not change, no matter which part of the sky you point your scope to. It does change if you bump into your tripod or if the ground is soft and the legs sink in. A good time save is to jam 3 metal rods into the lawn, low enough not to ruin your mower, and put the tripod legs into the ends. If you can carry your tripod with the mount on you just place it there and go.

Important point: if you are not guiding the drift caused by polar error is your main concern. Polar alignment needs to be very precise. As soon as you use an auto guider the guide star is nailed at it's position. Now any polar error causes a rotation of the field that is orders of magnitude lower than the drift. In this case an error of 10 arcminutes (minutes!!) is fine for up to 15 minutes exposures.


Edited by the Elf, 07 December 2019 - 03:27 AM.

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#14 mtenboer

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Posted 07 December 2019 - 11:29 AM

Thanks - that's a great video. It's good to hear that one drift alignment is all that is needed.  Good idea using the metal rods. 



#15 SonnyE

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Posted 07 December 2019 - 11:59 AM

While waiting for my equipment to arrive, and waiting for it to come back from repairs, I watched all of Forrest Tenaka's video's.

Great points of reference.

I had forgotten how helpful he has been to me.



#16 hcf

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Posted 07 December 2019 - 12:17 PM

 

Please comment if you agree on the following alignment steps:

 

1.  Set my latitude and use a compass to aim at North.

2.  Do one of the iOptron alignment methods ( 3-star or Iterative )

3   Do a drift alignment.

 

Do I need to do more than one drift alignment in different areas of the sky?

It should be

1.  Set my latitude and use a compass to aim at North.  (Polar Alignment)
2.  Do a drift alignment. (Polar Alignment)

3.  Do one of the iOptron alignment methods ( 3-star or Iterative )  (GoTo Alignment)

 

The first two do Polar Alignment (moves the mount), and the third does GoTo alignment (moves the scope, but does not move the mount). Polar alignment needs to be done before GoTo alignment.

 

Drift alignment needs two corrections one for the Azimuth when you point towards the Meridian, and the other for Altitude when you point East or West. You might need to do these couple of times, to make sure correcting Altitude does not break the Azimuth alignment, but the adjustments after the first time are likely to be very small.


Edited by hcf, 07 December 2019 - 12:18 PM.


#17 Stelios

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Posted 07 December 2019 - 01:53 PM

1.  Can I expect to get GOOD imaging results using alternative alignment techniques like plate solving, assuming I buy a decent equipment like:  a CEM60 mount, a triplet refractor,  a decent guide scope, and the software to run all of it ?  

Good imaging results require more than just decent acquisition equipment. They require proper use of that equipment--and they also require a lot of time and effort studying and applying processing techniques. 

 

It's very easy to read a lot about this hobby and think you know something. It's like reading a couple of books about bowling, watching some pro videos, ordering a bowling ball, and thinking you'll go out and shoot a 300 game. 

 

You may never shoot a 300 game, and your first shot will 99.99% be a gutterball. 

 

A little feeling of apprehension about the difficulty of our hobby (AP, not bowling :)) is a good thing :) It sets you up for the *significant* investment in time (and $) required, and the many sleepless nights resulting in little more than pictures somehow smudged for reasons unknown to man or beast :(



#18 SteveInNZ

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Posted 07 December 2019 - 03:38 PM

3.  Do one of the iOptron alignment methods ( 3-star or Iterative )  (GoTo Alignment)
 

These (3-star and Iterative) are different things to achieve a different purpose.

Iterative is a tool using the goto capability of the mount to get polar aligned, while 3-star is setting up the goto alignment.

In this context, Iterative alignment is an alternative to drift, DARV, polarscope, etc.

 

Steve.


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#19 doolsduck

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Posted 07 December 2019 - 04:10 PM

I have a 20 minute video to explain drift alignment with the aid of a model here : https://www.youtube....gUdEF_zI&t=272s and two memory jogging videos for just the adjustments to make in the NH and SH here https://www.youtube....6n7n64b5uk&t=2s and here https://www.youtube....asFk7NEg&t=2s.  I think they are boringly narrated but the easiest for a beginner to understand.



#20 mtenboer

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Posted 08 December 2019 - 10:42 PM

Thanks everyone. 

I'm aware I have a large learning curve ahead despite my up-front efforts to research this.  I'm a CPA so my tendency is to analyze things into paralysis.  But several years ago I jumped into astronomy on a whim and it was a mistake.

 

I have been fascinated with astronomy since childhood, but I assumed that the city lights where I lived would ruin the view from any consumer telescope.  So I waited to buy one until I retired and moved to the mountains with clear skies.    With great expectations and without doing any research I bought a Celestron Evolution 8 on sale.  I thought that such a large 8" telescope with the goto controls would certainly show me clear views of the Universe like I had seen in pictures over the years.   And I would see them in real time!   I was totally disappointed in the results  - except for seeing Saturn and Jupiter with their moons.  I put it away for a long time and only brought it out to show visiting friends and family views of the planets.  I was very close to selling it and moving on, but I just I didn't want to throw in the towel without finding out why this went wrong.   I found Cloudy Nights and quickly learned about the differences between visual and AP astronomy.  I saw a posting here for a book by Alan Hall Getting Started Budget Astrophotography.   After reading it I understood why my telescope could not show me the views I had hoped for, and that AP could, but it requires a rigorous, time consuming process, with potentially expensive equipment, that has a significant component of trial and error.  This is not unlike my main hobby - woodworking - which I have been learning for over 40 years.   It also takes many years to learn, is very time consuming to perform, and requires expensive equipment.   SOOO, AP looks right down my alley.    

 

I will be back with many more questions. Thanks for your patience.


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#21 the Elf

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Posted 08 December 2019 - 11:50 PM

All you write is true. Let me add two more points:

a) at any given time you can say decent amateur equipment can now do what was reserved to professionals 10 years ago. This is due to the fast development of electronics and sensors and the improvment in numerical simulations when calculating optics. It is also because of the rapid growth of consumer goods production in China. Some good parts are available at low prices now that have been handmade for the pros a decade ago. Restarting after a few years puts you into a new world.

b) salesmen continue seeing you as a source of money and don't care about what you want. A lot of the mass produced goods from china - especially in the entry level segment - is advertised in a manner that lets your expectations go up high while the equipment is not capable of doing it. The limitations are never printed on the box. So the first obstacle is to understand each products limit and find the one that acually fits your needs.

 

Although I'm working in image processing as an engineer and although I have some experience in photogrphy (35 years now) I failed understanding b) and sold all my initial equippment within the first two years. Every single part was updated in order to reach decency level. If you continue reading posts at CN you will find the mount, the mount and the mount again is the key and that is true. Short focal lenths are forgiving. Take your time and don't be afraid of asking others to select your bits and pieces according to your needs, expectations and abilities. Your list (CEM60 mount, a triplet refractor,  a decent guide scope) looks like a good choice to me though it needs refinement. The mount gets good ratings here. I cannot comment on it, I'm using the Skywatcher EQ6-R and I'm happy with it. I also use a triplet refractor and that's a good choice. Well, mine is a quadruplet actually, the field flattener is already inside. This is what you need to select: either a scope that comes with a flat field, a so called astrograph or qudruplet refractor or you get a standard one that comes with a curved image. For AP you need to add a field flattener or a reducing flattener. Reducing makes sense when your camera chip is smaller than full format. Be reducing the image size it gets brighter. So the task is to find a pair that results in the focal lenght you need and that fits your camera.

Most folks use a computer. It has the potentail to do a lot for you: take the image, do platesolving, automatic focus, guiding, change filters, etc. Often beginners start and a minority continues to image without computer which makes the hobby more like photogrphy or handcrafted work. In this case you pick a stand alone auto guider (e.g. lacerta MGEN-II) and a DSLR. I do because I hate computer trouble. Last but not least the question is if you go for a DSLR a OSC (one shot color) astro camera or a mono camera and a filter wheel. Not going into detail here, you will find a lot of information about it here and in the web. Guiding is essential. I don't recommend to start without guiding. If you want to take a look at my website (see signature) that is what you get for an investment of about 7k$. You can easily spend 20+ if you like...

 

best

the Elf


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#22 mtenboer

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Posted 10 December 2019 - 02:21 AM

Elf - your images are terrific.  Thanks for the info on triplets vs quads.   I will watch your YouTubes soon, as well as many more before I buy.  I am not in any hurry since winter is here and I don't want to struggle in the cold with the early learning curve.  Overall my goal is to buy equipment that will carry me a long time, not be upgaded in a short time.  

 

I have read that each piece I buy will have trade-offs and will benefit or restrict the objects I can target.  But I have not yet found sources to guide me as to what optical equipment is minimally necessary, preferred, or is perfect for photographing specific objects.  I have Alan Halls book on the Messier objects which is terrific for using an APS-C camera and gives time of the year, number of exposures, length of time, ISO and many additional comments to get good results.  It shows the size of the object using three focal lengths.   But  other than that book I haven't found reference material for other DSO's, or for using CCD cameras, OSC vs mono+ filters, or other criteria.  Are most all DSO's capable of being photographed by a single setup, but more expensive equipment would provide clearer detail, sharper stars, more color (ie filters), etc ?   I know that many of you have multiple telescopes - how did you decide?  I guess the question is how do I learn about the trade-offs and apply them to my equipment decisions ?



#23 the Elf

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Posted 11 December 2019 - 12:10 PM

Thank you!

Choosing equipment starts with the question what you want to image. If it is planets you need a very long focal length, a decent barlow lens, a camera with a small chip that can capture movies and no cooling is needed. I can't help in this case. If you want to image Deep Sky Objects (DSO) you will probably evolve from easy to more challenging. Easy means the object is large and bright. Examples are the Great Orion Nebula and the Rosette Nebula and the Pleiades. The smaller and the fainter the target the more difficult and the more demanding for the equipment. The most difficult one that comes to my mind is the Cat Eye Nebula. There are some large and relatively faint objects the require special filtering: narrow band. There is no one fits all optics so you will have to start with one and add more later. Re focal length overview, I'm talking about a APS-C sized sensor: for a shot of the whole milky way you would use a 12 or 16mm camera lens, a prime. For large fields like Heart and Soul in one frame or the Spaghetti Nebula or the Cosmic question mark 135 or 200mm lenses. Then comes a transition point where a lens is as expensive as a real telescope, this is roughly from 300 or 400mm up. The above examples (Orion, Rosette, Pleiades) require 300-400mm, the Andromeda Galaxy as well. If you go for the smaller objects, namely the galaxies outside of our local group you need a far longer focal length like 1000+ mm. This needs a far more expensive mount and is in general far more difficult to handle than the smaller ones. So you have to decide if you want the large objects and have technically good images with a moderately priced rig (3 to 4k$). If you want all the galaxies you should add 1k$ to the mount.

The next thing to decide is OSC (one shot color) or mono and filters. OSC can be a DSLR that you perhaps own right now or an astro camera. Mono is more effort, you need a filter wheel, a set of filters and need to dealt with 3-7 sets of date to compose one image. See my video "which camera is best for me". A popular cooled mono with filters is in the 2k$ range. Cooling is not always needed. Noise comes from a warm sensor and noise also comes from a bright sky background. So cooling makes sense under very dark skies or if you image narrowband. Under a city sky with an OSC you might not see much of a difference. For details find 2 videos by Robin Glover, linked in the Media section of my website. OSC and a relatively short focal length below 500mm is the easy route with early success but a limited selection of objects. If you want more of a challenge go for a longer focal length and mono and filters. Only you can tell if you tend to get along well with tasks that consist of many steps and if you can handle tricky situations. Many start simple and by the time sell things and get new stuff to grow. With the mono you have an option for narrowband. The bright stars and the sky background is dimmed down while the nebulae stand out. This works well under light polluted skies. It requires long exposure times and thus a good mount a a long over all exposure time. One narrowband image can easily take you several nights.

Telescopes: a newton gives you most aperture for the money. It is as long as it's focal length, it needs to be collimated (easy) and it need a coma corrector. It must be designed for imaging and even then not all of them work with a DSLR because the sensor is too far away. Refractors do not need collimation, they are also as long as the focal length and they need a flattener. You can attach any camera without problem. If you want a fast refractor with a long focal length it will be expensive. That is where a folded design like a Schmidt-Cassegrain, a Richey-Chretien, a classic Cassegrain, a Dall-Kirkham and so on will be cheaper with the same parameter or offer the desired fastness at all. They all need collimation and this can be difficult. They are short, a bit more than 1/3 of the focal length and some of them come with a flat field, so you need no more correctors. The Celestron Edge is one example, RCs can be used without flattener when cropping a bit of the image. Less common designs like maksutov-newtonian also come with a flat field. If you have damp conditions all scopes with a glass element at the front (corrector plate in case of Schmidt-Cassegrain and Mak-Newt, lenses for refractors) can fog up and need dew heaters. Scopes without a front Element like RC or Newton do not fog up. The mirrors in the tube are protected from the wet air. The idea of using the same telescope for AP and visual is not a good idea. I got myself a pair of binoculars to watch the sky while the scope is taking the image. Some have a cheap large dobson for "family fun". All telescopes have some sort of image errors. For more money you get better corrections. The most important one for refractors is chromatic aberration (CA). In general a triplet lens can correct this better than a doublet, though there are some excellent doublets on the market as well. Mirror scopes don't have CAs. They only occur if the light goes through glass, not when it is reflected on a silver mirror.

Answering your question: my first rig was an RC6 on an AVX and a DSLR. The AVX was the biggest mistake. The RC6 is regarded as a monster by many but mine did not cause much trouble. I invested a few hours in the first collimatiotion and then it was fine. Reasons for choosing it was large aperture for the price, short an compact, light weight and camera at the rear end plus no fogging trouble and no CA. Many here will recommend not to start with an RC and tell you horror stories about collimation. I decided to sell it in order to update to it's larger brother, the RC8 carbon. A carbon tube reacts less to temperature changes. You don't need to refocus that often. There are carbon refractors as well. I decided to get the SW EQ6-R as an intermediate step because right now I don't want to invest in a premium mount but going on with the AVX was an absolute NoGo. I also started with a book of Messier objects. Alas many of them are too large for the RCs and I got the small refractor. I decided to get the quad to save me from trouble with flattners and I decided to get a small one, 65mm aperture only because the faster it is, the more accurate you need to focus. Of course I wanted a triplet front element to avoid CA. My decision was not to use a computer outside because I hate computers and the trouble they cause. That in turn made me invest quite a large sum of money in a mono conversion of a DSLR and a stand alone auto guider. This is really a minority path. I am a multiple minority member anyway, so why not continue here? My biggest mistake was the AVX and my biggest boost was the mono camera.

Hope this helps!


Edited by the Elf, 11 December 2019 - 12:20 PM.


#24 mtenboer

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Posted 12 December 2019 - 01:56 PM

That's a huge help !!  Thanks for taking the time to provide such a thorough explanation.  This raises many more questions, but I've moved off topic - polar alignment - so I  I will start a new post on equipment selection in a few days after I have thought about your suggestions.  I will check out the Robin Glover videos.  Thanks again.



#25 the Elf

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Posted 13 December 2019 - 02:37 AM

Have you got this book?

https://www.amazon.c...,aps,328&sr=8-1

It covers everything you need. This one is a less know alternative, imho as good as the above:

https://www.amazon.c...,aps,236&sr=8-1

If you know that you are going to use a DSLR this one is more to the point. Unlike the other two it offers a guideline how to step into this hobby:

https://www.amazon.c...,aps,236&sr=8-2

 

Either of the three will probably answer most of your questions.




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