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Frequently Asked Questions (FAQ)

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#1 Madratter

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Posted 03 October 2014 - 11:51 AM

The same questions get answered here over and over again. I have put together the start of a FAQ on my website.

 

http://astroimages.w...-questions.html

 

Certainly not everyone is going to agree with all my answers. Clearly, these are my opinions. Here is the current version:

 

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1. What mount should I get?

How much money have you got to spend? In general, the longer the focal length of your imaging system, the better the mount that is required. Unfortunately marketing hype is often very misleading. My recommendation is that for any imaging system with a focal length over about 400mm, at the very least a mount should have roller bearings in BOTH the RA and Declination axis. It is possible to use mounts without a roller bearing at longer focal lengths, but I think you will find tracking very frustrating. In particular a number of popular mounts (the CG-5 and the AVX) do not have roller bearings in the declination axis.

 

This advice gets ignored on a frequent basis. It is your money. I would rather spend a little more once and get something that won't end up getting replaced.

 

(To be clear I consider the CG-5 and AVX mounts excellent visual mounts for up to 8" SCTs).

 

My recommendation is to get at least a Orion Sirius or Orion Atlas. The Sirius is the lighter and is good for up to around 6" RCs. The Orion Atlas is considerably heavier, and what I would consider barely portable. I use it with an 8" RC working at around 1190mm.

 

Some people also recommend the IOptron ZEQ25 as a lower cost alternative to the Sirius. I have heard mixed things about it. On the plus side it is certainly very portable.

 

One reason I really like and recommend the Orion Atlas and Sirius mounts is that EQASCOM (often called EQMOD) is available free for it. This allows ASCOM control of the mounts and is very well done. It gives you easy access to things like multiple park positions, position limits, tracking rates, game pad control, creating a pointing model, etc. Another is that they use stepper motors, which are quieter than servo motors and less likely to burn out if you stall one. One caveat is that Orion warranties only cover the original owner so resale value can be effected.

 

2. DSLR vs CCD built for Astro Imaging?

 

In my view if you have the money, the CCD wins hands down on EVERY count except cost. That said, reasonable results can be had from DSLRs, especially when it isn't quite as warm outside. There are people who produce truly excellent pictures using DSLRs.

 

One of the big advantages of the  typical CCD is setpoint cooling. The ability to cool the camera greatly reduces the amount of dark current. This is noise that builds up over time. The dark current is halved with every roughly 5 to 6 degrees Celsius in temperature. Since the average CCD can usually cool to around 35 degrees Celsius colder than ambient this makes a significant difference. The advantage of setpoint cooling is that the sensor is maintained at a near constant temperature. This greatly facilitates matching lights to darks.

 

3. Mono CCD vs OSC ?

 

The main advantages of the OSC camera are cost, form factor, and ease of use. Cost is significantly less than getting a mono CCD, especially once you factor in the cost of the filters and filterwheel. The OSC cameras can often be found in friendlier form factors (such as being round) making them more suited for use on Hyperstar configurations. They may weigh considerably less because of the lack of filters and filter wheel as well. As for ease of use, you don't need to worry about switching filters, and you don't need to worry about combining the channels of separate data.

 

Advantages of the Mono CCD over OSC are sensitivity, more friendly to narrowband imaging, and resolution. Because of differences in the filters that are used the Mono CCD will typically bring in a significantly larger share of the desired photons. This is the so-called Quantum Efficiency (QE) that gets talked about. OSC cameras almost always have a Bayer Matrix. Typically there is a filter pattern that overlays the pixels. Normally there is one red, two green, and one blue filtered pixel per 4 pixels. The result is that if you are shooting HA data, which is a deep red, only 1 of 4 pixels will capture any light at all. The other three pixels are essentially noise. That same thing effects the resolution and cuts it, in that case in about half. OIII data isn't quite as bad since it is blue green in color and so might be recorded by 3 of the 4 pixels. However, HA data is probably the most commonly used narrow band filter.

 

One advantage sometimes cited for OSC cameras is that the amount of exposure time needed to create an image of a given quality is higher for the Mono CCD. This is a myth that I have addressed elsewhere.

 

 4. Reflector vs Refractor?

 

In general reflectors are much cheaper for the aperture. 6" is the cutoff for many people where beyond that a refractor is just too expensive to be affordable. But refractors have many advantages including easy collimation, and the lack of diffraction spikes (although some people like them). For people interested in widefield imaging, the choice is often a smaller triple apochromat refractor like an 80mm f//6. Shooting galaxies, except a few really big ones, requires more aperture and focal length and you will see people gravitating towards 8" and larger reflectors.

 

5. What is the difference between an Achromat, ED, and Triple Apochromat Refractor?

 

The difference is in the glass used. A refractor relies on refraction to focus light. Unfortunately, the way it does this is wavelength specific so it is difficult to get all the visible wavelengths to focus at the same time. An achromat typically uses two glass elements to focus the light. When used for imaging for broad band imaging, the stars will be bloated with fringes of blueish purple.

 

ED Refractors usually still use 2 elements. One of the elements is made of Extra low Dispersion glass (hence the ED). This does a better job of getting all wavelengths focused at the same time. Still some fringing may be noticeable or you may find spot sizes are larger for some color channels than others.

Triple Apochromats also use ED glass but they have three elements and they are usually very well corrected with almost no fringing noticeable even when imaging. However, the exact type of ED glass still makes a difference. Currently the best ones tend to be made with FPL53 glass.

 

Note that if you are using the refractor for narrow band imaging, even an achromat may do quite well. Still most people will want to do at least some RGB imaging and then the triple apochromat shines.

 

6. What is a modified DSLR?

 

DSLRs normally come with an IR cut filter. Unfortunately, this usually strongly attenuates the Hydrogen Alpha (Ha) light that is dominant in many emission nebulae. And those nebulae often are great widefield targets ideal for beginners and experienced imagers. A modified DSLR removes this IR cut filter. A non-modified DSLR will typically only bring in 20 to 30% of the HA light that is available.

 

7. What is a sub?

 

A sub is short for Sub Exposure. Typically imagers using DSLRs or CCDs take many sub exposures and then stack them together to create an exposure that has a longer effective exposure time (often called integration time).

 

8. How long should I expose my subs?

 

This is actually a fairly complicated subject. If you expose your subs for too short a time, then there will be lots of unnecessary noise. If you expose for too long, then you lose dynamic range (stars will tend to get over exposed). If you take a look at a histogram of an exposure, it will typically have a large bump. At very least you want that bump well separated from the left. DSLR users often try to get it out about 1/4 to 1/3 of the way from the left. Make sure that ALL colors are separated.

 

Some programs such as Sequence Generator Pro have built in exposure calculators and they will suggest ideal exposure times based on test exposures.

 

9. What Is Guiding?

 

Unfortunately, until you get into very expensive mounts, they do not track the stars exactly enough to result in round stars in images unless taking short exposures (how long depends on focal length). This inaccuracy is due both to polar alignment errors which cause drift that effects both Declination and RA, and gearing errors, which results in something called periodic error that effects RA. As a result, most imagers end up guiding. The basic idea is that if you have another camera that is taking short exposures, it can detect this movement and send corrections to the mount compensating for the polar alignment errors and periodic error.

 

There are two common methods of doing this. One uses a separate guide scope pointed in approximately the same location as the main scope (the more accurate, the better). The guide camera then takes images through this guide scope. A very common choice for a guide scope is a 50mm finder/guider. They can be had for under 100$. The main advantages of this method is cost and typically a large choice of stars to guide with. A disadvantage is that if there is any play between the optics of guide scope and the main scope, then the guiding is inexact and you can get stars that are no longer round. This is called differential flexure. This is an especially serious problem for users of SCT and RC telescopes since the main mirrors are often subject to some movement under the influence. (So called mirror flop with SCTs).

 

Another method is to use a prism to pick off some of the light that is off axis as it heads towards the main camera. This is what is called an Off Axis Guider (OAG). Because it is using the same optical train, it is far less subject to differential flexure. However, since the light is off axis, it is often dimmer. Also, it is typically at longer focal lengths. The net result is that it can be harder to find guide stars. People using OAGs usually use very sensitive guide cameras as a result.

 

Most people will start with a guide scope and camera and if they are using a short focal length refractor, they will probably be quite happy with the result as long as the guide scope is mounted rigidly. However, I would recommend that users of SCTs (other than those with mirror locks) or the inexpensive RCs such as the AT8RC or AT6RC strongly  consider using an OAG. Very sensitive guide cameras such as the QHY5L-II mono are now available in the 320$ range. This is only slightly more expensive than less expensive guide cameras such as the Orion SSAG (currently 279.99$) , which is less sensitive. Actually I recommend the QHY5L-II mono over the SSAG even for folks who will be using a guide scope.

 

10. What would you suggest for someone starting out?

 

It really depends on how much money they have and how sure they are they are going to stick with the hobby. An 80mm triple apochromat using FPL53 glass, an Orion Sirius or Atlas Mount (depending on how portable you need things to be), a modified DSLR, a 50mm guide scope, and a QHY5L-II guide camera make an excellent starting system. Unfortunately, that is well North of 2000$. I personally think you are better off spending a little more for the gear upfront and avoiding the aggravation of using poor equipment. Your results will look better and you are more likely to be successful.

 

I would also strongly suggest you find a mentor or mentors either in person or online.

 

11. What about processing my images?

 

There are three popular alternatives:

 

a ) Photoshop - expensive but there are lots of tutorials available.

 

b ) PixInsight - made specifically for astroimaging and excellent at what it does. It is considerably cheaper than Photoshop. It can be rather daunting to learn. There is some help available. Harry has put together some excellent videos. There is also a tutorial on my website.

 

c ) StarTools - again this is made specifically for astroimaging. It is the cheapest but is still quite capable.

 

I own, like, and still use all three. If I could own just one, it would be PixInsight. But beginning imagers may find either StarTools or Photoshop easier going (especially if they already own and are familiar with Photoshop).


Edited by Madratter, 04 October 2014 - 05:03 PM.


#2 SergeC

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Posted 03 October 2014 - 12:23 PM

Very nice, good information here, very little with which to disagree.



#3 lily4ever

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Posted 03 October 2014 - 08:31 PM

4. Reflector vs Refractor?

In general reflectors are much cheaper for the aperture. 6" is the cutoff for many people where beyond that a refractor is just too expensive to be affordable. But refractors have many advantages including easy collimation, and the lack of diffraction spikes (although some people like them). For people interested in widefield imaging, the choice is often a smaller triple apochromat refractor like an 80mm f//6. For those interested in shooting galaxies, except a few really big ones, that requires more aperture and you will see people gravitating towards 8" and larger reflectors.

 

The last sentence is a little confusing to me. Does the "except a few really big ones" refer to galaxies beyond the capabilities of amateur astrophotographers? I assume the "that" after the comma refers to "those interested in shooting galaxies," but it looks like it's referring to the few really big galaxies.



#4 nitegeezer

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Posted 03 October 2014 - 08:35 PM

This looks like a good start and if it continues to grow I will add it to the "best threads" thread.



#5 Madratter

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Posted 03 October 2014 - 08:42 PM

 

4. Reflector vs Refractor?

In general reflectors are much cheaper for the aperture. 6" is the cutoff for many people where beyond that a refractor is just too expensive to be affordable. But refractors have many advantages including easy collimation, and the lack of diffraction spikes (although some people like them). For people interested in widefield imaging, the choice is often a smaller triple apochromat refractor like an 80mm f//6. For those interested in shooting galaxies, except a few really big ones, that requires more aperture and you will see people gravitating towards 8" and larger reflectors.

 

The last sentence is a little confusing to me. Does the "except a few really big ones" refer to galaxies beyond the capabilities of amateur astrophotographers? I assume the "that" after the comma refers to "those interested in shooting galaxies," but it looks like it's referring to the few really big galaxies.

 

 

You're right that is poorly worded. Thanks I'll fix it to make the intent clear.



#6 Goofi

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Posted 03 October 2014 - 08:44 PM

You left off ...

 

How much does this hobby cost?

This will cost 4 times whatever you have budgeted, and then it increases each additional year you stay in the hobby.

 

 

(Okay, a little tongue-in-cheek, but not too much!)



#7 proteus5

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Posted 03 October 2014 - 10:27 PM

Great info, and very well written. I wish all this info had been in one place when I started out. I was able to avoid the common mistakes through the advice of the great people here at CN. I progressed very closely to the way you have outlined. I started with an Atlas, Skywatcher 80ED, Canon T2i DSLR and an Orion 50mm guide scope with a Orion Starshoot Autoguider. I have since added an AT8RC and have an old Meade 10" SCT. I plan to get my DSLR modded sometime in the future. I doubt that I'll ever go to a CCD though. With my perpetually bad seeing, I don't know that it would give me that much improvment to justify the cost. At this point I think that my seeing, and light pollution are limiting me more than my equipment.

 

As always MR, thanks for the great article, and all that you do for the community. 

 

Oh, and Goofi is right, AP is a money pit, but it sure is fun when you see the images as they are downloading from the camera to the laptop!



#8 Lenbo

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Posted 04 October 2014 - 09:06 AM

Really pretty much on the money. I can relate to much of what is said. I may not have thought that going in but it pretty much reflects what I've learned at this point. Started with Atlas mount. Upgraded from 8" newt to RC8. Started with cheap DSLR and self modded it. Graduated to a CCD. Next step up will most certainly be the mount for me. Not sure what at this point. Been careful with money I've spent but stepping up from here gets more expensive for sure. My admiration goes out to MR for his willingness to share his experience with others and doing the work it takes to get it done. A tip of the hat sir! :bow:  Len



#9 terry59

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Posted 04 October 2014 - 09:40 AM

More excellent information. Two things jumped out at me. Including EQMOD as part of the "why Atlas/Sirius" discussion may help and I would include smart used purchases in the starting out section. I would add the "Orion issue" caveat there as well.

 

Definitely sticky-worthy IMO



#10 josh smith

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Posted 04 October 2014 - 10:06 AM

#9 is what I would consider about the most common question and the best answer... I personally would've preferred to have been told to start off with an oag though.  I would say that could almost be its own number.  OAG's aren't more challenging at reasonable focal lengths and let you jump over many issues a separate guide scope can introduce.   



#11 Footbag

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Posted 04 October 2014 - 10:25 AM

Great writeup!  Definitely sticky worthy.



#12 Madratter

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Posted 04 October 2014 - 04:14 PM

Thanks everyone. There are definitely some great ideas for extending it/modifying it too.



#13 Madratter

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Posted 04 October 2014 - 05:04 PM

I have now incorporated suggestions by Terry, and influenced by Josh have added a section on guiding.



#14 TimN

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Posted 05 October 2014 - 10:59 AM

I think this is an excellent idea and I certainly agree with most of the information here. I would add something to the mount area . I have an AT8RC which is a popular OTA. I have no problem with it taking 10 minute subs on my HEQ5 Pro (Sirius EQ-G) even after adding a heavier auto focuser (Moonlite) as well as a 50mm guider and a fairly heavy dew shield. Before I went this route I found a lot of other imagers - especially in Europe - use a similar configuration. However, the ones I checked with were all using DSLR's so you are probably correct in that 6" RC's are probably the maximum if you are using a CCD with a filter wheel. The only reason I mention this is that I wouldn't want an HEQ5 Pro (or Sirius equivalent) owner that is using a DSLR to feel that they can't use the popular AT8RC. However, if they uprade to a heavier CCD with filter wheel, they may also have to upgrade the mount.


Edited by TimN, 05 October 2014 - 11:20 AM.


#15 nitegeezer

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Posted 05 October 2014 - 11:03 PM

Rather than making this thread a sticky, it is now mentioned in Best Threads in Beginning Imaging



#16 Madratter

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Posted 06 October 2014 - 06:15 AM

Thanks Chuck.



#17 Poochpa

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Posted 08 October 2014 - 12:26 PM

Very good! I would add ImagesPlus to #11.



#18 SteveInNZ

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Posted 10 October 2014 - 10:12 PM

I'd like to pick up on "CCD wins hands down on EVERY count except cost". You knew someone would, right ? :)

 

The DSLR wins on familiarity. There is a lot to learn when starting out in astrophotography and starting with a DSLR takes a huge 'unknown' out of the learning curve.

 

Steve.



#19 TimN

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Posted 11 October 2014 - 09:18 AM

The DSLR also wins on flexibility. Not only can it be used outside astronomy but it can be used for DSOs as well as planetary within astronomy. While the DSLR is also a better value due to its flexibly and mass production, I agree that it can't match a dedicated CCD for DSO's. However, that advantage diminishes somewhat as light pollution and warmer temperatures diminish.



#20 guyroch

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Posted 11 October 2014 - 10:06 AM

Moderators... this thread should be pinned.  It is full of useful information for the beginner.

 

Well done Maddrater.

 

Guylain



#21 targetshooter

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Posted 11 October 2014 - 11:55 AM

I agree with everything. 

I would like too add that your mount is more important than your OTA. 

When I was buying my setup the sales man kept referring to the mount.

I realized he was doing it and ask why he was doing so and not mentioning the scope (size/type)

He never said why, he would just shrug his shoulders. So I just left puzzled.

Later I found out (each time I setup my equipment) that the mount and it's ability to track is everything.

And your setup and alignment is the 2nd most important item.

This holds true for visual observing also.



#22 Madratter

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Posted 11 October 2014 - 11:57 AM

Steve, I'll grant you the familiarity angle although I think it is minor since whether using a DSLR or CCD, you need to learn the software that will be running it. It is possible to run a DSLR without software but very tedious IMO.

 

Tim, I should have made clear that this FAQ is in the context of DSO astrophotography only.

 

Thanks Guylain.



#23 khobar95

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Posted 11 October 2014 - 01:38 PM

I'd like to ask a question about H-alpha filters.

 

I've read H-a are needed for some objects (Horse Head?), but I've also read they are useful for observing the sun. For observing/imaging the sun (solar prominences, solar flares, solar corona, etc). what's the difference between using a solar filter that fits over the end of the scope + an H-alpha filter for a couple hundred $$$ and something like a Lunt Solar 50mm H-Alpha Filter - B600 for 2" Focusers that costs approx. $1350? I see some H-a filters well into the thousands!


Edited by khobar95, 11 October 2014 - 01:45 PM.


#24 Madratter

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Posted 11 October 2014 - 03:26 PM

Using a non solar HA filter for the sun could be very dangerous because all the light from the objective is concentrated on the filter. If it cracks, the results could be disastrous.



#25 khobar95

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Posted 11 October 2014 - 06:16 PM

Madratter, on 11 Oct 2014 - 1:26 PM, said:

Using a non solar HA filter for the sun could be very dangerous because all the light from the objective is concentrated on the filter. If it cracks, the results could be disastrous.

 

Absolutely. To be clear, I was wondering about using a full-aperture solar filter in conjunction with an HA filter that threads onto the EP and how that differs from using a solar HA filter costing several times more.




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