DSLR lens vs Achro refractor for low-cost astrophotography

[astrochumak]

Hello everyone,

I'm currently building my low-cost astro setup for deep sky imaging and was wondering what people tend to use more.

There are a lot of lenses that can be potentially cheap and good enough for astrophotography, but in general situation is completely different when people discuss achros for imaging. 

So, in your opinion, do lenses really outperform cheap glass scopes, or is it just a widespread misconception?

Say, would you get better results (better resolution, image quality etc) with Orion/Sky Watcher/Celestron 70-80mm achro scopes than with some general (not "L":) 70-200 or 70-300 f/4-5.6?

Or is worth to trade field flatness of DSLR lens for refractor's better reach, if there is any?

 

 

P.s. I've seen some good results with ST80, but there's really a few, so I'm interested in anyone's experience:) Be free to post here any of your thoughts too! Thank you!

Edited September 25, 2021 by astrochumak

[Nik271]

If you already own a DSLR just go for a second hand lens. There are a lot of these on the market and will be even more  as photographers start to transition to mirrorless and sell their old gear. You don't need image stabilization and auto focus, manual lens design was already close to perfect decades ago.

A cheap doublet fast refractor will not be able to beat an average lens in my opinion.

Edited September 25, 2021 by Nik271

[vlaiv]

Really depends on what you want to image.

Lens already have good focusing mechanism and direct attachment to DSLR (provided you get lens for camera model you have).

Their downside is that they are not diffraction limited, often suffer CA and other aberrations wide open and they come in short focal lengths.

Achros on the other hand - have CA issues, their stock focusers are not up to task of holding DSLR (weight issues) and require much larger mount. They are however diffraction limited optics and are meant for astronomy.

Most people don't like to think of using Achros for astronomy due to chromatic aberration - but CA is really not that big issue compared to lens.

Let me show you this on example. Here is an image that I took with SkyWatcher StarTravel 102mm F/5 scope:

(you can barely make out Crescent nebula in that image).

This image is actually 1280x1024 in size (or there about) - but I resized it down to show you what 100mm lens would create with above camera (which was really small planetary camera).

CA is barely noticeable in this configuration.

Using same telescope with DSLR would give you this FOV:

On the other hand, if you know few tricks - you can take achromat scope and remove chromatic aberration from it completely for a bit higher resolution work. This image was done by using aperture mask and wratten #8 yellow filter:

(again, using small planetary type camera - ASI185 this time). It is 2x2 mosaic

As comparison, with similar sized sensor of ASI178, and Samyang 85mm F/1.4, you can get image like this:

This was taken wide open - at F/1.4 and it shows - stars are "blobs" rather than points and some have very pronounced halo:

Here is crop from color version of that image (but not nearly as stretched as above version):

Both red and blue / violet halo shows around brighter stars.

Lens like Samyang 85mm or 135mm will outperform any achromat scope - but you'll be limited to that focal length. Both lens need to be used stopped down at say F/2.8 to give best results, although you can use them wide open if you want to do very wide mosaics and are prepared to bin your data / lower resolution, or you use filters and mono camera.

Telescopes will have better performance on same targets and same resolution, but they inherently have longer focal length and you definitively need to use mosaics and binning with them to match FOV and resolution of the lens. You can shoot higher resolution images with them, but then you need to be prepared to use some tricks - like using filters and further stopping down aperture and so on.

I guess that in order to decide what is best for you - lens or scope, you need to answer few questions first:

1. What mount will you be using for imaging (this is really important bit)

2. What type of targets will you be interested in (wide field low resolution work, or narrower field medium resolution work - high resolution work is really not for beginners - at least not long exposure, planetary is fine).

[astrochumak]

Thanks for your input, @Nik271! I already have Canon 600D (LPF 2 removed) and Sigma 70-300 f/4-5.6 APO DG (bought specifically for AP)

Wow @vlaiv , thank you so much for such a detailed reply! 

As mentioned above, I have a DSLR and lens, and it is mounted onto Az-GTi (EQ), so pretty stable. Also, I'm interested in galaxies mostly, although I understand that 1) Light pollution is an issue as narrow band filters wouldn't work for them, and 2) I'm pretty limited in choice, as there's a few bright and large enough to catch with 80mm aperture. So probably as up-close to faint fuzzies possible with cheap glass scope is my thing

The thing is, I started thinking about the aperture I am shooting with, and, being a visual astronomer before, I got a little bit confused.. If we were to shoot at say 200mm f/5, we would get an effective aperture of 40mm (..right?). Isn't it too small to compete with the resolution of even 80mm f/5 achro, stopped down to f/6-6.5? We get at least 20mm more aperture

If chromatism can be removed in post, then the only issue with any refractor is field curvature? 

[Alien 13]

I like lenses, much kinder on the mount requirements and they are ready to go with there inbuilt field flatteners etc but you pretty much get what you pay for in performance. The new breed of scopes like the RedCat based on camera lens designs are interesting too.

Alan

[rnobleeddy]

Having played with a few random old and new lenses, definitely do some research before buying. All lenses are not equal, and the cheaper zoom lenses that come with DSLRs have been pretty awful in my experience. On the flip side, some older prime lenses have performed well. If buying new, there are some threads here that cover some common good choices, like some of the Samyang prime lenses.

[Dazzyt66]

+1 for the prime lens suggestions. I'm now only using lenses for AP - and using vintage 135mm and 200mm primes sourced from eBay (M42 fit). I'm finding that they are great for my needs from a Bortle 7 back garden where I do most of my imaging. I used to use my ST80 but get far better results from the lenses. If you look around the images on the forums you'll find examples of loads of different types from cheap to very extravagant!

[astrochumak]

My main concern is that with lenses you are limited in aperture, as I said above, if I am correct. How would it affect the image resolution? 

[astrochumak]

@Dazzyt66 that's good news, thank you! Do you have some examples on AstroBin, maybe? 

The thing is that I'm trying to decide whether I should buy 80mm achro to get that extra bit of aperture, or should I stay with the lens?

[Dazzyt66]

1 minute ago, astrochumak said:

@Dazzyt66 that's good news, thank you! Do you have some examples on AstroBin, maybe? 
 

Heres an example of M81 from my ST80:

And M31 from my 200mm prime with no auto focus:

And also NGC7000 using my auto focuser with the 200mm

Forgive my processing skills as I'm still learning!

Hope that helps.

Daz

 

[astrochumak]

These are great images, Daz! Whenever it'll be comfortable for you, could you please specify the exposure time and light pollution level for M81 with ST80 and North America with 200mm lens?

 

If anyone has other examples of achro vs lens comparison photos, please be sure to post them here, thanks!

[Dazzyt66]

35 minutes ago, astrochumak said:

These are great images, Daz! Whenever it'll be comfortable for you, could you please specify the exposure time and light pollution level for M81 with ST80 and North America with 200mm lens?

Thanks! That means a lot!

All the images were taken from my back garden (Doncaster, UK) which is Bortle 7/8. Right by my back garden is an LED streetlamp and various neighbour security lights that pop on and off during the night, so never really dark at all.

All the images were taken with my Canon 1100d (astro modded) at iso1600. The ST80 subs were done over 3 nights before my camera was modded (around 500x30s in total - probably used the best 85%) using just bias and flats. The M31, after camera mod, was just 2 hours of 60s subs again with bias and flats around 90% useable with the lens wide open at f3.5. NGC7000 was just a few nights ago with 180x60s again 80% used with just bias and flats but with the lens at f5.6. I don't use any kind of LP filters.

Hope that helps 😀

Edited September 25, 2021 by Dazzyt66

[happy-kat]

You can enter all your details of kit in here with a selected target to understand the FOV you would get. The link is from the resources tab top of this website.

http://astronomy.tools/ 

[vlaiv]

3 minutes ago, astrochumak said:

The thing is, I started thinking about the aperture I am shooting with, and, being a visual astronomer before, I got a little bit confused.. If we were to shoot at say 200mm f/5, we would get an effective aperture of 40mm (..right?). Isn't it too small to compete with the resolution of even 80mm f/5 achro, stopped down to f/6-6.5? We get at least 20mm more aperture

Aperture is important in astrophotography, but it is not only part of the equation.

40, 50, 60mm of aperture can quite happily image celestial targets, provided you take care of other part of equation.

Speed of imaging setup depends on "Aperture at resolution". If you think about it - coming from visual - if you use same telescope, hence same aperture, but you raise magnification - target will become dimmer. This is because same amount of light is spread over large surface. Similarly, speed at which image is taken (how much time you need to spend imaging your target) will depend on aperture size, but also - on working resolution.

Working resolution depends on focal length and pixel size, and can be changed by using binning (which is combining multiple pixel into single pixel - like group of 2x2 or 3x3 pixels to act as single pixel having x4 or x9 effective light gathering surface).

3 hours ago, astrochumak said:

If chromatism can be removed in post, then the only issue with any refractor is field curvature? 

Chromatism can't be removed in post processing effectively - but there are tricks to lessen it in capture time.

1. Use of aperture mask

2. Use of filters that remove specific parts of spectrum.

Field curvature is something that depends on focal length of telescope, and larger telescopes have less of it. There are also field flatteners that deal with it.

[astrochumak]

20 minutes ago, vlaiv said:

There are also field flatteners that deal with it.

I'm aware of field flatteners, but unfortunately they tend to cost as much as the cheap glass scope itself 😅
Thanks for clearing things up tho!

[astrochumak]

So I was browsing through the AstroBin and found a ton of stunning pictures taken with Canon 70-200 L lens, which is obviously a marvelous lens by itself (for those wondering: https://www.astrobin.com/search/?q=Canon+70-200&d=i&t=all&date_published_min=2011-11-09&date_published_max=2021-09-25 ) There is a cheaper analog to it, Sigma 70-200 f/2.8, which seems to not be as popular, but quality-wise is said to be +- the same 

Also, I was comparing the photos taken with DSLR + lens to the ones taken with Z61, and I gotta tell you that there's not that big of a difference that I can tell (although it mostly depends on the one who takes the photo)

So what do you think, is it worth to spend extra money on a fancy glass scope when you can buy similar gear, at least specifications-wise, for 60% of the price? 

P.s. Yes, Z61 has more focal length, but there's also Sigma 100-300, which is closer to refractor's FL, but still cheaper

 

 

 

Edited September 25, 2021 by astrochumak

[BCN_Sean]

2 hours ago, astrochumak said:

There is a cheaper analog to it, Sigma 70-200 f/2.8, which seems to not be as popular, but quality-wise is said to be +- the same 

I've a Sigma 70-200, and it's not too bad up to the 180mm mark when stopped/masked down to F:/4.  However with the design of the lens as the zoom element moves backwards in the barrel it can start throwing odd internal reflections if there are bright stars within the field of view with the light being reflected off the rear element leaving which appear like Newton's rings and can be quite a pain to clear up; maybe be different on other generations of the lens, but I'm not going to risk your money on recommending a "maybe".  If I'd not bought the lens quite a few years before I started with astrophotography, I would have been disappointed with it, whilst it's handy to have a few focal lengths covered in the same optic (70, 135 & 180) something like the Samyang lenses which vlaiv mentions above, or something like the Nikon 180mm IF-ED with a mount convertor on it would rip it to shreds quality wise.

[astrochumak]

Hmm, never thought that, @BCN_Sean, thanks! Do you have any examples of your photos taken with this lens? And what are you using now for low FL astrophotography?

[Dazzyt66]

9 hours ago, astrochumak said:

So I was browsing through the AstroBin and found a ton of stunning pictures taken with Canon 70-200 L lens, which is obviously a marvelous lens by itself (for those wondering: https://www.astrobin.com/search/?q=Canon+70-200&d=i&t=all&date_published_min=2011-11-09&date_published_max=2021-09-25 ) There is a cheaper analog to it, Sigma 70-200 f/2.8, which seems to not be as popular, but quality-wise is said to be +- the same 

Also, I was comparing the photos taken with DSLR + lens to the ones taken with Z61, and I gotta tell you that there's not that big of a difference that I can tell (although it mostly depends on the one who takes the photo)

So what do you think, is it worth to spend extra money on a fancy glass scope when you can buy similar gear, at least specifications-wise, for 60% of the price? 

P.s. Yes, Z61 has more focal length, but there's also Sigma 100-300, which is closer to refractor's FL, but still cheaper

 

 

 

Although relatively new to AP, I'm a definite lens convert. There is plenty of info out there on how to get some decent images with a lens DSLR combination rather than spending considerably more on dedicated AP equipment, here is a great example:

https://astrobackyard.com/budget-astrophotography-lens/

As I've just said in another thread, it really depends on what you want from your images. If you are happy just exploring whats out there to a modest level, then a DSLR and lens combo is hard to beat (IMHO). Having read up on it quite a bit, its also worth trying to stick to prime lenses rather than zooms as there are less optics to contend with (as mentioned above). I'm having the time of my life with just cheapo 50mm, 135mm and 200mm primes - and thats what having a fulfilling hobby is all about isn't it? 😀

[wxsatuser]

19 hours ago, rnobleeddy said:

Having played with a few random old and new lenses, definitely do some research before buying. All lenses are not equal, and the cheaper zoom lenses that come with DSLRs have been pretty awful in my experience. On the flip side, some older prime lenses have performed well. If buying new, there are some threads here that cover some common good choices, like some of the Samyang prime lenses.

I would agree, very few lenses fit the bill for the best images and most are expensive.

I cannot comment on other makes but the ones I have tried and bought.

Canon 200mm f2.8 prime
Canon 70-200mm f2.8, the expensive ones.
Samyang 135mm f2

I have tried various Sigmas, 105mm macro is reasonable if you like widefield.

[BCN_Sean]

3 hours ago, astrochumak said:

Hmm, never thought that, @BCN_Sean, thanks! Do you have any examples of your photos taken with this lens? And what are you using now for low FL astrophotography?

At the moment I'm using it up to 180mm and also use a 300mm F:/4 which has its own set of curiosities (mainly coma on the left side).  I don't have any images from either lens on this computer at the moment but I'll try and dig some out in a bit but I'm in the middle of ripping the network to pieces in the apartment and rebuilding it so it could be a bit of time.

[BCN_Sean]

@astrochumak, finally got around to getting some files off the computer; I left the other half supervising the new lines going in as I'd pulled an emergency call out, and they'd installed everything to the wrong side of the building to the network cabinet and the uninterruptible power supplies.  Three days of routing network connections and power cables through the place, and hopefully (off to test it tonight) it's not caused any fun with the astro gear.

This here is the sigma 70-200 at 200mm in length, with the rings I was saying about above, seems to be caused by reflections off of one of the elements, and it's noticeable in single unprocessed subs; I think it's when the zooming portion of the assembly (as it's a reverse zoom design) is picking up light that is reflected off the rear element.   Whilst it's not impossible to correct out, it's a bit time consuming.  This here is just colour balanced, stretched and size reduced.

[astrochumak]

Hmm, interesting.. Thanks for sharing, Sean! That's an awesome shot 

I was recently attending Nico's from Nebula Photos Zoom conference and there I asked him the question brought up in this topic. Basically, considering nebulosity, you won't get much of a difference between small ED frac and some nice lens. But obviously the difference is in the stars. So if you are going to stick with astrophotography for quite some time then refractor will better suit you. So I think I'm going for WO' Z61 analog called Sky Rover 60ED 

[900SL]

Samyang 135mm f2 is a great astro lens if you get a good copy. Might be a bit too widefield for M31 etc though. I use it at f2.8. 

The Nikon ED 180mm f2.8 is also a good astro lens, stopped down to f4. You can find these used, they were in production for many years and some don't use ED glass so check before you buy.

I looked at the Nikon 70-200 f2.8 but it costs more than a good refractor with field flattener / reducer.