Cheap Newtonians and a coma corrector are actually pretty cool!

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Andy Wray avatar
Ignore my processing, but I wanted to highlight to newcomers on here that you don't need to spend thousands on your first scope (OTA) to get nice sharp images.  The equipment here is a Skywatcher 200 DPS 8" Newtonian together with the Skywatcher 2" coma corrector.  Total cost for the two is about £530.  The first photo is a near-full frame of the crab nebula and the second photo shows the corners and centre.  Any elongation of the stars is more likely my poor guiding/mount rather than the OTA and Coma Corrector.  I used to think my relatively modest equipment was limiting my ability to take good photos, but I now know that it is other things like the mount, guiding and my pre and post-processing skills.  No doubt I will upgrade the scope one day as I too have equipment envy like everyone else.  In the meantime, I have much to learn elsewhere.

Also, just as a side-note:  I only ever worked hard to collimate this scope once 3 years ago.  Since then, I've probably only done a quick laser collimation on the main mirror every six months or so.  As long as you handle a Newtonian gently (i.e. don't bump it/drop it) , I don't believe you need to re-collimate very often at all.


Corner enlargements:



Crab Nebula SHO using a cheap newtonian
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Alan Brunelle avatar
Very nice image Andy!  

It is just my guess, but I would bet that there is no reason that a decent Newt should not be sufficient for most imaging when compared to others within it aperture class.  I think you demonstrate that here very well.  Getting a good coma corrector to do the job for the whole frame, might be a bit of a hit or miss.  But really, seeing will likely be the biggest issue for most.  For visual, and maybe a bit for astrophotos, the lack of central obstruction in refractors will improve contrast.  But who has an 8 inch refractor!  However, like yours, I have seen so many very wonderful images from Newts posted here, I am at a loss that more are not being put into service.  (Actually, for an answer to my last point, see below, since I have been waiting on my 12 inch Newt for 8 months now!)

I didn't see mention of the focal length of this scope.  I guess I could look it up, but I am too lazy.  Just interested because I have been waiting for 8 months on my 12 inch f4 Newt to come from Germany.  TS optics.   It is more expensive that yours, but that likely is a simple translation to that aperture.  My fingers are crossed that I get as good a result!

I did get a coma corrector and spacers for both imaging and visual.  I also got a very small secondary just for visual use as well.  For imaging, I will want to do nice photos from time-to-time, so the coma corrector will need to work well.  But for the IR work I mostly bought this for, I will only be sampling a small area right in the center of the field, so will ditch the coma corrector.
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Andy Wray avatar
Alan Brunelle:
But for the IR work I mostly bought this for, I will only be sampling a small area right in the center of the field, so will ditch the coma corrector.


I'm intrigued.  What will you be observing in IR and what kind of camera will you be using?
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Andy Wray avatar
Alan Brunelle:
I didn't see mention of the focal length of this scope


Sorry, missed this question.  It has a native focal length of 1000mm, but this is reduced to 906mm with the coma corrector.
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Mike Dobres avatar
Andy, 
I would go even further, its not just the OTA, you don't even need an expensive mount to produce half way decent images. I'm a relative newby, and purchased my first (and only scope) a 14" SW Newtonian - GOTO Dobson (bought used for 700 euros).  That price included the 2" coma corrector! Combined with a good camera, (ASI294MCpro)  a good dual - narrow band filter (L extreme), I'm now able to produce some pretty passable images of nebulas such a the Wizard, Monkey Head, Tadpole Nebulas etc..... with some pretty short integrations (10-40 mins).  All with no guiding.  Using short 1-4 sec exposures seems to work just fine. Below is just one example of a 10 min integration of the Monkey Head Nebula. As was stated above, a lot depends on seeing, and your processing skills.  And, obviously, the aperture really helps in my case!

I'm a bit more fussy on collimation, especially when it comes to imaging planets ,where regular laser collimation and occasionally double checking with a a star-test is necessary for the best results. 

Overall, Im extremely happy with the purchase, I have a scope that can be used for  visuals, galaxies, nebulas and planets, and now delivers pretty good images  with very little fuss. 

Just curious - you dont mention the mount you  used for the Crab Nebula image. I assume it is the HEQ5  I see you used for other images on your page?  I ask because I picked up a used HEQ5pro-mod on the cheap - and have been considering putting an 8" newtonian on it (instead of a smaller refractor) - but thought it might be too heavy?  Thoughts?

Mike

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SemiPro avatar
I feel like this shows how much the technical fundamentals play a role in this hobby. You got the back focus down, the optics are collimated and corrected, the guiding is good and everything is nicely focused. 

In addition to improving processing skills, getting technical skills down will do way more for people than upgrading to a Takahashi I think.
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Andy Wray avatar
Mike Dobres:
I picked up a used HEQ5pro-mod on the cheap - and have been considering putting an 8" newtonian on it (instead of a smaller refractor) - but thought it might be too heavy?  Thoughts?


I am using an HEQ5 pro with 8" newt and all the guiding, camera, filter wheel, coma corrector and autofocusser gear on it.  I have 11.1 Kg on it at present and I think it is just coping and as such am thinking about upgrading to an EQ6-R Pro when funds allow.  My guiding is currently between 0.7 and 1.2 arc secs right now.  I did do the rowan belt drive upgrade.  I have noticed that my RA axis was a bit slack, so have just tuned that and have also just ordered a better guiding camera to match my setup.  I'm hoping/expecting that I can achieve 0.6 arc secs guiding with careful setup and tuning of the HEQ5 Pro.   11.1Kg is at its limit for astrophotography though and I did have to add a third counterweight.
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Alan Brunelle avatar
Andy Wray:
Alan Brunelle:
But for the IR work I mostly bought this for, I will only be sampling a small area right in the center of the field, so will ditch the coma corrector.


I'm intrigued.  What will you be observing in IR and what kind of camera will you be using?

Hi Andy,

Thanks for the question!  It gives me a chance to rehash to myself, just what it is I plan on doing and yet explain to someone who is actually interested.

I am interested in observing the light curves of young stellar objects (YSOs) and the like.  I have some fool idea that I may actually see transits of such objects such as planets, dust from planetary disks, and nascent planets and star companions.  Particularly ones that are more in the formative stage of developing solar systems.  In fact, I would find it interesting to just find stars being eclipsed by rather dense dust clouds as well.  There are a number of transiting planets that are now well known and fall into the category of "visible to the amateur" that can be followed.  I point you to @sky-watcher (johny) and his posts as someone who is very active at showing the AstroBin community his captures of variable star light curves.  And he has also posted on planet transiting light curves.

But rather than just attack objects that have already been reported, I hope to do more of a saturation survey of starfields that the pros have identified as being particularly rich in YSOs.  This will also include some well known and not so well know star-forming clouds.  I hope to also follow some of the interesting well-known variable nebulae.  By taking this approach, I hope to find some objects that have yet to be identified or to better characterize some of those that are already listed.  Bu the discovery is the fun stuff for me.

To do this, I plan on imaging in both a visible channel and a NIR channel.  This falls within the limitations of my currently planned setup.  The camera I plan on using is the QHY5III462c.  It is a pretty inexpensive small sized CMOS sensor that has physically deeper silicon elements that extend its sensitivity well into the NIR.  Please see

New Toy(s), "First Light", IR camera 

for my first light with this camera.  While the literature for this camera cuts off the performance data at 1,000nm, I found that when I use a strong 950 nm Edge filter (which strongly eliminates any light below 950 nm), I still get good sensitivity.  Please also see

IC 1396A: Study Comparing Images from QHY5III462C (IR950 edge filter vs. UV/IR cut "visible"), and RASA QHY268C and
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SH2-106, A Study in False Color with Combination of a NIR Channel 

two examples of imaging I did with this camera and the 950 cut filter using my 5 inch Mak Newt.  For sure the hoped-for 12 inch will collect a lot more light.

My design for the 12 inch, f4 is to place a dichroic beam splitter in front of focus.  Not unlike the On Axis Guiders used by some here on AstroBin to guide with higher end telescopes.  The plan is to have the dichroic splitter do its job of separating the vis image from the NIR image and send them to two identical cameras that I mentioned above.  That way, I can image real-time, both the vis and NIR bands I want to use simultaneously.  Most variable star studies look at stars that show magnitude variations that have periods of hours, days or longer.  I have seen some studies that have shown variation "features" that can change in minutes or less.  Certainly, my 12 inch will not be able to see changes of seconds.  But I am hoping to try for short as possible.  And being able to image IR and vis simultaneously will be a great help in working out seeing issues as well as possibly discriminate between occluding planets and dust.  NIR is less affected by dust than say violet of the vis, so such differential responses can say something about the occulting objects.  Aside from the hardware challenges, I will need to learn to software processing in order to automate as much as possible the compiling of the data, since I hope to survey a good number of stars at the same time.  I am not sure I am up to the task, but it will be fun trying.  I would certainly like to reach further into the IR than 950-1100 nm.  And there are cameras that work up to 1700 nm or even higher.  But these are pricey and I will need to work my chops on the simpler setup first.  What I do like about the QHY5III462 is its capability to work at both vis and IR.  With identical image frame and pixel dimensions, it makes it really nice to compare the vis and IR data with a simple alignment.  But unfortunately, the vis has a Bayer color layer.  That limits its vis sensitivity if I work with one color band.  So I may have to scrape off the Bayer filter, which is doable.

Probably more than you wanted to read!  But you asked!  I just got an email from my telescope vendor today.  With a couple simple mods to go, the scope should be ready to ship soon!

Edit:  I should have stated that I chose to use a Newtonian because the light path is 100% reflective.  For the IR work, I wanted no glass between my subject and my sensor.  And the filters that I do place between subject and filter, I define and can ensure that I do not lose signal to the wrong kind of glass, or optical coatings.  I also was careful to ensure that I got an aluminum coated primary and secondary.  Many of the reflective optics are now dielectric coated.  And the manufacturers and vendors absolutely will not give you data proving reflectivity/transmissivity across the wide range of wavelengths I needed.  In fact, some dielectric coatings have sharp cutoffs just longer than red.  Finally, with the aluminum, I can easily strip the coating and do my own silver if I desire.  Silver is a truly superior reflector for wide range light work, though it is painfully delicate and does not last long!
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