Schmidt Newtonian for astrophotography?

[Space Cadet]

Hi
Wondering about opinions for suitability of a 10" f4 Schmidt Newt on an EQ6 goto, mostly planning for deep sky Astrophotography.
Is this OTA a decent option for an ambitious newbie?

Other option i am looking at (not much cheaper) is an ED80 on an EQ5 goto
Words of wisdom please?
Thank you
Greg

[peter_4059]

I'm using a Meade SN10 for astro photography. It has undergone a few mods to make it suitable and for the price works quite well. My Astrobin page has a few images I've taken with it. I initially had it on an EQ6 but recently upgraded to an EQ8.

[doppler]

With astro photography bigger is better, most people want more aperture and lower f ratios. The only downside is that bigger is harder to work with. The ED 80 is a great starter scope but is wide field and you'll soon be wanting a bit more focal length.
Can't comment on the Schmidt Newt, they don't seem to be a popular choice when compared to the amount of fast newts and RC scopes being used by people here? Finding the right scope is hard, which is why a lot of us have more than one.

[ZeroID]

The advantage of a newt is no CA
The disadvantage of a newt is coma, especially at f4. I hope you have factored in an expensive coma correction solution.
I have a seldom used 8" f8 astrograph. The long focal length means it is almost coma free, the long focal length means it's a logistical problem in my small ob to swing it around as it is nearly 1.6 meters long.

[doppler]

I think that the Schmidt Newt design makes it pretty well coma free due to the corrector plate on front , with the added advantage of no diffraction spikes.

[peter_4059]

Greg, which 10" f4 Schmidt Newt are you looking at - is it a Meade LXD55 or LXD75?

[Wavytone]

For an ambitious newbie I'd suggest either of the two 6" maksutov-newtonians that were listed in the IIS classifieds would be a better choice, and frankly I'm surprised no-one snapped up the Explore Scientific one as IMHO it is a bargain if AP is your thing.

[peter_4059]

They do look good although they are a smaller aperture and focal length to the SN10. I was after around 1200mm f/l as this suits a lot of galaxies nebulae. For wider fields I have the 80mm f6 triplet.

I'm not at all familiar with the Mak-Newt - I assume they don't need a corrector?

[Wavytone]

Tne corrector is the big soup-bowl lens at the front !

[peter_4059]

Why didn't I think of that! Actually the reason I ask is because I find that despite the corrector plate, the SN10 is not flat to the edge of the field with my QSI683 sensor and I'm therefore using a Paracorr. I haven't determined if this is causing other issues yet as I'm still trying to resolve other challenges like collimation, tilt, flexure and guiding.

[Wavytone]

If you take a look at the ad for the ES 6” mak-newt the seller has linked to his site with quite a few photos taken through it. Judge for yourself, it’s a quite capable scope.

[clive milne]

With respect to Schmidt Newtonians...

They are not coma free, they just have 1/2 the coma of a Newtonian.
If you were to reposition the corrector plate at the radius of curvature of the primary, THEN and only then will it be coma free.

And no, you cannot use an off the shelf coma corrector to make the system aplanatic.

[peter_4059]

What can you use?

[clive milne]

Either a custom fabricated coma corrector designed specifically for it, or, rebuild the tube assembly, moving the corrector away from the primary.

When the corrector is placed at double the focal length distance, the system will be anastigmatic. Essentially, you will have just turned it in to a classical Schmidt camera.

Two things to be aware of:
The focal plane will still be curved with a radius equal to the system FL.
And, the Achilles heal of any Schmidt is internal reflections. Basically the light that bounces off the CCD (or at least a portion of it) will be collimated by the primary, returning up the tube assembly until it hits the Schmidt plate.
A portion of that light will be reflected by the front and back surfaces of the corrector back down to the primary which refocuses this light to form ghost images on the CCD.

In short, a Newtonian with a coma corrector is cheaper and suffers none of these issues.

best
~c

[Merlin66]

Clive,
I think you’re being overly harsh on the Schmidt-Newtonian design.
I have an original Meade Comet Catcher 6” f3.5 from Halley days.
I can’t remember any significant issues with this scope....

[clive milne]

Ken,
I also had a Celestron 5.5" comet catcher.

I really enjoyed using it and don't recall any issues with it.
That being said, I maintain that there are better performing telescopes
more suitable to astrophotography using current CCD's.

best
~c

[troypiggo]

“Better performing scopes for AP” - of course if budget is no issue. I’d be interested to see your recommendations for the same cost as SN10, since that’s what the OP was asking about.

[Camelopardalis]

f/4 newts are a dime a dozen these days, throw in a coma corrector and your still under $1000.

Alternatively, use smaller pixels on a smaller scope. Less weight = less grief.

[peter_4059]

There's always a compromise. I had an f5 newt but this was not so good for qld astrofest conditions due to dew forming on the primary during the night which would end the imaging session. I tried various approaches to resolve this without success. The attraction of the SN is a dew strap around the corrector means no more due issues. Not sure if the new GSO truss designs address this but they are $2k plus in any case.

I could get a smaller aperture refractor however it would be at least $4k for a reputable triplet around 130mm aperture / 1000mm focal length. There are many cheaper refractors however CA is not something I want to deal with. The SN10 costs around $1k.

Then there's sub exposure / integration time vs aperture. There is a lot of advantage in 10" aperture in acquiring enough data in a reasonable time.

What would be really helpful is some more information about a coma corrector optimised for the SN since it won't be practical to move the corrector.

[Camelopardalis]

Duckadang does seem to be dew/fog prone, I’ve not suffered that fate in other sites I’ve been to in SE QLD, luckily.

Plain old newts suffer their own set of problems of course. I’m not a fan of the average secondary assemble, I’d assume a corrector plate would resolve that and not flop around when slewing.

[ausastronomer]

The Meade SN10 out of the box isn't a very good telescope. The focusers and the mirror cell / mounting sytem leave a lot to be desired. Further, some of them have serious optical issues. I was able to observe Mars in stereo without binoviewers during the August 2003 opposition using long time forum member Tony Hitchcock's (Itchy) Meade SN10. The telescope was unique in that in 45 years of observing it is the only telescope I have ever used that was able to provide stereo images of every bright target it was pointed at. It provided 2 very fuzzy balls of Mars side by side with almost no detail of any description visible.

As one of the earlier posters said, his Meade SN10 has been somewhat modified to the stage where it's now a decent telescope.

An 8" or 10" F4 Newtonian is a whole lot easier and a lot more likely to be a decent performer without a good bit of work and modification.

Cheers
John B

[peter_4059]

John,

To be fair I think you would find a lot of people purchasing GSO or similar newts would replace the focuser and probably also make other mods to stiffen the area where the focuser attaches. They might also replace the primary colimation springs and flock the inside of the tube. They would also need to purchase a coma corrector. I did all of these things with my GSO newt and have done similar mods to the SN10 so I think it is a bit of a stretch to suggest these newts work well out of the box.

Here's a good thread on the topic from someone that is achieving outstanding results:
http://www.iceinspace.com.au/forum/showthread.php?t=111441


The biggest challenge with the sn10 is getting a suitable coma corrector in my opinion. For me the jury is still out on that and I might be in denial however I'm planning to keep trying as the advantage of being dew free is a big plus based on my years of astrofest experience with the newt.

Peter

[Space Cadet]

Thanks for the discussion guys, i am learning lots.
My main issue on a shoestring/dental floss budget is what to initially buy. Although the Mak Newts appear better, then i need a mount, etc, so $$$ are still an issue. i am currently considering 2 options
1. A HEQ5 mount with a ED80, 2"diagonal and 10x focuser and 1 eyepiece for $1600, local area, or
2. A EQ6 mount with f4 10" Schmidt Newt, with 80mm short tube finder with autoguider, plus telrad, motofocus, variety of eyepieces for $2K, with a 30hr road trip

i am happy to do the road trip if it is worth it, but dont want to waste that effort if not. I am also wondering if this option is getting too ambitious?
If either of these options are going to give me a good intro into the game with the potential to then buy better quality glass later that could also work for me.

I am told the EQ6 is a better mount option for the future potential to add a larger scope as my aperture envy develops. Makes sense to me, however there seems to be a myriad of EQ6 and EQ5 options, so my confusion is growing. Some black some white, then H versions N versions, pro versions, ahhh! A nice comparison matrix would be helpful Skywatcher!

Maybe i just need to learn some patience! :)
More wisdom and opinions please?
Cheers
Greg

[Space Cadet]

BTW the Schmidt Newt is a Meade LXD55.
How much should this be worth on its own as an OTA?

Cheers
Greg

[peter_4059]

Greg,

Here's my thoughts:
There have been various versions of the HEQ5 and EQ6 however most of the changes have been cosmetic. There is a newer EQ6 called an AZEQ6 that is different and has some upgrades including belt drives, a larger diameter counterweight shaft and easier adjustment of altitude. It also allows the mount to work in both equatorial and AltAz mode.

There is also a belt mod kit for the HEQ5 and EQ6 and many of the mounts coming up for sale have had this mod done. This reduces some of the errors in the reduction gearing if it is installed well. The belt mod kit comes from Rowan Astronomy in the UK.

http://www.rowanastronomy.com/products.htm

The good thing about these mounts is you can take them apart and adjust their internal workings if you are so inclined. Search/Google "EQ6 Hypertune".

The EQ6 will give you more options than the HEQ5 if you decide you want to use a heavier scope in the future. Many people end up wanting more focal length which usually means a heavier scope. If you overload the mount you will have difficulty getting accurate tracking and end up with stars in your images that look like eggs.

In terms of optics, if you want something easy that you can get good results with on large objects then go for a refractor like an ED80. These are quite forgiving and you will get good results quickly however you will be limited to wide fields of view/large objects. You get what you pay for with refractors and the cheaper ones produce violet halos around bright stars. Ideally go for a reputable triplet to avoid this. You will most likely need to purchase a flattener also -something like this:
https://www.bintel.com.au/product/orion-field-flattener/

Here's a good example of what you might be looking at:
http://www.iceinspace.com.au/forum/showthread.php?t=163203

If you are up for a challenge and like to tinker then go for a Newt or the SN10. This will be a more difficult scope to achieve good results with however it has twice the focal length of a typical 80mm refractor and 10x the light gathering due to the difference in aperture. This reduces the time you need to spend exposing the image. Because it has mirrors rather than lenses you don't get the violet halos - even in low cost ones.

The LXD55 is what I have. For astrophotography the minimum you will need to do is replace the original plastic focuser which is definitely not up for the task. I replaced mine with a motorised Moonlite focuser. For either the newt or the SN10 you will also need a coma corrector - something like this:
https://www.bintel.com.au/product/baader-mpcc-mkiii-2-inch/

I believe you would expect to pay $800-1000 for an LXD55 in good condition. These are quite hard to come by and the one I found was in good condition so I was prepared to pay the top end of that range for it.

No matter what you go for, if you are on a shoestring budget you will be making a compromise and will want something better as you progress (that's the path I've been on). The trick is to minimise regret spend so you can upgrade over time without having to offload too many things hence the reason I suggest the bigger mount.

Hope that helps,

Peter

[Space Cadet]

Thank you Peter
That does help

[clive milne]

The MPCC is a really bad idea.
For one, it will over-correct the coma, leaving you with as much negative coma as you originally had positive coma.

Not only will it do nothing to sharpen up the outer field, it will destroy the centre of the field as well from the spherical aberration it adds.

Even as a Newtonian corrector, it is a poor choice.

https://www.cloudynights.com/topic/554686-coma-corrector-compariosn/

A simple field flattener would be better.

best
~c

[peter_4059]

I have the MPCC type 1 and 3 and the paracorr. I'm tempted to get a GPU and do a comparison for myself.

[peter_4059]

Clive,

I should have asked, can you recommend a simple field flattener that would work well? I'd be keen to try that option also.

You might find this interesting ...

https://optcorp.com/products/ba-mpcc-multi-purpose-coma-corrector-mpcc-mark-iii