Schmidt Newtonian for astrophotography?

[Space Cadet (Greg)]

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 (Peter)]

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 (Rick)]

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 (Brent)]

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 (Rick)]

Originally Posted by 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.

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 (Peter)]

Originally Posted by GregJohns69

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

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 (Peter)]

Originally Posted by 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.

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]

Originally Posted by peter_4059

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

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

[peter_4059 (Peter)]

Originally Posted by Wavytone

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

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 (Peter)]

Originally Posted by clive milne

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

What can you use?

[clive milne]

Originally Posted by peter_4059

What can you use?

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 (Ken)]

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 (Troy)]

“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 (Dunk)]

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 (Peter)]

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 (Dunk)]

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.