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New galaxy scope recommendation


ninjageezer

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Ok so thinking of a new scope around maybe 1000mm FL for galaxies.

camera is a APS-C size imx571 sensor . 

the simpler the better i must be able to acheieve i good flat field .(hate bad stars on edge lol)

i have EQ6R pro mount .

i can go up to 2k ..

currently use a 400 mm refractor for nebula 

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A simple newtonian of about 8 - 10", if you don't mind the star spikes and the occasional collimation. If you want stars to look good over the entire field, as well as somewhat easier handling, a slower model seems a safer bet. So, 8" @f/5 rather than 10" @ f/4.

Opt for a good quality focuser, and a Losmandy plate to replace the more common Vixen dovetail.

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I'd consider the MN190 as well as a Newt.  Because I don't like endless fiddling with optics I would second Wim's advice in favour of F5 for a Newt. The Quattro can be be made to work but it won't necessarily be easy.

Olly

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19 minutes ago, ollypenrice said:

I'd consider the MN190 as well as a Newt.  Because I don't like endless fiddling with optics I would second Wim's advice in favour of F5 for a Newt. The Quattro can be be made to work but it won't necessarily be easy.

Olly

I like the MN190 a friend has one but the focuser is a terrible wobbly bit of kit .

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Refractors are easiest but expensive for this sort of focal length, so newt or SCT probably the way to go (from my experience SCT does sometimes give soft focus likely temperature related). What about a 200PDS? The price difference will allow purchase of a good coma corrector.

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7 minutes ago, Elp said:

Refractors are easiest but expensive for this sort of focal length, so newt or SCT probably the way to go (from my experience SCT does sometimes give soft focus likely temperature related). What about a 200PDS? The price difference will allow purchase of a good coma corrector.

200pds is on the shortlist ..i have an edgehd 8 but not very suited to my APS-C camera and i did get it last year for planetary ,but i think i will sell it .only used it in sept/oct last year sat here doing nothing.

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Up to you. If you've already got the C8, no harm in getting the F6.3 reducer and using an uncooled camera and trying it. I've been doing it recently with my C6, it's perceived to be noticeably slower than my F5.9 refractor, but the resolution increase is great. I suspect the perceived slowness has more to do with signal spread out over a wider area on the camera sensor more than anything.

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2 hours ago, ollypenrice said:

I'd consider the MN190 as well as a Newt.  Because I don't like endless fiddling with optics I would second Wim's advice in favour of F5 for a Newt. The Quattro can be be made to work but it won't necessarily be easy.

Olly

The reason I didn’t recommend it is that it isn’t a beginner’s scope. Imo, it’s better to have some experience with a standard Newtonian before tackling the MN190. It’s also heavy, compared to a standard Newtonian (not necessarily if compared to a large apo), and better suited for a permanent setup.

2 hours ago, ninjageezer said:

I like the MN190 a friend has one but the focuser is a terrible wobbly bit of kit .

It’s wobbly until you manage to fix the extension tube. Shimming it solves the issue. But yes, this shouldn’t be necessary to begin with.

I have a Feathertouch on order to replace the stock focuser. Ordered it last August and so far hasn’t materialised.

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1 hour ago, wimvb said:

The reason I didn’t recommend it is that it isn’t a beginner’s scope. Imo, it’s better to have some experience with a standard Newtonian before tackling the MN190. It’s also heavy, compared to a standard Newtonian (not necessarily if compared to a large apo), and better suited for a permanent setup.

It’s wobbly until you manage to fix the extension tube. Shimming it solves the issue. But yes, this shouldn’t be necessary to begin with.

I have a Feathertouch on order to replace the stock focuser. Ordered it last August and so far hasn’t materialised.

I used to have 250 newt .collimation was very easy on that. but yes i agree it should be necessary on the MN190.

this is what he has ordered.https://www.ebay.co.uk/itm/144895854948?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=c2lihuirrvk&sssrc=2349624&ssuid=QgHl1DARS2y&var=444115764875&widget_ver=artemis&media=MORE

along with this: https://www.deepspaceproducts.com/product/starlight-instruments-adapter-2-for-mk190-maksutov-newtonian-intes-alter-mn84-mak-newton-es8-astrograph

will see how it is when its done.

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4 hours ago, ninjageezer said:

I used to have 250 newt .collimation was very easy on that. but yes i agree it should be necessary on the MN190

The MN190 is trickier to collimate because of the corrector plate which puts extra constraints on distances and coinciding rays and axes.

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8 hours ago, ninjageezer said:

have an edgehd 8 but not very suited to my APS-C camera

Why is that?

I was going to recommend EdgeHD 8" as potential candidate - but since you have one, what makes it unsuitable?

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6 hours ago, vlaiv said:

Why is that?

I was going to recommend EdgeHD 8" as potential candidate - but since you have one, what makes it unsuitable?

it causes some outer edge issues with vignetting and poor stars ,apparantly it does not play well with APS-C or full frame cameras....especially when used with the reducer...

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1 minute ago, ninjageezer said:

it causes some outer edge issues with vignetting and poor stars ,apparantly it does not play well with APS-C or full frame cameras....especially when used with the reducer...

well, don't use reducer.

I was also going to ask - why limit to 1000mm.

That is not galaxy FL with APS-C sized sensor. Most galaxies are small enough to fit central 1/5th of sensor or there about at 1000mm.

There will be a lot of sensor "wasted" on surrounding space. 2000mm is quite decent FL - and you don't need reducer for galaxies - again same reason - even without one, there will be still plenty of surrounding space captured for context.

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10 hours ago, vlaiv said:

well, don't use reducer.

I was also going to ask - why limit to 1000mm.

That is not galaxy FL with APS-C sized sensor. Most galaxies are small enough to fit central 1/5th of sensor or there about at 1000mm.

There will be a lot of sensor "wasted" on surrounding space. 2000mm is quite decent FL - and you don't need reducer for galaxies - again same reason - even without one, there will be still plenty of surrounding space captured for context.

thank you 

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1 hour ago, SamAndrew said:

800mm focal length gives you 1" per pixel, going above this won't likely give you any more resolution. 

Wider FoV is good for galaxy clusters.

1600mm also gives 1"/px, so does 2400mm and 3200mm.

I agree that one should not aim for higher than 1"/px, and in reality, most of the time 1.5"/px is enough (which is by the way - what 2000mm gives if one properly treats the data).

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Here is the thing - let's look at it from different perspective.

Some of the largest galaxies are say - like M101 - less than half a degree x half a degree - but most are sub 10 arc minutes in extent.

But for the sake of argument - let's go with 40 arc minutes as the width of the fov (to have some room around the target).

40' = 40 * 60 = 2400"

If we want to image at say 1.5"/px - that is 1600px across.

IMX571 has 6000+ pixels in width - that leaves us with option to bin at least x4 the data and have 1500+ px across.

That is the path I would take - I'd take that EdgeHD 8" - sort out any collimation issues and tilt to get good star shapes across the sensor.

According to Celestron spec sheet - spot diagram should be good for 40mm so 28mm should not be a problem:

image.png.261995f4d0bb16390dcfb531eb1c7d74.png

here is spot diagram out to 28mm for EdgeHD. While not perfect - it is not disaster either. I'd expect round stars in regular or even good seeing conditions. I don't think scope would be capable of 1"/px - as combined spot diagram (across all colors) seems to be bigger than airy disk size - so I'd limit it to say 1.5"/px for very good sharp results if mount / guiding and seeing allows.

I would do split debayering on the data and additional x2 bin on top of that (or just bin x4 regularly debayered data)

 

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6 hours ago, vlaiv said:

1600mm also gives 1"/px, so does 2400mm and 3200mm.

I agree that one should not aim for higher than 1"/px, and in reality, most of the time 1.5"/px is enough (which is by the way - what 2000mm gives if one properly treats the data).

Is there an advantage to binning over picking the FL to give you optimum resolution? if you're binning 4x4 you're adding 16x the read noise and 16x dark current noise (surely that starts to become significant at some point)

The disadvantage is massively reduced field of view, so you will be limiting your composition choices (Leo Triplet a good example, but even M81 and M82 might not fit on an APS-C chip at 2m+ focal length)

Practically there might be some reasons to pick the F10 edge over an F4/F5 newt, but it's pricey.

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12 minutes ago, SamAndrew said:

Is there an advantage to binning over picking the FL to give you optimum resolution?

You don't have to bin - just use camera with larger pixels.

Adding x16 read noise is not as bad as it might seem. It only increases read noise by factor of x4 over regular read noise, so if you have camera with say - 1.4e of read noise - it will act as camera with 6.4e of read noise - but here is the thing - you don't have to do anything about it (many old CCD cameras had read noise that was double that - like 12e-13e of read noise).

Read noise needs to be swamped by background signal right? When you sum your pixels - you also sum background signal as well as read noise (and everything else) - so ratio of read noise to LP signal / background signal remains the same. If you exposed properly for your sky conditions - binning won't change anything.

Advantage is that it is easier to get aperture / faster system at those focal length. Faster in terms of light gathering and not F/ratio. With shorter FL - you need to use F/ratio "faster" system - and that means better optics and more careful collimation - corrective optics must be very good not to add aberrations of their own and so on.

For galaxies - it really does not matter.

M101 and Leo triplet are just two targets - but there are like thousands of galaxies that are very small, so FOV is not very important (if one really wants to have "galaxy" scope).

 

 

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Ill add to the discussion that the exposure times required to have read noise dealt with are likely quite long for the f/10 scope so the mount needs to behave probably for 5-10 minutes in normal operation.

Vlaiv has made good points and suggestions, other than M31, M33, and some galaxy clusters such as the Virgo and Coma clusters you will fit every galaxy in a 2m fl scope and an APS-C chip. That leaves you with dozens of good targets and hundreds of difficult ones.

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5 minutes ago, ONIKKINEN said:

Ill add to the discussion that the exposure times required to have read noise dealt with are likely quite long for the f/10 scope so the mount needs to behave probably for 5-10 minutes in normal operation.

Yep, going after galaxies does require good mount for both resolution and longer exposures.

Above 5-10 minute is correct for darker skies SQM 21 and higher, but for Bortle 4-5 for example (SQM ~20), exposures in 3-4 minute are ok. In heavier LP - you can get by with even shorter.

I used 1 minute exposures on my F/8 scope in Bortle 7 skies (SQM 18.5) with 1600mm FL and 3.8um pixel size.

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