'Galaxy' scope for imaging.. choices

[900SL]

21 hours ago, vlaiv said:

I would think so.

Don't be afraid to bin, and also - do pay attention that you need larger aperture to hit certain pixel scale. I would not go below 1"/px, and even approaching that, you would need 8" or more.

6" is more suited for ~1.4-1.5"/px range as upper limit of sampling rate.

Can I bin a 533MM Pro? Mono camera, 3000 x 3000 pixels at bin 1, so 2x2 bin would bring this down to 1500x1500? With a 150mm F8 newtonian the pixel scale would be around 1.3"/px

With my 90mm f6 I can get 1.4 "/px, so presumably not a lot of point in binning to use the f8 newt at 1200mm?

[vlaiv]

Just now, 900SL said:

With my 90mm f6 I can get 1.4 "/px, so presumably not a lot of point in binning to use the f8 newt at 1200mm?

Point is faster system with 150 F/8.

Both setups will have roughly the same pixel scale 1.4"/px vs 1.3"/px - but 150mm will have much more light gathering surface over 90mm.

That and, if both scopes are of decent optical quality - 150mm will produce ever so slightly sharper image.

[900SL]

Thanks Vlaiv. I'm tempted to take a chance on the Bresser. I have a 10% off voucher, and it won't need a coma corrector. Might make a good lunar/planet/galaxy scope with my existing set up.  It's a long aluminium tube so focus might change a bit over the course of an evening..

[vlaiv]

2 minutes ago, 900SL said:

Thanks Vlaiv. I'm tempted to take a chance on the Bresser. I have a 10% off voucher, and it won't need a coma corrector. Might make a good lunar/planet/galaxy scope with my existing set up.  It's a long aluminium tube so focus might change a bit over the course of an evening..

That's 150mm F/8 newtonian right?

Aluminum has about thermal expansion of about 22um per meter Celsius. You have about a 1.2 meters, so you get something like 26um of expansion for every C.

F/8 system has critical focus zone of about 156um, so you should be good for about 3C change in temperature (give or take - it might be even a bit more, depending where you landed with original focus).

Slower scopes are harder to get knocked out of focus due to temperature change because critical focus zone grows as square of F/ratio.

Btw, Coma free zone of F/8 newtonian is about 5.7mm radius, so 11.4mm diameter circle. That is shy of 16mm diagonal of ASI533, but you'll likely not notice much coma to the edge of the sensor as it is very small at that F/ratio.

[900SL]

On 16/04/2024 at 16:36, vlaiv said:

That's 150mm F/8 newtonian right?

Aluminum has about thermal expansion of about 22um per meter Celsius. You have about a 1.2 meters, so you get something like 26um of expansion for every C.

F/8 system has critical focus zone of about 156um, so you should be good for about 3C change in temperature (give or take - it might be even a bit more, depending where you landed with original focus).

Slower scopes are harder to get knocked out of focus due to temperature change because critical focus zone grows as square of F/ratio.

Btw, Coma free zone of F/8 newtonian is about 5.7mm radius, so 11.4mm diameter circle. That is shy of 16mm diagonal of ASI533, but you'll likely not notice much coma to the edge of the sensor as it is very small at that F/ratio.

Thanks Vlaiv.

I'm trying to see what the downside is here. Surely there must be something I've missed

 

A 150mm f8 is twice as fast (with 2x2 binning) as my current set up (90mm F6 APO and 533MM Pro)

An f8 newt is more tolerant of collimation, has a flatter field and doesn't need a coma corrector on a 533 MM sensor

2x2 binning gives me a pixel scale of 1.3"/px, which suits my likely seeing and is well within my mount and guiding capabilities

What's the catch?

 

 

 

[vlaiv]

3 minutes ago, 900SL said:

What's the catch?

There is no one.

You might argue that the catch is size of the tube and mount requirements.

90mm F/6 apo is very small scope and it's easily mounted. 6" Scope - while not as big of a sail as 8" scope - is still very long and prone to wind issues - it has ~1200mm of length and arm moment is big at those distances, so you need better mount and better mount connection (and possibly fair weather) to image properly.

Some of those issues can be worked around with compact scopes - like 6" F/9 RC - but people are afraid of RC collimation for the most part. Then there is 6" SCT (not sure if there is EdgeHD version - but I'd go with that one for imaging rather than regular + corrector) - and finally 6" CC - which is F/12 so you'll need to bin x3 instead x2 - but with larger sensor that should not be problem either.

[alacant]

On 12/04/2024 at 11:27, 900SL said:

f8 150mm newt is twice as slow as my f6 APO

A 90mm telescope collects more light than a 150mm telescope? Really?

Do a comparison. Take say, m101. Image with each using the same camera and exposure. Now process and  make the same size image. The reality will be immediate.

I too used to think that f ratio decided speed until I saw the results of my 80mm f6 lenses with the guy along side me's 150mm f9 rc. In his you could see the galaxy even on individual frames! 
Cheers

Edited May 23 by alacant

[900SL]

2 hours ago, alacant said:

A 90mm telescope collects more light than a 150mm telescope? Really?

Do a comparison. Take say, m101. Image with each using the same camera and exposure. Now process and  make the same size image. The reality will be immediate.

I too used to think that f ratio decided speed until I saw the results of my 80mm f6 lenses with the guy along side me's 150mm f9 rc. In his you could see the galaxy even on individual frames! 
Cheers

Pixel scale also affects SNR. Just quoting the maths: 

"My simple understanding is that the relative performance of a scope and camera can be approximated by net Aperture squared x pixel scale squared (plus adjustment for losses)

So my 90mm f6 at 540fl with 533MC has objective area of 6360mm2 and pixel scale of 1.4"/p, which gives

6360 x 1.4 x 1.4 = 12900

150mm f8 has net aperture of 15700mm2 assuming 50mm secondary, and pixel scale with 533MC is 0.64'/p

15700 x 0.64 x 0.64 = 6437 neglecting losses at mirrors

So f8 150mm newt is twice as slow as my f6 APO, unless I bin"

Using my mono canera and binning 2x2 should speed it up significantly, and improve pixel scale

Edited May 23 by 900SL

[900SL]

On 09/04/2024 at 12:03, alacant said:

Hoping this reply will also be of use to the OP, if not apologies for the noise and @devs please remove it.

The main issue with the long sw is the awful 1-1/4" focuser. A better choice for imaging would be the  Bresser 150 f8 which comes with a hefty 2-1/2" rack and pinion model. Great for galaxies. Don't cringe the f8 though; you'll find it just as good if not brighter than any 5" refractor.

Oh, and this may be important to the OP. It's only just over 6kg.

Cheers and HTH.

 

Hi A

Do you know if this f8 newt has a parabolic or spherical mirror? I note that Bresser avoid any mention of this, unlike the shorter focal length newts, which are all parabolic. 

There is also a comment from a user on astrobin who has a 150L/1200, who mentions a spherical mirror.

My understanding is you need a higher f number (like f12) to be OK with a spherical mirror

 

[alacant]

59 minutes ago, 900SL said:

parabolic or spherical mirror

Parabolic

[900SL]

11 hours ago, alacant said:

Parabolic

Interesting. Bresser just replied to my query to confirm they use spherical mirrors in the f8 scopes, which presumably is why they don't state parabolic in the technical listing

[Ags]

On 09/04/2024 at 21:05, Clarkey said:

I think I must be the only person on SGL that likes RC scopes. I'll be burnt as a heretic next! 🤣

For the record, I love my RC6. Collimation was simple in the end , just use a camera and defocus a star. 

[alacant]

11 hours ago, 900SL said:

spherical mirrors

That's changed recently then. What's the difference between a sphere and a parabola at 1200mm I wonder...
Here are a few dslr images:

      

[900SL]

This is a good thread on the relationship between f ratio, focal length, mirror type etc and a quick glance seems to indicate that spherical at f8 is 'ok', not quite as well corrected as a parabolic but close enough. Which backs up what Bresser said in reply to my query

https://www.cloudynights.com/topic/275394-f8-parabolic-vs-f8-spherical/

 

[alacant]

The parabolic models are still available:
https://astrotelescopios.com/products/telescopio-nt-150l-1200-ota

IIRC, the spherical model with the flimsy focuser -and plastic tube?- are the ones shipped with a mount:
https://www.astrocity.es/telescopios-bresser/640-messier-nt-150l-1200-exos1-eq4.html?search_query=nt150l&results=3

HTH

EDIT** Sorry. It looks to be the other way around. Parabolic, aluminium tube, hexagon focuser: 
https://www.bresser.de/es/Astronom-a-bresser/Telescopios/BRESSER-Messier-NT-150L-1200-EXOS-2-GoTo-Telescopio.html

Edited 8 hours ago by alacant
vínculo

[900SL]

21 hours ago, alacant said:

The parabolic models are still available:
https://astrotelescopios.com/products/telescopio-nt-150l-1200-ota

IIRC, the spherical model with the flimsy focuser -and plastic tube?- are the ones shipped with a mount:
https://www.astrocity.es/telescopios-bresser/640-messier-nt-150l-1200-exos1-eq4.html?search_query=nt150l&results=3

HTH

It was this one:

https://www.bresser.de/en/Astronomy/Telescopes/BRESSER-Messier-NT-150L-1200-Hexafoc-Optical-Tube.html

You will note they do not mention mirror type, so I raised the query. Some resellers do say this is parabolic, but that would appear to be incorrect. 

Edited 13 hours ago by 900SL

[ollypenrice]

I don't think you'll find much difference in resolution between 0.9"PP and 1.3"PP. I had both from a pair of TEC 140s on a dual rig, with a guide RMS of around 0.33 arcsecs. The cameras had different pixel sizes. I used the smaller pixels for the luminance but, quite honestly, I never found any obvious difference in resolution. I never did a meticulous comparison but a casual one was distinctly underwhelming, even when shooting Ha in both barrels.

Have you considered the Mn190?

Olly

[Peter Drew]

A Newtonian parabola is no guarantee of improved performance over a sphere at F8 in a relatively entry level telescope.  Parabolic accuracy can range between 1/2 and 1/20 wave depending on how much you are prepared to spend.  A "cheap" parabola may also include a turned edge, zones or surface roughness, a good sphere which is potentially easier to produce would likely outperform a so-so parabola.     🙂

[900SL]

18 minutes ago, Peter Drew said:

A Newtonian parabola is no guarantee of improved performance over a sphere at F8 in a relatively entry level telescope.  Parabolic accuracy can range between 1/2 and 1/20 wave depending on how much you are prepared to spend.  A "cheap" parabola may also include a turned edge, zones or surface roughness, a good sphere which is potentially easier to produce would likely outperform a so-so parabola.     🙂

Thanks Peter

[900SL]

36 minutes ago, Peter Drew said:

A Newtonian parabola is no guarantee of improved performance over a sphere at F8 in a relatively entry level telescope.  Parabolic accuracy can range between 1/2 and 1/20 wave depending on how much you are prepared to spend.  A "cheap" parabola may also include a turned edge, zones or surface roughness, a good sphere which is potentially easier to produce would likely outperform a so-so parabola.     🙂

Peter, is it correct that a spherical f8 newt has no coma, but some minor spherical abberation as a result of the sphere? 

[Peter Drew]

That is my understanding.    🙂

[vlaiv]

19 minutes ago, 900SL said:

Peter, is it correct that a spherical f8 newt has no coma, but some minor spherical abberation as a result of the sphere? 

No, off axis aberrations remain with spherical mirror as well.

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm