90mm f6 or 102mm f7

[900SL]

I was about to push the button on the 90mm f6 TS CF APO when TE dropped the prices on some of their other scopes yesterday. The 102mm FPL55  CF APO f7 is now only 200E more. 

So, decision time:

Am I correct in assuming the 90mm with a 0.8 reducer (f4.8) will image 50% faster than the 102mm (f5. 6) on the same camera (533MC Pro) for wide field objects?  Or does the increased aperture somehow offset this with the 102?

The pixel scale of the 90mm is OK at 1. 8"/px, I think the framing suits the 533MC on the usual DSO wide field targets and I can use my existing 6Aiii field flattener / reducer so I'm pretty set on the 90, but the 102 is a good price right now and might be handy farther down the road. 

 

 

[vlaiv]

2 minutes ago, 900SL said:

Or does the increased aperture somehow offset this with the 102?

Larger aperture does collect more light - but longer focal length spreads that light over larger area.

If you don't do anything to offset this (like change pixel size by using different camera or binning pixels) - then 90mm at F/4.8 will be faster than 102 at F/5.6

but it will also have different sampling rate - namely 1.8"/px vs 1.35"/px

At those aperture sizes (around 4") - I would prefer closer to 2"/px than to 1"/px - so 90mm seems like logical choice both in terms of speed and in terms of sampling rate.

 

[900SL]

Thanks Vlaiv. That swings it. Faster is better given my current location, and seeing likely to be poorer as I'm close to the sea. Best regards

Edited December 4, 2022 by 900SL

[Mr Spock]

If you've been following the thread on the StellaMira 90mm f6 you'll see the kind of results you can get.

[900SL]

5 hours ago, Mr Spock said:

If you've been following the thread on the StellaMira 90mm f6 you'll see the kind of results you can get.

Just spent some time looking through the thread. Some great images in there. 

 

[Moonlit Knight]

7 hours ago, 900SL said:

Faster is better given my current location, and seeing likely to be poorer as I'm close to the sea. Best regards

Really, is that a thing?

I live near the coast myself, near enough to be able to gather seaweed for the garden and walk back with it anyway.

[vlaiv]

2 minutes ago, Moonlit Knight said:

Really, is that a thing?

I live near the coast myself, near enough to be able to gather seaweed for the garden and walk back with it anyway.

It is, but it also depends on rest of geography.

Sometimes being near the sea can be really beneficial for seeing, rather than cause poor seeing. Some of the best observatories are situated at vulcanic islands.

In these cases large area of ocean makes air flow rather laminar in nature (smooth) - which gives very nice seeing.

Shallow see next to large land masses on the other hand is acting like heat source. It causes air to swirl. Especially if there is ocean current passing near by. That causes poorer than otherwise seeing.

[Louis D]

I've always associated being near the coast with excellent seeing based on the reputation of the Winter Star Party in the Florida Keys for having really stable seeing conditions..

[900SL]

[900SL]

15 hours ago, Moonlit Knight said:

Really, is that a thing?

I live near the coast myself, near enough to be able to gather seaweed for the garden and walk back with it anyway.

I'm now based in Helsinki, it's at sea level and pretty high humidity so I assumed the seeing would be poor. It's good to hear that is not the case. 

Now waiting for the clouds to go.. maybe January 🌧️🌧️🔭

[newbie alert]

Why do people see the f-ratio first.. I see it as a 90mm scope with a 540 FL.. or a

102mm with a 714mm FL

F ratio is relative to the Fl divided by the aperture... Since you can't enlarge the aperture you can only shorten the FL to give yourself a lower f ratio or in the case of planetary increase the FL with a Barlow which gives you a higher f ratio

[900SL]

3 hours ago, newbie alert said:

Why do people see the f-ratio first.. I see it as a 90mm scope with a 540 FL.. or a

102mm with a 714mm FL

F ratio is relative to the Fl divided by the aperture... Since you can't enlarge the aperture you can only shorten the FL to give yourself a lower f ratio or in the case of planetary increase the FL with a Barlow which gives you a higher f ratio

I'm still getting my head round the differences between telescopes and lenses, coming from a photography background. I'm mainly into imaging of DSO nebulas, so 'faster' is better for a given focal length and sensor as I understand it, up to a limit. 

The relationships between aperture, focal length, pixel size, resolution, seeing etc requires a fair bit of study. In fact, after reading to 20 odd pages on Cloudy Nights on this subject, the one conclusion I've come to is that there are twenty different opinions on the subject and trying to wring out the truth is somewhat challenging.

Is there a math based illustrated primer that covers the whole subject anywhere?

 

 

Edited December 5, 2022 by 900SL

[wongataa]

38 minutes ago, 900SL said:

I'm still getting my head round the differences between telescopes and lenses, coming from a photography background.

There isn't anything different between them in the numbers that specify them.  A telescope is a type of lens.  Aperture and focal length are properties of a lens/telescope that mean exactly the same thing for both of them.  Your photographic knowledge applies to telescopes.

Things like seeing also apply to daytime photography but is far less important so no one really cares about it there.

[vlaiv]

21 minutes ago, wongataa said:

There isn't anything different between them in the numbers that specify them.  A telescope is a type of lens.  Aperture and focal length are properties of a lens/telescope that mean exactly the same thing for both of them.  Your photographic knowledge applies to telescopes.

Things like seeing also apply to daytime photography but is far less important so no one really cares about it there.

Problem is that "photographic" knowledge is limited in its context.

Most who have photographic background will assert that say F/5 telescope is faster than F/10 telescope, but in reality:

F/5 can be faster than F/10

F/5 can be equally fast to F/10

F/10 can be faster than F/5

There is important part of equation missing when you only mention F/ratio. This is deliberately done in daytime photography, because most talk about F/ratio of objective lens assumes you'll be using it on the same or similar camera.

In astrophotography - we have a luxury of large range of different sensors and thus pixel sizes, as well as binning - which can make all the difference.

[pipnina]

8 minutes ago, vlaiv said:

Problem is that "photographic" knowledge is limited in its context.

Most who have photographic background will assert that say F/5 telescope is faster than F/10 telescope, but in reality:

F/5 can be faster than F/10

F/5 can be equally fast to F/10

F/10 can be faster than F/5

There is important part of equation missing when you only mention F/ratio. This is deliberately done in daytime photography, because most talk about F/ratio of objective lens assumes you'll be using it on the same or similar camera.

In astrophotography - we have a luxury of large range of different sensors and thus pixel sizes, as well as binning - which can make all the difference.

I think a prime example of this effect is seen with the HST and JWST. They might take a half hour integration and have a nice clean result, while being f22! Meanwhile my f5 130mm telescope on the ground would take several hours to approach that level of SNR. Difference being that those scopes have 85mm square (not diagonal) sensors onboard, whereas I only have a 24mm width sensor.

Maybe at that size aperture has something to do with it, and for sure they have the whole being in space advantage which probably works in their favour rather strongly, but I was shocked to discover that both of those space telescopes were above f20, which is often considered unusably slow for many normal photographers as that aperture reduction tends to soften the image and usually lets in too little light.

[vlaiv]

1 minute ago, pipnina said:

Maybe at that size aperture has something to do with it,

Aperture is the key at any size.

If you fix arc second per pixel (sampling rate) - aperture dictates speed

[ONIKKINEN]

18 hours ago, 900SL said:

This place will likely have good seeing once/if the shallows close to shore freeze solid. Like it is now there will be a little thermals rising during the night so low elevation targets are probably not worth the effort.

If its windy it might be good anyway. Not a bad horizon at all really.

[newbie alert]

To change the f ratio to the scope you either have to change the aperture ( not possible to increase the size but some decrease this with solar imaging)

Or increase the fl with a Barlow or decrease with a reducer

With camera lenses you can increase or decrease the aperture blades to change the f ratio as the focal length is fixed unless it's a zoom lens

[newbie alert]

2 hours ago, 900SL said:

I'm still getting my head round the differences between telescopes and lenses, coming from a photography background. I'm mainly into imaging of DSO nebulas, so 'faster' is better for a given focal length and sensor as I understand it, up to a limit. 

The relationships between aperture, focal length, pixel size, resolution, seeing etc requires a fair bit of study. In fact, after reading to 20 odd pages on Cloudy Nights on this subject, the one conclusion I've come to is that there are twenty different opinions on the subject and trying to wring out the truth is somewhat challenging.

Is there a math based illustrated primer that covers the whole subject anywhere?

 

 

Depends on how complex you want to make it, .. different rules to what type of imaging you want to do ... Seeing is the limit that you have no control over , light pollution is another one that you can't control unless you change location ( it has an effect if you're taking long exposure deepsky images but if you're planetary imaging its not the same problem)

You can drive yourself completely gaga in the world of f-ratio and sampling.. or you can just get on with what you have

 

[vlaiv]

2 hours ago, 900SL said:

The relationships between aperture, focal length, pixel size, resolution, seeing etc requires a fair bit of study. In fact, after reading to 20 odd pages on Cloudy Nights on this subject, the one conclusion I've come to is that there are twenty different opinions on the subject and trying to wring out the truth is somewhat challenging.

Opinions are not relevant.

Facts are on the other hand straight forward, no ambiguity there.

[900SL]

4 hours ago, ONIKKINEN said:

This place will likely have good seeing once/if the shallows close to shore freeze solid. Like it is now there will be a little thermals rising during the night so low elevation targets are probably not worth the effort.

If its windy it might be good anyway. Not a bad horizon at all really.

I've been looking at the car park south of Kirkkonummi that you recommended @Onikkinen on Google maps. Only 40 mins away. You set up near the car park? When the clouds clear and assuming it's not -20c I'll give it a go

[ONIKKINEN]

1 hour ago, 900SL said:

I've been looking at the car park south of Kirkkonummi that you recommended @Onikkinen on Google maps. Only 40 mins away. You set up near the car park? When the clouds clear and assuming it's not -20c I'll give it a go

I set up on the car park as i cant carry the 60kg mountain of stuff far anyway. Not much traffic after dark and most of that is other astronomers. SQM 21 or slightly better skies to anywhere but north/northwest where the Helsinki nebula is still bright. South is obstructed to 25 degrees or so, which means M42 is not visible here.

I have another common spot i use to the north: https://www.taivaanvahti.fi/locations/show/120

Since that is north of Helsinki it can be used to image northern targets better than the Kirkkonummi one. This is a less windy spot too, but not by much. Have had good seeing here a couple of times too. Also is a carpark so winter friendly.

By the way since clouds have a warming greenhouse effect, once they go away the weather turns cold really fast. Shouldnt expect to have warmer than -10 clear nights befire spring and should expect to get many -20 nights in January-March so best get used to it if you want to image at all 😬.

[900SL]

I ordered the CF 90 in the end, after going around the houses with the FLT91 (700 euro more expensive!), TS 102 CF, 106 photoline, 115 photoline, and a raft of others

Pixel scale is good, but what swung it is the framing and speed. Plus I can use my WO 6Aiii 0.8 reducer flattener

Of course, now we have been hit by a blizzard and -20 wind chill, so it will be a while before first light 😂

 

[Louis D]

7 hours ago, 900SL said:

Of course, now we have been hit by a blizzard and -20 wind chill, so it will be a while before first light

Well, you do live just 6 degrees latitude south of the Arctic Circle, so you're sort of asking for it. 😉

I live just 7 degrees latitude north of the tropics, and I'm fully aware of how hot it gets here for 8 months of the year.  It makes for very hot and muggy nights of observing.  When we do get a blizzard as in Feb. 2021, we shut down entirely and lives are at risk due to hypothermia.

7 hours ago, 900SL said:

I ordered the CF 90 in the end, after going around the houses with the FLT91 (700 euro more expensive!), TS 102 CF, 106 photoline, 115 photoline, and a raft of others

I'm sure you'll enjoy your CF 90.  I really like my TS-Optics 90mm FPL-53 Triplet APO.  After actually handling it, I'm glad I didn't go any bigger as it's quite dense.  My DSV-2B mount has zero issues with it, though.

[900SL]

I'm looking forward to the postman arriving Louis. I asked TS to check collimation, focuser and alignment before dispatching and that takes a week at the moment so unlikely to receive before Xmas.

Were it not for Brexit, I would have gone for the Stellamira 90 but that's the way it goes these days