Help choosing a longer focal length??

[Skipper Billy]

My current setup is a Tak 106 paired with an Atik16200 mono and I have enjoyed wide field imaging for several years

For next season I am thinking about moving to a much longer focal length and possibly dabbling with a colour CMOS camera to attack the smaller nebulae and some galaxies.

I am somewhat constrained by the size of my obsy and I will need something fairly compact like an SCT or RC.

Weight isnt an issue - it will be perched on a Mesu

Budget isnt an issue.

It must be abe to accept an electronic focuser.

Are there any 'golden combos' ???

I have been casually looking at the 9.25 Edge HD with its 0.7 reducer and the ZWO 6200 

Happy to listen to any suggestions!!

 

Edited March 28, 2022 by Skipper Billy
Typos

[Stuart1971]

I think the RASA’s are very popular and great for the type of target you want to image…👍🏼

[vlaiv]

45 minutes ago, Skipper Billy said:

I have been casually looking at the 9.25 Edge HD with its 0.7 reducer and the ZWO 6200 

Maybe best way to approach it would be to set your working resolution first?

ZWO 6200 has small pixels at 3.76um, and odds are that you'll have to bin that camera extensively with long focal length instrument.

Say you opt to go with 1.2"/px, Your base focal length is 646mm

This means you need to go in increments of that:

Bin2 1292mm

Bin3 1938mm

Bin4 2584mm

C925 with reducer is smack in the middle of that with 1645 - neither here nor there.

Why don't you just go with 9.25 Edge HD without reducer? That is better option. With Bin x4 it will give you 1.32"/px and if you are feeling lucky - x3 will give you 1"/px (you do have mesu after all).

If you are tempted to get reducer to speed up things - don't.

Best way to speed up things is to find your wanted working resolution / sampling rate - like say 1.2"/px and then throw as much aperture as you can that will be well corrected for 43mm and will fit in your obsy.

I'd rather use something in 12" RC - 16" RC range than 9.25". Maybe look at CCF and Officina Stellare scopes since money is not issue - these seems to be better built with less issues than GSO RC scopes.

https://www.teleskop-express.de/shop/product_info.php/info/p13582_TS-Optics-RC12-Pro-304-mm-f-8-Ritchey-Chretien-Telescope---Mechanics-by-CFF.html

In any case - 12" RC will have 2400mm of focal length - so same as Edge HD 9.25 - so same sampling rates for bin factors - but it gathers x1.7 more light.

Another option would be for example this scope:

https://www.orionoptics.co.uk/product/odk14/

2380mm of focal length and 14" of aperture. Seems to be well corrected for 52mm diagonal.

Remember - you'll be using 4 x 3.76 = 15um effective pixel size - so these spot diagrams are even smaller compared to pixel size then the image suggests.

 

[vlaiv]

2 minutes ago, Stuart1971 said:

I think the RASA’s are very popular and great for the type of target you want to image…👍🏼

Rasa is very poor choice for small targets as it is very short focal length and far away from diffraction limited optics.

[Skipper Billy]

Thanks @vlaiv - I only mentioned the 6200 as I have read good reviews but I realise its probbaly not the best choice.

I am limited to a scope of 9" max due to the design of my oby. I would love a 12" - 14" RC but that will have to wait for obsy v2!

 

 

[vlaiv]

18 minutes ago, Skipper Billy said:

I only mentioned the 6200 as I have read good reviews but I realise its probbaly not the best choice.

That camera would be my choice for large aperture (and focal length) scope.

I don't really care much that it has small pixels - there are really no downsides to binning it - even 4x4. You can think of it like this:

Do you know full frame CCD camera that has 15um pixel size, ~90% QE, 6e read noise (with 1.5e read noise and bin 4x4 - you get effective read noise of 6e with cmos) that costs $4000?

In any case - I stand by my advice from above - figure out realistic sampling rate for small targets, figure out focal length and bin factor to get that sampling rate with 3.76um pixel size and get the largest aperture at that focal length that is corrected over 43mm field and that you can effectively mount (given your mount and obsy limits).

[DaveS]

I have to say that I do like the ODK 12 . When I was at Astrofest I visited the OOUK stand and had a chat with John after looking at the spot size data, which made it look like the 12 was the sweet spot. He agreed, and said they had computed the 12 originally and scaled the others from that. I promptly put down a deposit there and then!

Now I just have to quell a ruinously expensive hankering after a FLI 4040  .

[davies07]

On 28/03/2022 at 14:18, DaveS said:

I have to say that I do like the ODK 12 . When I was at Astrofest I visited the OOUK stand and had a chat with John after looking at the spot size data, which made it look like the 12 was the sweet spot. He agreed, and said they had computed the 12 originally and scaled the others from that. I promptly put down a deposit there and then!

Now I just have to quell a ruinously expensive hankering after a FLI 4040  .

Forgive me, but I wouldn't go for a OO UK ODK 12. The optics are nice but the mechanics leave a lot to be desired. A friend of mine has had nothing but pain from one his12"ODK. They are 'collimated' on assembly and should you dismantle the primary mirror assembly - say to clean the mirror - then the chances are that you won't be able to collimate the scope on reassembly. There is no facility to adjust the position of the primary mirror. Moreover, the method of attaching the primary mirror is a quite primitive mechanical arrangement and if you are really unlucky - as was my friend - the primary mirror might shift sideways slightly due to poor machining and fit of the central mirror support in the hole in the mirror.

My own recommendation would be one of the newer GSO 10" RCs, such as the FLO's StellalLyra giving a focal length of 2000mm. Anything bigger would be a real handful to manage. Stay away from the older design Mk1 RCs such as the RC8 and older RC10s and 12s; collimation of these is possible but a challenge.

For a camera, one of the modern 16-bit CMOS cameras would suit but, as already mentioned, you would need to bin 2 x 2 or 3 x 3, to bring the image scale to around 1 arcsec/px. For that reason I would go for a mono camera which can be binned in hardware. That then leaves the question of filters; you would need to spend some money on these to get filters with good colour cutoffs and good matching of green and blue gains across 500nm, the OIII waveband.

David

[vlaiv]

48 minutes ago, davies07 said:

For that reason I would go for a mono camera which can be binned in hardware.

Only difference as far as data goes is level of read noise.

With modern low read noise CMOS cameras - even if you bin x4-x5 you will still be at or below read noise levels of CCD sensors (say 1.7e read noise binned x5 will produce equivalent pixel with 8.5e read noise - that is still in CCD territory).

Binning in software allows you to decide on bin factor depending on conditions of the particular session and you can choose to utilize "split" bin rather than regular bin - which produces ever so slightly sharper results (removes pixel blur) if image is properly sampled.

[DaveS]

QHY600 could be quite nice, bin 2x2 for a slightly dodgy 0.73"/px, or 3x3 for a more sensible 1.1"/px. But I don't like the 3:2 ratio, and my 2" mounted filters would vignette.

No, the QHY 461 is *not* a sensible option.

[iapa]

On 28/03/2022 at 12:13, Skipper Billy said:

My current setup is a Tak 106 paired with an Atik16200 mono and I have enjoyed wide field imaging for several years

For next season I am thinking about moving to a much longer focal length and possibly dabbling with a colour CMOS camera to attack the smaller nebulae and some galaxies.

I am somewhat constrained by the size of my obsy and I will need something fairly compact like an SCT or RC.

Weight isnt an issue - it will be perched on a Mesu

Budget isnt an issue.

It must be abe to accept an electronic focuser.

Are there any 'golden combos' ???

I have been casually looking at the 9.25 Edge HD with its 0.7 reducer and the ZWO 6200 

Happy to listen to any suggestions!!

 

No room for a 10"f4 quattro

 

[iapa]

On 28/03/2022 at 13:18, Stuart1971 said:

I think the RASA’s are very popular and great for the type of target you want to image…👍🏼

Thing is, even the 11" RASA FL os only about 620mm

[Stuart1971]

1 hour ago, iapa said:

Thing is, even the 11" RASA FL os only about 620mm

Yes, my mistake I totally got it all the wrong way around….👍🏼

[wornish]

I have a Celestron 925HD Edge bought from FLO  just over a year ago.

It's not getting any use and is in virtually "as new" condition.

I have struggled with the long focal length on my AZEQ6 mount as I need to set up every time.

It's a great scope but I now realise its beyond my capabilities

I have only used it for Lunar and a few Planetary images a handful of times. See my posts on here.

Its spends all its days inside in my study wrapped up.

It deserves a good home on a good mount!

Open to a sensible offer if you are interested.

Edited April 4, 2022 by wornish

[jaspalchadha]

Yup Stay away from Orion Optics UK, I brought a 14 inch ODK and it performs like a Svbony!! The primary has tilt etc etc 

[ollypenrice]

Although I wouldn't have said this a few years ago, I'd now consider a flat field SCT very seriously.  The RCs are better on paper but they can be a nightmare to collimate, I suspect because of variability in the machining and mechanical side. I had a totally unsuccessful time trying to help one of my robotic clients with his and, in the end, we gave up. It did not behave as described on a wealth of different internet sources.

What's changed recently lies in the processing: my objection to the SCT was its tendency to produce large stars in DS imaging but with modern star removal/replacement processing this needn't be the issue it was. The real positive is the ease with which the SCT can be collimated, thanks to its spherical primary, and the ease of long term maintenance with a sealed tube.

I concur with those members above who advise against OO UK. Feel free to PM if you wish.

Olly

[dan_adi]

When I wanted more focal length, I purchased a 12 in SCT and put it on a Mesu 200 mount. Without going through all the story, I got rid of the SCT and got a longer focal length refractor from CFF. It is plug and play, no hassle, great optics. Last summer I finally started imaging with the 8 inch refractor, after 2 years  optimizing/tinkering the SCT. I can't wait for the weather to calm down and start imaging again  

My recommendation: stick to refractors. If the cost is prohibitive, then at least try to get a 10-12 inch RC from CFF or one of those 12 inch Planewave CDKs. 

The image below is a test image with a 1300 mm FL , 8 inch refractor. Target is Abell 2218, around 40 hours exp time in Luminance. Just some DBE and stretch, nothing fancy yet ... much to learn in PI .  There are lots of awesome small galaxies and by the the end of summer I hope some gravitational arcs in the middle

Edited May 2, 2022 by dan_adi

[Skipper Billy]

Thanks for the info dan_adi but I am severely space limited - it has to fit in a 4' x 4' box!

My current setup just fits.

 

 

[dan_adi]

1 hour ago, Skipper Billy said:

Thanks for the info dan_adi but I am severely space limited - it has to fit in a 4' x 4' box!

My current setup just fits.

 

 

Then maybe it's time for upgrading to a obsy 2.0 in order to make a little more room for a new scope. My refractor barely fits in my obsy too, but I don't see myself upgrading from a 8 inch frac to a 10 inch one, although one can dream :).

[Skipper Billy]

22 minutes ago, dan_adi said:

Then maybe it's time for upgrading to a obsy 2.0 in order to make a little more room for a new scope. My refractor barely fits in my obsy too, but I don't see myself upgrading from a 8 inch frac to a 10 inch one, although one can dream :).

Our business is for sale and when it sells we will move back to England and Obsy V2 whill be much bigger and able to house something very tasty!!! 😉 

[gorann]

I stumbled on this thread and may be a bit late. As @ollypenrice pointed out don't rule out a 12 or 14" SCT, especially if you get an Obsy v2 that could accommodate one. I would suggest the Meade ACFs with a proper focuser added to the back. Then you can have the primary permanently locked and no mirror movement to worry about. The Meade ACF (in contrast to EdgeHD) are very forgiving about sensor distance since they achieve the flat image with the corrector plate and mirror rather than a set of correcting lenses (Edge HD) that demand a rather short and precise back focus. And as also pointed out by Olly, it is a sealed tube so no worries about dust on the mirrors like in RCs. As you see in my signature I have both types of SCTs, and the EdgeHDs are also lovely scopes, and have the advantage of lower weights but in a permanent obsy that is not an issue. Both brands can at least fill a full frame (24x36) sensor with photons. Here is an example of what my old 14" Meade can achieve:

https://www.astrobin.com/kqe06r/C/

 

Edited August 21, 2022 by gorann

[Skipper Billy]

Tack Goran.

Useful info to keep up me sleeve - I have decided to wait for Obsy MkII before upgrading as we are likely to move house in the next 18 months.

Tack sa mycket !

[gorann]

1 minute ago, Skipper Billy said:

Tack Goran.

Useful info to keep up me sleeve - I have decided to wait for Obsy MkII before upgrading as we are likely to move house in the next 18 months.

Tack sa mycket !

I trust it will be a move to a dark site! Fortunately I have a farm with space for more than one obsy (I am just finishing my 4th) so my 14" Meade has its own obsy and most nights its stays closed unless there is really great seeing. On average-seeing nights I will use my shorter FL scopes (RASA8 or Esprit 150) in the other obsies. So, unless you find a site with really steady skies I think that only having a 2-3 m FL scope could be a bit frustrating. My dual RASA8-rig (400 mm FL) does not care about seeing at all. Only clouds can stop it.

I am impressed by your Swedish - I hope it was not just google translator😄

[Skipper Billy]

Thanks Goran

Its likely to be a Bortle 2 -3  site - or I am not going!!!

Our son married a Swedish woman and they live in Alingsas - all our grandkids are Swedish so I have a smattering!

 

[Adam J]

If I was able to get a huge scope for small targets I would look at this camera to go with it an go with short exposures. 

https://www.firstlightoptics.com/zwo-cameras/zwo-asi-432mm-usb-3-camera.html

https://www.firstlightoptics.com/stellalyra-telescopes/stellalyra-10-f8-m-lrc-ritchey-chrtien-carbon-truss-telescope-ota.html

Something like the above.

 

Adam