'Galaxy' scope for imaging.. choices

[900SL]

Not necessarily a full galaxy scope, but I'm looking for something with a longer focal length, so my M51's and similar sit a little larger in my FOV

I currently have a 90mm f6 APO @ 540mm, and an SXD2 mount, 14 kg max imaging capacity

Currently have 533 MM / MC Pro cameras so looking at 750mm to 1000mm FL for my typical seeing and pixel scale and guiding

After a week of wandering the web, I think my options boil down to:
 

APO:

130mm f7 TS Photoline APO. 930mm FL, FPL53
Cost 2000E (I can use an existing TSFLAT2 as a 1.0 flattener)
Pro's: Reportedly good
Cons: Slower than a Newt, expensive-ish and 9 kg long OTA so towards the top end of the mount capabilities

Newt:

8" f5 Clone Newt, 1000mm FL
Cost 500E plus another 850E to cover a coma corrector and farkles/tools to get it up to scratch for imaging, say around 1500E all in
Pro's: Aperture, speed, cost
Cons: Collimation, weight and susceptibility to wind. Again at top end of the mounts capabilities
 

RC:

8" Carbon RC, FL 1600mm reduced to 1050 mm
Cost 1500E inc. reducer and collimation tools
Pro's: Weight, cost, speed, manageable size
Cons: Collimation, Collimation, Mirror/focuser interaction and tilt, plus I've not been that impressed by images I've seen

SCT:

Edge 8", 2000mm reduced to 1400mm  
Cost 2500E inc. 0.7 reducer
Pro's: Weight, flexibility
Con's: slower at f7, Dew magnet, oversampled, and I am unlikely to use a hyperstar

 

I've pretty much dismissed the SCT, mainly due to cost. The RC piqued my interest but as far as I can tell these can be a pig to set up right due to the focuser / back plate / mirror interaction.

Which leaves the Newt and the APO. The speed of a Newt appeals, the FL is spot on for my circumstances, but it may overtax my mount (SXD2)

The 130 mm Photoline also appeals, because frac but it's more than a stop slower than the Newt

Anything I've missed in my considerations?

 

 

[Clarkey]

I'll give my opinion - but it is only that....

APO - yes they are good but pretty large for the mount. I had a 130mm photoline but had to sell it as it did not quite fit my mini observatory. I was planning on putting it on the AZ-EQ6. I think 'speed' for galaxies is slightly less of an issue due to the concentrated brightness. 

RC - great scope for the money and my main galaxy scope for the last 3 years. It will almost certainly need a new focuser (I use a Steeltrack). Mine is reduced to F6 and I bin2 which gives pretty good results. I have been lucky with mine as the mirror optical axis seems pretty close to that of the focuser, so collimation is not too much of any issue. Also, it only needs doing occasionally.

Newt - As you say, lots of scope for the money, but a little more 'faff'. I am currently considering an F4 250mm to give me a fast a system as I can currently afford. Again, you may well need a new focuser. The standard Baader MPCC or Skywatcher coma correctors are fine at F5 so not too expensive.

SCT - I have no experience, but your comments seem valid enough.

One other possible option I'll throw in the mix is the 115mm refractor. I have the Altair version with FPL51, but the correction an imaging is excellent. Slightly lighter for the mount. I use it with a bog standard 1x field flattener from Stellamira which gives good results. There is of course an FPL53 version which would be better in theory. (I now combine my RC8 and 115mm for galaxies)

HTH's.

[WolfieGlos]

I think I posted about this last year, and I think it might have been clarkey in fact who recommend an RC! For me, it would have been a 6" due to my HEQ5, but I did find this 0.67x reducer which might be of interest. Essentially, the 8" RC you might have considered would become f/5.3 and reduced to 780mm. I ended up going for the Starfield 102, less focal length but stuck with the frac for no collimation. I have no regrets with this, other than the loss of focal length to be honest.

But, lately I've been considering a Sky-watcher 150PL. Although at f/8 it is relatively slow, 1200 focal length for £149 (currently on offer, down from £189) seems a very good option. Reviews suggest it'll work on a HEQ5 (I haven't been able to locate the weight of the OTA, but it is available with an EQ3). The only downside, is that I cannot locate many images on astrobin, which perhaps suggests it might not be suitable for imaging. Those that are there, are pretty good for short integration DSLR images.

Failing that, a Seestar might be of interest 😉

HTH.

[Elp]

When I just had my Z61 I first went with a 130pds as a second scope but the FL wasn't that much different and I didn't accept that modifications would need to be done to it to get it into a prime imaging spec. So I wanted more FL, so I went with a C6. The PDS soon went out the door. In my case the prime reason was so I could use a HS with it. So now it's kind of a do all scope, visual native 6 inch aperture and 1500mm FL, plantery imaging with that too, F6.3 1000mm DSO imaging, and F2 300mm fast imaging. Once I used the HS no other system is good enough for me now, I'd attribute it to the aperture increase also so I can only imagine what a RASA 8+ would be like. I've done comparisons to F6.3, the latter does provide a lot lot more resolution but it also takes so so much longer to get similar intensity signal as it's spread out across more pixel area. With the weather I wouldn't make much progress at F6/7 with my other scopes on a lot of targets where imaging time is in precise small windows of opportunity.

If I could get an apo refractor which could do the same that would be my choice, but A it wouldn't be cheap and B it'd be large. I like to keep to a particular weight class also (sub 10kg all loaded), so at a push I'd likely do it again with a RASA 8 or for more flexibility an Edge HD8. My current upgrade refractor choice would be an Askar 120 triplet.

Newts do give good bang for buck but are large in box volume so a bit more faff to setup and shield from wind vibration as they act like sails, RCs tend to be heavy (if you've got the mount plus sturdy tripod for either then you're okay).

For those small targets you haven't really got much choice, you need the aperture your seeing and mount can deliver if you want the resolution and the FL, you do have some control over the speed based on your camera, binning, reducers etc, as well as the initial F ratio spec.

Edited April 8 by Elp

[IDM]

Last year I bought a C11 for planets which had a Celestron dew ring heater and it has been really successful in controlling dew. Indeed I have been using it without a dew shield with no problems. Just the other week I picked up a C8 with an F6.3 flattener/reducer for galaxies. I have only tried it once (I just posted my M51 image in the DSO imaging section). I haven’t sorted all its issues particularly with the reducer but it seems a good scope. I bought my C8 secondhand it was cheap and it’s quite light.

cheers

Ian

[900SL]

13 hours ago, Clarkey said:

I'll give my opinion - but it is only that....

APO - yes they are good but pretty large for the mount. I had a 130mm photoline but had to sell it as it did not quite fit my mini observatory. I was planning on putting it on the AZ-EQ6. I think 'speed' for galaxies is slightly less of an issue due to the concentrated brightness. 

RC - great scope for the money and my main galaxy scope for the last 3 years. It will almost certainly need a new focuser (I use a Steeltrack). Mine is reduced to F6 and I bin2 which gives pretty good results. I have been lucky with mine as the mirror optical axis seems pretty close to that of the focuser, so collimation is not too much of any issue. Also, it only needs doing occasionally.

Newt - As you say, lots of scope for the money, but a little more 'faff'. I am currently considering an F4 250mm to give me a fast a system as I can currently afford. Again, you may well need a new focuser. The standard Baader MPCC or Skywatcher coma correctors are fine at F5 so not too expensive.

SCT - I have no experience, but your comments seem valid enough.

One other possible option I'll throw in the mix is the 115mm refractor. I have the Altair version with FPL51, but the correction an imaging is excellent. Slightly lighter for the mount. I use it with a bog standard 1x field flattener from Stellamira which gives good results. There is of course an FPL53 version which would be better in theory. (I now combine my RC8 and 115mm for galaxies)

HTH's.

Thanks Clarkey.  I initially thought the RC was the best option, but reading up on user issues put me off. Your experience has put thus back on the table. The 130mm APO is also likely undermounted as you note. The 115 would work, but it's not a significant jump from my TS CF 90mm f6. I'll do some more digging on the RC and the Newt options.

[900SL]

13 hours ago, WolfieGlos said:

I think I posted about this last year, and I think it might have been clarkey in fact who recommend an RC! For me, it would have been a 6" due to my HEQ5, but I did find this 0.67x reducer which might be of interest. Essentially, the 8" RC you might have considered would become f/5.3 and reduced to 780mm. I ended up going for the Starfield 102, less focal length but stuck with the frac for no collimation. I have no regrets with this, other than the loss of focal length to be honest.

But, lately I've been considering a Sky-watcher 150PL. Although at f/8 it is relatively slow, 1200 focal length for £149 (currently on offer, down from £189) seems a very good option. Reviews suggest it'll work on a HEQ5 (I haven't been able to locate the weight of the OTA, but it is available with an EQ3). The only downside, is that I cannot locate many images on astrobin, which perhaps suggests it might not be suitable for imaging. Those that are there, are pretty good for short integration DSLR images.

Failing that, a Seestar might be of interest 😉

HTH.

Interesting that you raise the 150mm f8. I have been looking at a 150mm f6, being a good match for the mount.

F8 is too slow for me, and I think the SW 150 has a 1.25" focuser so probably better suited to observing. Not sure of the illuminated field? 

I think TS do a UNC 150 f6. I've also been pricing up parts from TS for a build your own scope. Not cheap if you go for a good tube, cell, spider and focuser though!

[Clarkey]

14 hours ago, WolfieGlos said:

but I did find this 0.67x reducer which might be of interest. Essentially, the 8" RC you might have considered would become f/5.3 and reduced to 780mm.

This is the reducer I use. As it is a variable reducer depending on distance, I don't reduce the full 0.67 as I find it better at around 0.75. At F6 and bin 2 it gives a relatively 'fast' system.

CCDT67 & 27TVPH (astro-physics.info)

25 minutes ago, 900SL said:

I initially thought the RC was the best option, but reading up on user issues put me off

I know there are people who have really struggled with the RC's. I may have just been lucky and got a 'good one'. If I get the secondary perfectly aligned with the focuser, the primary can just be adjusted with a star test. Generally, the primary adjustment is so small it does not need any more than this. I do have a 6" F4 newtonian which I find more difficult to get perfectly collimated than the RC. I think this is down to the flex in the system that does not exist with the RC. 

 

14 hours ago, Elp said:

RCs tend to be heavy

I'm not sure they are that heavy for the FL and aperture. Having said this, the 8" would be at the limit of the mount. I have used mine on my HEQ5 and it will just do it - but the guiding was not that good.

I have just checked the C8 weight and I was surprised how light it is. Maybe the SCT with reducer is the way to go?

 

32 minutes ago, 900SL said:

The 115 would work, but it's not a significant jump from my TS CF 90mm f6

The reason I raised this was down to the pixel scale. For my set up this gives about 1 arcsec/px which is about as low as I can go (and probably lower than I should). A 910mm FL with a 0.8 reducer would give a similar scale with a bit of extra 'speed'.

 

Ultimately, unless you have unlimited resources, whatever option you go for will be a compromise in terms of aperture / FL / mount limits.

[ollypenrice]

I'm sure you wanted your list to get shorter rather than longer but there is a conspicuous absentee in the form of the MN190. https://www.firstlightoptics.com/maksutov-newtonian/skywatcher-explorer-190mn-ds-pro-mak-newt-astrograph.html

This has both the right FL and an excellent aperture for high res imaging using a modern CMOS camera with small pixels. The faff factor is hard to predict and I've never owned one of these, but I think it would be less than with the RC. (I'll restore the list to its original length by saying that efforts to help one of my customers with an 8 inch RC flagged up a 'Never Again' alert in my mind. We gave up on it.) The MN190 weighs 10kg so I don't know if you could stay within payload with that but I think you might.

Why all this talk of reducers? Just make your effective pixels bigger by stacking in superpixel mode and/or resampling downwards before processing. Reducers are best seen as field of view wideners and my experience of imaging at high resolution (about 1"PP, say) is that I ended up cropping almost every image I shot in order to present it at full resolution without the need for click-to-zoom. The last thing I wanted was a wider FOV. (Ironically, the shorter the focal length, the more often I do mosaics with it.)

Refractors are certainly the easiest in what is a tricky business at the best of time. I have lots of nice results (I think) from my TEC140 F7 at 0.9"PP but I would shoot about 20 hours per galaxy.  The MN190's extra 50mm of aperture would almost halve that exposure time. If you factor in what you will really capture, aperture may outweigh pixel peeping perfection...

A big refractor 'plus' used to be the high quality stellar images but, with modern processing, you can get decent stars out of other systems as well.

Olly

[Clarkey]

12 minutes ago, ollypenrice said:

(I'll restore the list to its original length by saying that efforts to help one of my customers with an 8 inch RC flagged up a 'Never Again' alert in my mind. We gave up on it.)

As I said, I seem to have got a good one. Because they are built to a price point, I am sure the quality varies from one to another. New focuser is a minimum for all of them. I have heard the horror stories - it made me think twice about buying one. For once I seemed to have got lucky!

15 minutes ago, ollypenrice said:

Why all this talk of reducers? Just make your effective pixels bigger by stacking in superpixel mode and/or resampling downwards before processing.

I agree to a point. However, when imaging natively at <0.5 arc secs /px I found the results better with a reducer and bin2 (giving about 1.3"/px) compared with bin3 at 1.5"/px. The reducer also acts as a flattener too. Just some 'fine tuning' really.

[alacant]

15 hours ago, WolfieGlos said:

I've been considering a Sky-watcher 150PL

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.

 

Edited April 9 by alacant

[900SL]

7 hours ago, ollypenrice said:

I'm sure you wanted your list to get shorter rather than longer but there is a conspicuous absentee in the form of the MN190. https://www.firstlightoptics.com/maksutov-newtonian/skywatcher-explorer-190mn-ds-pro-mak-newt-astrograph.html

This has both the right FL and an excellent aperture for high res imaging using a modern CMOS camera with small pixels. The faff factor is hard to predict and I've never owned one of these, but I think it would be less than with the RC. (I'll restore the list to its original length by saying that efforts to help one of my customers with an 8 inch RC flagged up a 'Never Again' alert in my mind. We gave up on it.) The MN190 weighs 10kg so I don't know if you could stay within payload with that but I think you might.

Why all this talk of reducers? Just make your effective pixels bigger by stacking in superpixel mode and/or resampling downwards before processing. Reducers are best seen as field of view wideners and my experience of imaging at high resolution (about 1"PP, say) is that I ended up cropping almost every image I shot in order to present it at full resolution without the need for click-to-zoom. The last thing I wanted was a wider FOV. (Ironically, the shorter the focal length, the more often I do mosaics with it.)

Refractors are certainly the easiest in what is a tricky business at the best of time. I have lots of nice results (I think) from my TEC140 F7 at 0.9"PP but I would shoot about 20 hours per galaxy.  The MN190's extra 50mm of aperture would almost halve that exposure time. If you factor in what you will really capture, aperture may outweigh pixel peeping perfection...

A big refractor 'plus' used to be the high quality stellar images but, with modern processing, you can get decent stars out of other systems as well.

Olly

Thanks Olly. I did consider the MN190, but it sits on the upper limit of the mounts capacity and I'm sure it will act as a sail. I also have pretty severe temperature gradients from indoors to outdoors (like 40 - 50C in winter) and dew.

Plus, and I shouldn't say this, but I don't buy Skywatcher stuff after previous experiences with their materials and QA/QC. A Mak Newt requires very accurate assembly, and I don't trust SW to come good.

The good thing is you've convinced me to knock the RC off the list, so you've actually shortened it! Merci Beaucoup!   

[900SL]

21 hours ago, IDM said:

Last year I bought a C11 for planets which had a Celestron dew ring heater and it has been really successful in controlling dew. Indeed I have been using it without a dew shield with no problems. Just the other week I picked up a C8 with an F6.3 flattener/reducer for galaxies. I have only tried it once (I just posted my M51 image in the DSO imaging section). I haven’t sorted all its issues particularly with the reducer but it seems a good scope. I bought my C8 secondhand it was cheap and it’s quite light.

cheers

Ian

Very nice image of M51!  I think the focal length of an 8" SC would be too much for my seeing and guiding however, even with a reducer to 1400mm. Plus they are pretty expensive here, once I add in a reducer and OAG/focuser.  Thanks for the feedback though.

[900SL]

7 hours ago, 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.

 

That is interesting. No coma corrector required on a 533 sensor? Sufficient field illumination and image circle?

Is it intended more for visual, or set up for imaging? Ive been looking at a 150mm f6 newt to give 900mm fl that should work well with a 533 sensor

Edited April 9 by 900SL

[alacant]

1 hour ago, 900SL said:

No coma corrector required on a 533

No. It's fine. Even over aps-c. It also focuses fine with room to spare using a dslr.

I've never looked through one, so can't comment on its visual capability.

Edited April 9 by alacant

[ollypenrice]

1 hour ago, 900SL said:

Very nice image of M51!  I think the focal length of an 8" SC would be too much for my seeing and guiding however, even with a reducer to 1400mm. Plus they are pretty expensive here, once I add in a reducer and OAG/focuser.  Thanks for the feedback though.

None of this makes any difference if you adjust your true and final image scale at capture and then processing to what the seeing and guiding can realize. If I went for a flatfield SCT I wouldn't use the reducer, I'd capture in superpixel and/or resample before processing. This would mean I'd get the extra light per pixel onto the target and not onto background sky which I'd later crop out.

Olly

[Clarkey]

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

[ollypenrice]

13 hours ago, Clarkey said:

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

I think you're safe. It's been so wet... 

Olly

[WolfieGlos]

On 09/04/2024 at 10:03, alacant said:

The main issue with the long sw is the awful 1-1/4" focuser.

On 09/04/2024 at 08:11, 900SL said:

F8 is too slow for me, and I think the SW 150 has a 1.25" focuser so probably better suited to observing. Not sure of the illuminated field? 

The 1.25" focuser is another thing that was putting me off - and the very low price tag suggesting that something's amiss. Given the 533 can use 1.25" filters (I've been told) I thought it might have been worth a punt. But I'd agree with your other comment about Skywatchers QC. Whilst my mount is fine, my 72ED was pretty bad and feels very cheap compared to the Starfield 102.

3 hours ago, ollypenrice said:

I think you're safe. It's been so wet... 

Olly

Sat here, looking out at the rain whilst eating my lunch, I nearly spat my yogurt out when I read that 🤣

[alacant]

On 09/04/2024 at 09:11, 900SL said:

F8 is too slow

Hi

It isn't f-ratio which makes a telescope fast or slow.

Cheers

Edited April 10 by alacant

[ONIKKINEN]

I'd go with the 8" newt, but then again as a newtonian user myself i am biased.

Something to keep in mind is that you need to install a fan on the primary mirror to keep the scope acclimated throughout the night. Temperatures can easily drop 10 degrees at night here in winter, which is too much for just passive cooling to do the trick. Its not an expensive thing to set up but one more thing to fix (just a computer case fan + some DIY for mounting and power).

I am planning on getting rid of my Paracorr because it has some field curvature over an APS-C sized chip, but i think it would be very good for your 533 cameras. Let me know if you are interested in that if you do decide to go for the newt.

[900SL]

20 hours ago, ONIKKINEN said:

I'd go with the 8" newt, but then again as a newtonian user myself i am biased.

Something to keep in mind is that you need to install a fan on the primary mirror to keep the scope acclimated throughout the night. Temperatures can easily drop 10 degrees at night here in winter, which is too much for just passive cooling to do the trick. Its not an expensive thing to set up but one more thing to fix (just a computer case fan + some DIY for mounting and power).

I am planning on getting rid of my Paracorr because it has some field curvature over an APS-C sized chip, but i think it would be very good for your 533 cameras. Let me know if you are interested in that if you do decide to go for the newt.

Kiitos Oskari. Do you need dew heaters on the primary and secondary as well as the fan, or do you use a extended dew shield? I'll likely stick to a 150mm f6 with a smaller secondary for the 533, given the sensor size

And thanks for the offer of the Paracorr. Nothing decided yet and I probably wont get anything until later in the year, but I'll drop you a line if I go down that route. Have a collapsed storm drain to fix at the moment out in the yard, about 10m long to a blocked soakaway. There goes any astro-budget for the next couple of months. 

[ONIKKINEN]

41 minutes ago, 900SL said:

Kiitos Oskari. Do you need dew heaters on the primary and secondary as well as the fan, or do you use a extended dew shield? I'll likely stick to a 150mm f6 with a smaller secondary for the 533, given the sensor size

And thanks for the offer of the Paracorr. Nothing decided yet and I probably wont get anything until later in the year, but I'll drop you a line if I go down that route. Have a collapsed storm drain to fix at the moment out in the yard, about 10m long to a blocked soakaway. There goes any astro-budget for the next couple of months. 

I've no heaters, the primary is so far down the tube it really doesnt get dew. The secondary is kept dry by a dew shield and flocking applied to the back of the mirror (so it doesnt radiate heat away as much). I think the fan also helps with dew prevention since there is constant airflow over both mirrors and dew is less likely to stick.

[900SL]

On 10/04/2024 at 15:47, alacant said:

Hi

It isn't f-ratio which makes a telescope fast or slow.

Cheers

I've gone back to an earlier post where this was discussed in some detail

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 

I think 150mm f6 would be better match for 533MC/MM. Relative speed is 11600, so not too slow, and pixel scale 0.9, which suits my seeing and guiding

@vlaiv is my maths right there buddy?

Edited April 15 by 900SL

[vlaiv]

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

 

@vlaiv is my maths right there buddy?

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.