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Imaging galaxies and Messier objects with a F10 90mm achromat refractor?

Astrophotography DSO
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#1 IR656nm

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Posted 25 November 2021 - 03:25 PM

Hi Cloudy Nights!

 

I was hoping I could get an opinion on upgrading my imaging rig.

I have recently purchased a HEQ5 Pro with Rowan belt modifications. I am learning pH2 autoguiding and have got a choice of a 60mm F3.75 guidescope and a 30mm F4 Svbony guidescope.

 

I have an ASI120mm-S, ASI 174mm, Canon dSLR (astro modded) 100D, Canon 6D (stock) cameras.

I am thinking of upgrading also to a mono imaging with RGB and also Narrowband filters (I think 2 inch filter size)...

 

I have got a 70mm ED refractor F6 (Altair Astro) which I use for solar H-alpha/white light and wide field imaging. I have a 150 mm Newtonian reflector which I have not started to image with yet.

I have a 90mm F10 achromat refractor (upgraded to a GSO dual speed focuser) for mainly solar and planetary imaging. I have found unsatisfactory results from imaging Jupiter on a first attempt but I did not have a good colour camera (only an adapted Logitech webcam).  I think this is because the telescope is an achromat.

 

Question:

 

1) I was thinking if I can get decent results with the 90mm F10 achromat for imaging galaxies and nebulae, and Messier objects, NGC galaxies etc.?

 

Or 2) will I need a bigger ED or apochromatic refractor ? such as 102 mm or 120mm refractor. I have a 150 mm Newtonian reflector which I am preparing to use for imaging...

 

Many thanks for your help.

 

Magnus



#2 c2m2t

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Posted 25 November 2021 - 04:00 PM

Hi Magnus!

Very much of any response will be dependent on what you mean by "decent results"!! I do a lot of prime focus imaging with both achro, ED and APO refractors. Your 90mm F10 scope will do ok but because of the size of the optics, it will be magnitude limited...I am guessing clean star images to about mag. 13 to 13.5 with 30 sec to 1 minute exposures at ISO 1600...if the skies are clear and reasonable dark ie away from city lights in a rural setting. The issue with the achromat is that for bright stars, of which many clusters have bright enough stars, you will get violet fringing. This may or may not be an issue for you. My recommendation, if you are wanting to invest a lot of time, is that you consider the purchase of a scope with ED glass...they really do eliminate the fringing and in my opinion are the best bang for the buck. The ED design is a quality optic and can be easily resold on the used market if you feel you want to make an upgrade in the future to say an APO triplet. 100mm would be a minimum with a focal ratio of F8 to F10. 120mm would be better but the 120mm designs seems to be maxed out at F7, but a good online search would confirm that.

 

Before you buy, certainly put your reflector to the test. It with generate spikes on bright stars that most people find pleasing in an image...I would have to agree.

 

I wish you all the best as you head down the imaging road!!

 

Cheers, Chris.


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#3 IR656nm

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Posted 25 November 2021 - 04:45 PM

Thank you Chris ! for your thoughtful reply.

I must admit I should really test my 90mm F10 scope on imaging stars and star clusters in the first instance. Its hard to find a good stable weather, as it takes over 1 hour to set up my imaging rig, and then to have to take it down takes another 1 hour minimum.

 

Magnus


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#4 sbharrat

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Posted 25 November 2021 - 05:19 PM

...

Question:

 

1) I was thinking if I can get decent results with the 90mm F10 achromat for imaging galaxies and nebulae, and Messier objects, NGC galaxies etc.?

 

Or 2) will I need a bigger ED or apochromatic refractor ? such as 102 mm or 120mm refractor. I have a 150 mm Newtonian reflector which I am preparing to use for imaging...

 

Many thanks for your help.

 

Magnus

As mentioned by Chris, the issue with the achro will be the horrible stars. This was with an Orion ST120 (F5) achro:

https://astro.bharra...34-jan2021.html

 

If you are really good with post processing, you can clean up to some extend. But it is pretty time consuming IMO. 

 

The change to an apo was dramatic. These are with an 80mm (F6) apo:

https://astro.bharra...60-jul2021.html

https://astro.bharra...35-oct2021.html

 

Note I showed nebula pictures. For galaxies, you will eventually want a longer focal length scope. (I am hoping to get an Edge HD later). But then you are going to be pushing it on the HEQ5 (I have the Sirius which is an HEQ5 rebrand). 


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#5 IR656nm

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Posted 25 November 2021 - 05:35 PM

Hello Shaun,

 

Thats really helpful advice. Lovely TS80 images and colour, tremendous imaging and processing!

 

Was your TS80 a triplet apochromatic or only a ED80 (doublet)?,as I know triplets or quadruplets cost 10000 dollars/pounds!

 

Thanks.

Magnus


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#6 sbharrat

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Posted 25 November 2021 - 06:41 PM

Hello Shaun,

 

Thats really helpful advice. Lovely TS80 images and colour, tremendous imaging and processing!

 

Was your TS80 a triplet apochromatic or only a ED80 (doublet)?,as I know triplets or quadruplets cost 10000 dollars/pounds!

 

Thanks.

Magnus

The TS 80mm apo is a triplet. It was about $900 USD. It is a common design that is rebranded by several manufacturers. Ask on the equipment forum to get the equivalents model #s for other brands. 

 

I think the BIG difference will be moving from an achro to at least a good ED doublet. The incremental from an doublet to a triplet is much smaller to my understanding. But the achro was pretty bad on stars.

 

The other point I wanted to get across is that an 80mm (F6 or faster) is perfect for nebula... you really don't need to go up to 100. Especially since you are working with a HEQ5. 



#7 revans

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Posted 25 November 2021 - 06:43 PM

Hi Cloudy Nights!

 

I was hoping I could get an opinion on upgrading my imaging rig.

I have recently purchased a HEQ5 Pro with Rowan belt modifications. I am learning pH2 autoguiding and have got a choice of a 60mm F3.75 guidescope and a 30mm F4 Svbony guidescope.

 

I have an ASI120mm-S, ASI 174mm, Canon dSLR (astro modded) 100D, Canon 6D (stock) cameras.

I am thinking of upgrading also to a mono imaging with RGB and also Narrowband filters (I think 2 inch filter size)...

 

I have got a 70mm ED refractor F6 (Altair Astro) which I use for solar H-alpha/white light and wide field imaging. I have a 150 mm Newtonian reflector which I have not started to image with yet.

I have a 90mm F10 achromat refractor (upgraded to a GSO dual speed focuser) for mainly solar and planetary imaging. I have found unsatisfactory results from imaging Jupiter on a first attempt but I did not have a good colour camera (only an adapted Logitech webcam).  I think this is because the telescope is an achromat.

 

Question:

 

1) I was thinking if I can get decent results with the 90mm F10 achromat for imaging galaxies and nebulae, and Messier objects, NGC galaxies etc.?

 

Or 2) will I need a bigger ED or apochromatic refractor ? such as 102 mm or 120mm refractor. I have a 150 mm Newtonian reflector which I am preparing to use for imaging...

 

Many thanks for your help.

 

Magnus

First you probably need a good tracking mount that you can reliably polar align with and trust to track very well for hours in an imaging session.  Your reflector is the better bet.... your 90mm F10 refractor is better used for lunar or planetary photography.  Before buying anything, I'd give the reflector a go and it will tell you if your mount and your polar alignment are sufficient for further progress.  Eventually you may stay with the reflector or you may opt to get a shorter focal length refractor for deep sky.... a triplet is expensive and great, a doublet can be sometimes less expensive and very good or great depending on who makes it, and an achro will have some issues with halos and purple fringing around bright stars.  You can use a fringe buster filter but they work so-so.  You can also work on this problem with post processing.  All in all, it is better not to have to do that.  


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#8 IR656nm

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Posted 27 November 2021 - 03:04 AM

Thank you Rick,

 

I will prepare my Skywatcher 150 PDS over the next 6 months, hopefully will get some chances to try imaging. I have to consider a coma corrector, get practice with collimation, and to get autoguiding working...!

 

I was looking at some inexpensive triplet apochromatics 80 to 100 mm and then decided it was better to work on my autoguiding, processing and Mount control as well as polar alignment...

 

I bought the 90mm achromat for mainly solar as I was informed that it was alright for monochromatic imaging like H-alpha. Not sure about white light imaging. I was hoping it would also do planetary imaging, but I need a proper colour camera or a filter set for a mono camera. Maybe the ASI120mm-S could do planetary imaging, but I don't have a filter holder or filter set yet.

 

Magnus




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