Refractor for imaging?

[Andyk93]

Hi, at the minute I have a skywatcher 130pds on an neq6.

I'm now looking to get rid of the reflector and buy a 80mm refractor and would like some recommendations if possible please. I have a few that I've looked at but open to suggestions. Budget is £800 ish but I don't mind waiting and saving a bit extra. 

1. Skywatcher ed80ds pro, slower f ratio than my 130pds so longer subs needed. (Don't know how much longer) 

2. Ts photo line 80mm triplet? Not heard much about this

3. The 2016 Altair astro star wave 80edr. Looks like a great bit of kit. Really interested in this one

A bit past my budget is the Skywatcher esprit 80 and the WO star 71. The last one appeals to me because of the fact it doesn't need a field flattener. But it's got a very wide FOV which I can't decide if it's too wide for my liking. 

Thanks 

[laser_jock99]

What's wrong with the 130PDS?

[Pig]

5 minutes ago, laser_jock99 said:

What's wrong with the 130PDS?

That was my first thought too  

[iapa]

Wiith guiding, could get you more data than the 130pds over a night as your image would move less in relation to the image in your guide camera allowing longer exposures.

 

Option 1. Ais only one I can comment on. Appears to be a very common suggestion from people, I bought one on the basis of such recommendations as a replacement for 200mm f5 reflector.

wish I'd had the chance to really use it? But got my first M31 which my others OTAs had too narrow an FOV to get.

[Andyk93]

8 minutes ago, laser_jock99 said:

What's wrong with the 130PDS?

Sounds like a stupid reason but the more I get into imaging the more I don't like diffraction spikes. I like the nice clean round stars a refractor gives. collimation although easy is something I don't have to think about either and I still have a 200pds for the smaller targets

[Pig]

Hey its not stupid at all.... fracs sure do deliver pinpoint stars  and they are incredibly handsome scopes ......

[ecuador]

Just don't start adding diffraction spikes in post-processing after you get a refractor! It drives me nuts!

To answer your question, to get the same total light/pixel as your 130PDS @ f/5, you would theoretically need 2.25x the exposure with an f/7.5, or 1.56x the exposure with an f/6 (e.g. the Skywatcher f/7.5 with a 0.8x reducer/corrector). In reality the difference is smaller, as the 130PDS has a central obstruction and also some obstruction from the focuser tube, both of which reduce the light that passes through.

[Andyk93]

9 minutes ago, ecuador said:

 

To answer your question

Thanks. With the limited clear sky's we get I didn't want it to be slower than I've got now

[KyleStoke]

I would save up a bit more and go for a WO GT81 + reducer, faster than your current scope. I recently went the other way from ED80 to 130pds reason being speed, I would get the GT81 but the field of view was to wide for what im currently after.

[alacant]

Hi. There are some good refractors on the market these days for much less than the older ed models. Faster too. The ff costs more than the telescope but the results are really good e.g. andromeda and the heart with a €150 ar80s f5: built like a tank and comes with solid 2" focuser. Maybe worth considering, especially if you're used to a fast reflector. HTH.

[ecuador]

3 hours ago, alacant said:

Hi. There are some good refractors on the market these days for much less than the older ed models. Faster too. The ff costs more than the telescope but the results are really good e.g. andromeda and the heart with a €150 ar80s f5: built like a tank and comes with solid 2" focuser. Maybe worth considering, especially if you're used to a fast reflector. HTH.

Eh, you are talking about achromats. It is not the same thing as "older ed models", not even close. Sure, I am not against imaging with an achromat - I've done it with nice results, but when the OP says he wants to get rid of diffraction spikes do you think he means he'll trade them for halos around bright stars?

[Andyk93]

Still looking at the Altair Astro star wave 80ed-r 2016. Looks really nice but there are no reviews that I can find. Most things I read about their scopes are not great

[mountainmadman]

22 minutes ago, Andyk93 said:

Still looking at the Altair Astro star wave 80ed-r 2016. Looks really nice but there are no reviews that I can find. Most things I read about their scopes are not great

Hi Andy,

I've just bought an 80ed-r.  This is a review of one by an SGL member, and very well respected user.

http://astrofarmfrance.com/starwave-80ed-r-refractor-telescope/

I've only had mine out the once, and used that time to learn more about some of the software as opposed to the scope itself.  But, I did go out to see Andrew (the reviewer) and I was sold on getting my ED-R from seeing the previous model 80ED in action out at AstroFarm.  In fact, I tried to get hold of one when I got back to the UK, but there were none to be found.  So, I stumped up the extra cash and got the 80ED-R instead. 

Andrews review was written after I had made my purchase, but reading it, I would say that it's very good.  Have a read and see what you think.

All the best,

Tony

[Andyk93]

1 hour ago, mountainmadman said:

 

Hi Tony, 

Thanks for the link, it's a minefield really, I would love to have a look through one first but I'm 3 hours from the closest shop. If the clouds ever clear up and you get chance to use it, I would love to see some subs. 

Thanks, Andy

[Adam J]

It will be a double edged sword, as people have noted you have gotten rid of your diffraction spikes but at the expense of a slower scope.

Do you use a DSLR or CCD? The disadvantages of DSLRs (being less sensitive than CCD) are compensated by a fast scope in my opinion. With a CCD you can more effectively increase exposure to compensate.  Not always an option with a DSLR due to noise. 

Personally I would rather have better signal to noise and live with the spikes. But if you really don't like them...

 

[ollypenrice]

We have to be careful in comparing the exposure time because the F ratio rule does not necessarily apply in AP as it does with camera lenses. The rule says that if you double the F ratio you quadruple the exposure time. This may or may not apply in the case of taking astrophotos. The issue is discussed here: 

However, in short focal length imaging we usually fill the frame with 'wanted target' and only rarely shoot small objects which don't fill the frame. In this case the F ratio rule is applicable. A focally reduced ED80 will be considerably slower unless, of course, you needed to do a mosaic at the longer focal length! 

If using a DSLR you will, as others have said, feel the pain of the slower F ratio. It might be considerable pain! On the other hand I agree about diff spikes and the simplicity of refractor imaging.

The FL of the Newt is not long enough to get you into the galaxies but is long enough to restrict you on the larger nebulae. It is not an FL that I would want to be using, personally. The targets that are available tend to be large and more FOV would be good for them.

The whole charm of these threads is that you get to spend other people's money so I'd go for the Esprit 80 or the Star 71.

lly

Edited February 2, 2017 by ollypenrice
Spolling!

[Andyk93]

3 hours ago, Adam J said:

 

I use a modded canon 1000d at the minute but once I have the scope sorted I was going to start saving for a ccd. The most likely one I will buy is the esprit 80 so at f/5 it will be just as fast. I do wonder if the money would be better spent buying a ccd instead of the scope but I can see the ccd sensor and filters getting dust on them because of the open tube

[Andyk93]

3 hours ago, ollypenrice said:

 

The FL of the Newt is not long enough to get you into the galaxies but is long enough to restrict you on the larger nebulae. It is not an FL that I would want to be using, personally.

Thanks Olly, I'm learning new things all the time. 

I do agree with you on that point. It's not quite enough FOV to frame andromeda nicely. It always seems to clip the edges of the galaxy. 

I am going to have a look at an esprit 80 as it looks like a really well built scope

[ollypenrice]

48 minutes ago, Andyk93 said:

Thanks Olly, I'm learning new things all the time. 

I do agree with you on that point. It's not quite enough FOV to frame andromeda nicely. It always seems to clip the edges of the galaxy. 

I am going to have a look at an esprit 80 as it looks like a really well built scope

There are only three galaxies for short focal length, M33, M101 and M31 (which is too large for almost all scope-camera combinations.) Even with full frame CCD we needed two panels at 530mm FL.

Olly

[iapa]

3 minutes ago, ollypenrice said:

There are only three galaxies for short focal length, M33, M101 and M31 (which is too large for almost all scope-camera combinations.) Even with full frame CCD we needed two panels at 530mm FL.

Olly

Slightly off topic I suppose.

If you are looking to image M31, or the other larger targets, you would want a mosaic I expect; given comment above.

My question to that would be, if a 4 panel mosaic was to be done, then, would you do one panel per session to accumulate the data, or rotoate through each panel in each session?

I was just thinking that if one panel were to be captured per session, then you may end up with panels 1 & 2, and nothing for months. I am not half way up a mountain at one of the tropics  half a million miles from nearest street/house/security/other *£% light.

On the other hand, if you rotated through panels 1,2,3 & 4 each night, you may not capture enough information per panel, and still have a deficit of data.

Possibly a discussion for a separate thread.

[SilverAstro]

On 31/01/2017 at 19:59, Andyk93 said:

 I don't like diffraction spikes.

re-fit the 130 with a curved vane spider

 

 

Edited February 2, 2017 by SilverAstro

[Peter Drew]

If diffraction spikes can be added to a refractor image, can they not similarly be removed from a Newtonian image?

[Adam J]

15 hours ago, Andyk93 said:

I use a modded canon 1000d at the minute but once I have the scope sorted I was going to start saving for a ccd. The most likely one I will buy is the esprit 80 so at f/5 it will be just as fast. I do wonder if the money would be better spent buying a ccd instead of the scope but I can see the ccd sensor and filters getting dust on them because of the open tube

Dust has not been a problem for me, just use a rocket blower and if that does not work it is possible to clean a filter the surfaces of filters are actually very very hard due to ion deposition used in making them.  Having said this though, I really don't get that much dust in the scope, just put the cap on and make sure that you point the scope downwards when its in storage so that dust does not settle on the primary. 

[ecuador]

16 hours ago, Peter Drew said:

If diffraction spikes can be added to a refractor image, can they not similarly be removed from a Newtonian image?

If you shoot a photograph of a fire hydrant, you can easily add a car in front of it in post processing. If you take a photograph of a car in front of a fire hydrant, it is much harder to remove the car afterwards and see the fire hydrant 

And I hate adding diffraction spikes to refractor images... Very few cases where it makes sense to me (e.g. nightscapes for showing constellations etc).

[Peter Drew]

I like the analogy , I presumed it would be somewhat easier with a star, shape with spikes same shape without spikes. I agree, adding spikes to refractor images seems a bit odd.