Recommend me a refractor

[swannies1983]

Hi all,

I'm really starting to get into imaging at the moment after about 1.5 years of not even taking any of my telescopes outside. My current scopes are an 8" dob and a Tal200K. I have recently taken up prime focus imaging using my unmodded Canon 400D with the 8" Newt on an EQ6. Firstly, it was just some unguided shots (out to about 90secs). Now I'm learning the ropes to guide using an old 60mm tasco refractor and an unmodded toucam.

While sometimes frustrating, it has been a fulfilling experience, producing some pleasing results, at least the unguided pics as I've just started guiding. However, the whole 8" Newt setup is quite bulky and I was thinking of perhaps getting a dedicated refractor for imaging. The Tal200K is a fantastic scope and you only have to look at Bert's work on here to see what is possible with the scope. However, the accessories required to make it into an imaging scope are just not purchases I want to make. Therefore, I was thinking of perhaps selling the Tal and using the funds to purchase a refractor.

In regards to a refractor, I'm thinking along the lines of between 80-100mm, probably an apo (doublet), unless I can be convinced to get an achro. My budget will be around the $1000 mark, but it all depends on what I can get for my Tal. I'm a complete novice when in comes to refractors (my cheap tasco doesn't count :lol:), so any advice would be very much appreciated.

Daniel

[jjjnettie]

You can't go past the trusty ED80, it's a real work horse.
There's enough big bright targets up there to keep you busy with it for a couple of years.
Don't forget you'll need to buy a field flattener to suit, otherwise you won't be able to take advantage of the full field of view because you'll be cropping the coma from around the edges.

[dannat]

Dan what r you wanting to use the refractor for? just DSO photography? ed80 would be the most popular choice people start with
achro is OK for visual or if you are just shooting mono

[Poita]

A second hand ED80 can usually be had for under $400 and is a great place to start.

[swannies1983]

Yeah, I have considered the ED80 and it's certainly on the shortlist. I was perhaps hoping to get something with a little more aperature. However, accessories (flatterner) certainly will play a role in deciding on what to get.



Yeah, I will mainly use the refractor for imaging DSO. I will stick with the Newt + toucam for planets.

I was also informed about the William Optics Gran Taurismo 80mm or the William Optics 90mm Megrez.

I plan to stick with the 60mm f/11.6 refractor as a guidescope for now. I don't know if this will have an impact on what options I have.

[Waxing_Gibbous]

Yep. Hard to go by the ED80.
Though if you're prepared to splurge a bit, hands-down the best quality-for-price scope in this range is (IMNSHO) the Sharpstar (aka Astro-Tech) 106mm f6.5.
They run about $1500.
Glorious scope!

[TheDecepticon]

http://www.iceinspace.com.au/forum/showthread.php?t=79598

Try this.:thumbsup:

[swannies1983]

hhhhmmmmmm....very tempting Deception!!

[TheDecepticon]

It is right on for your request, but the final choice is yours.:question:

By the way, it is Decepticon, like transformers, not Deception.:)

[swannies1983]

haha, sorry, my mistake. Was written in a bit of a rush.

I'm not so sure about the ED100 now. A fellow IIS member has informed me there was a common problem with this scope that produces diffraction rings on some of the brighter stars when imaging, which has something to do with lens spacer. This issue has been discussed on cloudynights. Plus, if I were to get the scope you mentioned, I would probably have to upgrade the focuser to accommodate my DSLR and the slow f-ratio is a negative as well.

Too many options. On one hand, the ED80 seems like a good choice, and the cheaper cost will allow me to buy accessories. However, I do also like the William Optics 90mm Megrez. But it's closer to $1,000.

[g__day]

The other avenue you could consider possibly is guiding from your existing scope using an off axis guider into your guide camera, rather than use the 60mm tasco refractor.

[Terry B]

Have you thought about the 127mm APO from Gilman?

[swannies1983]

Yeah I had considered it but I already had the refractor and my dad made up some rings for it.

My guide camera is an unmodded toucam. I guess I would still run into problems finding a guide star using this setup despite more light gathering power.

[dannat]

there is the 90mm megrez in the classifieds -worth a look

[Hagar]

Hi Dan, I don't want this to sound bad but it seems you have all the answers already yourself. You asked for educated recommendations from those who supposedly know more than you and openly dismiss the recommendations based on some posts on another site.

The thing to be aware of with negative posts on forums is that only the negatives seem to get posted. Very rarely does someone start a thread raving about a good scope.

Depending on the focal length you are looking for, it use in field or observatory, your physical ability and your limitted budget I would suggest you look at the ED range of scopes, either new or second hand. You also have to bear in mind that imaging with a doublet or triplet and a DSLR will require the use of a field flattener of one kind or another. The focusers on these scopes are not fantastic but are quite capable of holding a DSLR. Lots of things can be done to improve the focusers and to dismiss them is a matter of dismissing some of the greatest scopes for imaging that have ever been made and are the only cheap alternative to scopes like AP, Tak, TEC and the like.

My suggestion is you get hold of people who are using them and talk to them. Look at their images. Then decide which way to go.

[RobF]

Another 9x50 guider you can modify as a finder/guider. Then you can use your 8" Newt for DSO imaging. Can be a bit of faffing around, but some people say you can get the odd decent image :whistle:

Of course you'd still have to figure out how to spend the $1000 you'd save, and also find some way to use up the spare imaging time when your larger faster newt sucks down light 2 or 3 times faster than the refractor :question: :D

:scared3:

[swannies1983]

Doug, my apologies if you thought my posts were quick to dismiss certain scopes. Only one person mentioned the ED100. I simply raised an issue that appears to have been reported by more than one person in regards to this scope. If this problem is not the norm, then I take back my comments.

Regarding focusers. I probably should asked if there any issues with the standard focuser when using the scope with a DSLR. Some people have mentioned there is slippage, particularly when the scope is pointed towards zenith.

I appreciate all the suggestions made in this thread and those made via PM. There are loads of opinions and I'm weighing up all the options. :)

[gregbradley]

I agree with the ED80 recommendation. I had one early on - doesn't everyone get one eventually?

I had a high end William Optics 80mm triplet super APO. Focuser was rubbish and produced terrible coma it was so badly out of alignment.

Perhaps mine was a lemon - but I'd check others feedback on William Optics before getting one. They are probably fine and I just got a bad one.

Astrotech are also very good value. I think a lot of these 80mm doublets I suspect use the same lens?? Astrotech also does an 80mm doublet and the little 66ED I use for autoguding sometimes is surprisingly well made for such a cheap scope. Microfocuser, locking mechanism, a rotatable focuser and nice finish.

There was an Orion 100mm triplet being sold at Opt (optcorp.com) for US$500 recently. I wonder if they have anymore of those. It seemed like a deal. Also there was a carbon fibre 100mm triplet for sale cheaply recently by I think it was Orion as well. My first refractor was an Orion ED80.

Greg.

[swannies1983]

Rob, an 8" Newt on an EQ6 is my current setup for deep sky imaging. I am using my 60mm as I had this spare but have certainly considered modifying another finderscope to act as guidescope. Do people get pictures equal to that of a larger focal length guidescope?

[swannies1983]

Yep, had I have seen it and emailed seller several times ;)

[swannies1983]

How many types of ED80s are there? Which one do people recommend?

[Hagar]

Dan, There are an awful lot of brand names out there but you will find most come out of the same factory in China (Synta).
I have not had anything to do with the 100mm version but can recommend either the 80mm or the 120mm refractors from this factory. (Orion, Skywatcher and astrotech) are just a few of the brands from Synta.

Which ever you choose you will need a field flattener but these are readily available.

If you have an 8" Newtonian you will search hard for a nicer imaging scope. Using a Comma corrector to fix the comma and an upgraded focuser will give you an excelent imaging scope with some money left over to celebrate your images with.

Good luck with your search.

[g__day]

Given that Bintel sell Meade OAG for $109 https://www.bintelshop.com.au/Product.aspx?ID=5443 it wouldn't be dear or difficult to try prime length guiding. If you could sell the Tasco could could probably come out almost even.

Full focal lenght guiding will show the ability of your guiding skills and your mount's capabilities; and consequentially be alot cheaper than a new OTA. For long exposures if you don't go OAG - then you'll need very rigid gear to guide well (to eliminate or at least minimise differential flexure). I spent over a grand on my mounting gear and same again on focusers that can be PC controlled.

If your intended targets are dim they will require long duration guiding. For this to be successful you need really, really sturdy amd rigid gear - throughout your entire imaging chain, and possibly a way to maintain precise focus as the night progresses and therefore cools and this changes your OTAs focus.

[swannies1983]

Would I need a coma corrector when using an OAG given I will be trying to find stars at the edge of the field? How does the addition of the OAG impact upon the point of focus? I was lucky in that I didn't have to cut my tube and change the position of the primary mirror as I could attach my DLSR directly to the thread where the 1.25" adapter plate is. I only have about 5mm of inwards focus remaining. What does the OAG do to the setup?

[Alchemy]

I had the gold tubed one, surprisingly good optics, although ED it really is APO the focuser worked fine no problems that I picked. It then became a guidescope, and as I wanted a faster guidescope I replaced it with a Williams megrez 88 f5.6 I sold the ed80 to another IIS member.... One who has replied to this thread and also recommended an ed80.

Would have to be the best bang for your buck around IMO.

[RobF]

It's unlikely the 60mm guidescope wouldn't be at least as good as a finder guider Dan. Could I ask what sort of imaging you lust after? What sort of objects (big nebs or smaller galaxies) and what sort of camera and sub length is the goal? Do you have a fixed set up?

I guess what I'm thinking without knowing more is that if you're setting up and pulling down all the time much of the frustration that has prevented you setting up in the last 1.5 yrs (polar alignment, etc?) may still be a significant issue in a refractor setup.

[swannies1983]

It's probably a combination of both. I understand that the refractors within my price range would be better suited for wide view imaging, which will probably be a good starting point. I still have my newt for smaller objects.



Unmodded Canon 400D. I would probably like to get out to 5 mins. I can get out to this length with some room to play with before skyglow becomes a problem. However, the plan is to eventually get a light pollution filter though.



No I don't.



haha, the main reason why I haven't gone out in the last 1.5 years is because the wife and I had a baby ;)

But yes, I was getting a bit frustrated setting up the newt each time and trying to get the correct balance. I was thinking a smaller scope would produce less problems, but maybe this isn't always the case.

[alistairsam]

Hi,
any thoughts from anyone on the long perng scopes at andrews.
I've not seen or tried them but they do seem to have good build quality.

I've been thinking about one of these.
http://www.andrewscom.com.au/site-content-section-10-longperng.htm

Achromatic 80mm f/6 OTA doublet refractor
$349.00 AUD

CARBON FIBRE ED 80mm DOUBLET
ED APO 80mm f/6.25 carbon fibre refractor with ED fluorite lens and Series 2 Crayford style 2" 4 kg. capacity 11:1 microfocuser!
$599.00 AUD

ED APO 80mm f/6.8 OTA
$499.00 AUD
Includes FPL-51 fluorite ED lens


if you use a filter wheel with LRGB filters, a monochrome camera, would a doublet be good, as then you could get a bigger aperture for a lower price than the apo's and you wouldnt have chromatic aberration as you're shooting in monochrome?
is this correct? there's bound to be a catch somewhere with this?

Edit: Dan, I just noticed you're planning on using a 400D. but just wanted to ask the monochrome question nevertheless.

[Hagar]

Hi Alistair, Your question is quite a complicated one but the reality is that the use of an achromat and a mono camera should render the chromatic aberation null and void but would require a refocus with each and every change of the filters. This is at least in theory.
The ED scopes using either FPL-51 or FPL-53 glass, even in the doublet lens arrangement would correct most if not all the aberations on all but the very brightest of objects. Some aberations may exist on things like a full moon but even then much much less than the achromat.
The cost difference is well worth the quality you find in such lens systems.
The use of ED refractors is a massive step up to the achromat and is considered an APO or semi APO refractor. It';s use will also return good filters to par focal or at least very very close to this.

[g__day]

Programs like PHD can do a centroid find on a soft or coma affected star quite well. Back focus may be an issue - if the OAG is 30mm wide say - its has to be accounted for in your reaching accurate focus.

Its most common to see OAG used on long focal length tubes - like SCT which also have a serious lot of back focus available from their inate design of moveable primary mirror.

[gregbradley]

Achromats are no good for imaging and you will get blue halos around stars.

I am not 100% sure refocusing on each filter will stop the blue halos. I think not because you will probably get bloated stars in the blue subexposures but not 100% certain of this point.

Even so who wants to have to refocus for blue. A lot of extra work without automation software which will do it but it now requires an autofocuser software program and just another thing to bug up at 2am.

My scopes have the same focus for each filter. I checked.

Triplets outperform doublets generally. But the ED80 with the FPL53 (not FPL51) is the one that is used a lot and gives great results.

I read a thread about carbon fibre for refractors and Roland Christen seemed to be against it. Tube currents. Also aluminium tubes shrink with temperature but tends to match the shift in the lens with falling temps to some degree. Not conclusive. I wonder if those who are using a carbon fibre tubed APO can comment about tube currents and focus shift with temp changes?

ED80 is also super light which means it is easier for the mount to get better tracking results.

Greg.

[alistairsam]

As there are two types of chromatic aberrations - axial and lateral, that is different focal points along the optical axis and the other with different focal points on the same focal plane, I'm guessing achromats might result in lateral CA, so no need to refocus but there might be a slight lateral image shift with different wavelengths. So I wonder if re-positioning the blue images would correct the blue halo.

As all have mentioned, the semi-Apo's or ED series would be a good investment and a 102mm ED would be ideal.
The Black Diamond 102mm ED is 1199 at Andrews.

I'm going to try the Long perng ED's.
The ED APO 80mm f/6.8 OTA which Includes FPL-51 fluorite ED lens at $499.00 AUD is within my reach.
but a bit confusing because the description mentions this is a doublet which is two lenses, but also says its an Apochromat which is 3 lenses. might have to check with them.
Or the ED APO 80mm f/6.25 carbon fibre refractor with fluorite FPL53 ED lens at $599.00 AUD

Let us know what you end up choosing Dan.
A light portable AP setup is enviable.

[swannies1983]

It all comes done to funds and whether I sell my Tal. I have been following a thread on another site where they have been discussing changing the standard 1.25" focuser to a 2" one. It's not that easy and depends on the focuser that is used. Inwards focus is the problem as the baffle tube, just below where the focuser sits, is narrower then the 2" focuser drawtube meaning there's a limit as to how much the focuser can travel inwards. Bert's image train is another option but hopefully this alternative works.

Going by what's in this thread, I think the ED80 would be the best option. The lower price will allow for me to have some spare cash for a flattener. This will probably be my choice if I want a wide view imaging option.

[gregbradley]

The term APO has been hijacked in marketing hype to mean any scope with decent colour correction. It actually has a definition of 3 colours at the same focal point. Usually in practice that means a triplet but if it can be done with a doublet then it would meet the definition. Usually they don't but there are high quality doublets. The Tak FS series for example.

Orion ED may not be APO by definition but perhap best described as high end semi-APO. Tak FS series also.

If you can get a triplet for the same money you should be better off.

Fluorite is another marketing hype. Fluorite is a crystal not a glass. FPL53 has fluorite in it so marketing uses the favorable term fluorite to make it sound more marketable. Real fluorite is more expensive. TEC are one of the few scope makers who still do fluorite lenses. Officina Stellare are another.

Greg.

The ED APO 80mm f/6.8 OTA which Includes FPL-51 fluorite ED lens at $499.00 AUD is within my reach.
but a bit confusing because the description mentions this is a doublet which is two lenses, but also says its an Apochromat which is 3 lenses. might have to check with them.
Or the ED APO 80mm f/6.25 carbon fibre refractor with fluorite FPL53 ED lens at $599.00 AUD