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Author Topic: Which 4" refractor for mainlyastrophotography?  (Read 1042 times)

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Offline Leebert

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Which 4" refractor for mainlyastrophotography?
« on: 20:55:24, 17 May, 2013 »
I am considering a 4" or so refractor to set up an imaging rig on an HEQ5 Pro with a William Optics 66SD for a guide scope. I have had a look around and have a list of OTAs that look like they might suit the task.
They are...

Altair Wave Series 102mm F7 Super ED Triplet APO (102/715);
TS PHOTOLIN'E 102mm f/5,2 Flatfield Super APO telescope;
APM 107/700 F6.5 Super ED Triplet APO w 3" APM focuser;
William Optics FLT 98 APO Triplet;
Skywatcher Esprit ED 100 PRO 5-element.

These vary from about £1400 to £2300.

My budget is around £1700 but could increase for the right OTA (and flattener/reducer if required).

I only want to buy once and am prepared to wait. I am a relative newbie where Astrophotography and refractors are concerned. Looking at the short list, i have a few concerns from looking around the internet...
Altair - a bit slow at f7 and no dedicated reducer yet?
TS 102 - not found any reviews etc on them?
WO - Recent quality / build issues concern me.
Esprit 100 - Really stretches my budget and they don't seem to be in stock anywhere.

I would be interested in peoples thoughts on these or any other scopes...

Many thanks,
Lee

Online chris.bailey

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #1 on: 21:38:39, 17 May, 2013 »
Lee

Tricky choice. What CCD are you using with it. The FLT110 would be fine for smaller CCD;s but would need the TMB field flattener for larger one. The 110 is a great scope.

Chris
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Offline Leebert

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #2 on: 06:56:29, 18 May, 2013 »
Hi Chris,

I would be using a DSLR with an APS-C sensor initially but I am interested in getting a rig that 'I can grow into'. At some point in the future, an upgrade to a CCD will almost certainly happen. Possibly not for a while though after getting the OTA.

I am happy with the HEQ5 Pro and am not really looking to upgrade that. The weight is fine for me and I do like to cart it about to dark sky sites and star parties etc.

I had initially looked at ED80s and the like as they appear to be the OTA of choice generally for astrophotography when looking around the forums. I would like something with a little more aperture though hence looking for a 4". An 80 would also be too close in size to my WO 66SD too in my mind.

I haven't really considered the 110FLT along with other OTAs of that size (and upwards) really because of the weight for my mount and budget. There does seem to be some concerns over quality control at William Optics with some of their recent offerings. That had put the WO a fair bit down my list unfortunately.

Cheers,
Lee

Online chris.bailey

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #3 on: 07:03:31, 18 May, 2013 »
Lee

Dont discount it. The FLT110 is considered by many as being one of THE imaging scopes. Keep your eyes peeled on the second hand lists for one of the older TMB optics one (pre DDG), they do come up and though the focusser is a bit iffy, the optics are top notch. With a DSLR you will need the field flattener though.

If looking second hand a Tak Sky90 is a nice scope, a few issues with mechanics again but there a lot of very nice images out there taken with on.

The Espirit is not really 5 element and I would not touch it. Friend of mine had one and regretted it, good optics, poor mechanics.

The Altair 102 is a lot of scope for the money and takes a reducer nicely for wider field fast optics views.
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Offline astrograph

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #4 on: 14:06:47, 20 May, 2013 »
If I may throw my two pence worth in here.

Its important to understand that you CANNOT use a reducer with all of these refractors and that a reducer is a much more critical optical device than perhaps adverts would lead you to believe.

I must declare that I sell APM and I won't pass comment on this or the other scopes listed as I don't wish this to seem like I am touting for business, the forum needs to be kept free of people doing that, but if I may offer the following advice.

The focuser is important. It needs not only to be able to cope with the weight of camera gear and flatteners you hang off it and not suffer from focus creep etc but also be wide enough not to cause vignetting. When you have a faster F ratio you need to use a wider focuser. The APM is F6.5 and has a 3" focuser as standard. This means it will fully illuminate a 44mm image circle (35mm full frame). If it had a 2.5" focuser is would vignette. However if it was an F8 scope then a 2.5" would be fine for 44mm.

For imaging I think it makes sense to choose a scope that is designed to work with a reducer if needed, as these really do give you the benefit of two scopes in one and obviously reduce imaging time and place less demands on your mounts accuracy because of the reduced focal length.

However you can't just put a reducer on any scope. By reducing the focal length you effectively make the tube shorter and so you need a scope that has more back focus as standard. The APM 107 is designed for use with a reducer and has 180mm of back focus. From experience I would say that this is the minimum you need to allow for using a reducer and a 2" diagonal without extension tubes. Use of a regular field flattener may require an extension though as you can't have it both ways.

I know for a fact that some of the scopes on that list do not have enough back focus to work with a reducer so they will not focus. The tubes are too long. Where this is the case there is a way round it. Instead of fitting the reducer on the end of the focuser it can be fitted INSIDE the focuser if the throat size is large enough. However you have to be careful. The APM small reducer is about 2.5" in diameter so all things being equal it should fit inside a 3" focuser. We discovered recently that the 3" Feathertouch is not actually 3" inside because its baffle is concave. It still offers a 44mm image circle but about two thirds of the way inside the focuser tube it narrows to about 2.5" preventing the internal fitting of a reducer.

The other important factor about the reducer is that it has to be optimised for a small range of scopes and has a critical back focus of its own to provide a flat image circle. As an example the APM-Riccardi small reducer is 72mm long and requires 75mm from the back of it to the image sensor to give you a 40mm flat field. It is also supplied with a collimation flange (tilt adjuster). Because many CCD cameras are notorious for having sensors not aligned parallel to the mounting flange and because alignment of a reducer is critical you may need to tilt things. You may not need the adjuster but if you do this all adds more length meaning you need all the back focus you can get.

The image circle of a reducer will also suffer slightly from vignetting. The 40mm quoted is therefore OK for sensors up to APS size (26mm image circle max) but not for anything much larger. If you need more then there is a bigger version which has a 50mm image circle, but this is also longer and heavier meaning that the focuser has to be up to the job and the back focus adequate.

The last bit is that there are reducers and there are reducers. The off the shelf TMB reducer is NOT a TMB product. The only true TMB product comes from the APM/Thomas Back relationship and when Thomas Back was alive. This TMB reducer was only a TMB design and what is around now is not made to anything like the quality standard it would be if Tom was alive. As an aside, TMB Optical in the states is not connected with that reducer either and sells scopes of Toms design but made in China without the QC Tom insisted on.

You can get away with an off the shelf Chinese made normal flattener (The TS 2.5" is a good example of this and there is nothing wrong with it), because its not trying to do anything too clever and you only use the middle of the glass. Because of the extra light bend caused by a reducer it has to be matched to a limited scope focal length / F Ratio and made to a much, much higher standard. The APM reducers are triplet element true apochromatic designs with ED glass. Fitted to a 107/700 we know from user experience that this combination is still better colour corrected than a Tak FSQ-106 which is saying a lot.

So I hope this does not sound like an APM advert. Its just intended to outline the pitfalls of assuming a scope will work with a reducer.

The last point is that Petzval scopes like the Esprit model mentioned cannot be used with a reducer as they are already flat field.

Offline Leebert

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #5 on: 21:13:40, 23 May, 2013 »
Thanks for the very informative replies. Looks like I have a lot to think about!
Cheers,
Lee

Offline roundycat

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #6 on: 19:02:45, 24 May, 2013 »
A question to 'astrograph', (no name given);

I know nothing about the Esprit but have a Tak 106 which is a modified Petzval design. A reducer is available for the Tak. Either you can or cannot fit a reducer due to physical restraints but the reducer reduces. So long as the field stays flat where is the problem?

Dennis

Offline PaulB

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #7 on: 20:25:13, 24 May, 2013 »
Hello Lee,

For what it's worth. I would buy the Altair Wave Series 102mm F7 Super ED Triplet APO (102/715) I have heard some very positive noises about this telescope.

And you can use a dedicated field flattener with it.
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Online psjshep

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #8 on: 20:28:10, 24 May, 2013 »
A question to 'astrograph', (no name given);

FYI, Dennis...

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Offline roundycat

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #9 on: 09:12:30, 25 May, 2013 »
Sorry, that one has gone right over my head.

Dennis

Offline astrograph

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #10 on: 17:43:09, 25 May, 2013 »
Hi Guys

The names Rupert. At your service. The issue with a flat field reducer is that it is designed to not only reduce the focal length and thereby increase field of view but also provide a flat field. All scopes which are not Petzval designs will have field curvature to some degree and a flat field reducer assumes this. Tak make reducers for their scopes knowing that they are already flat field. That's OK but a Tak reducer would not be flat field on a non flat field scope any more than using the APM Riccardi Reducer on the Petzval Tak or Espirit would.

If you look at the reducer section of the APM UK site you will see the different optical designs of reducers. One size does not fit all with reducers...






Offline roundycat

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #11 on: 20:35:24, 25 May, 2013 »
In other words, don't pick'n'mix. Now I see what you were getting at.

Dennis

Offline FranckiM06

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Re: Which 4" refractor for mainlyastrophotography?
« Reply #12 on: 14:22:37, 02 June, 2013 »
If you want a good quality, you can buy the TSA 102 !
Takahashi TSA 120 refractor, ccd Atik 314L+ with filters wheel Atik and Baader filters LRGB & Ha, Canon 40d (unmodified). Mount: 10 Microns GM1000HPS.
For planetary imaging: DMK 21 AU 618 (from 2012) & Powermate x4
http://www.astrobin.com/users/FranckIM06/

 

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