Televue 85 as an imaging scope

[osbourne one-nil]

As I've mentioned elsewhere, I've got myself an Astrotrac to use, primarily, with my Olympus DSLR. However, I'm also getting my Porta II mount modified (or a plate made at least) so I can mount the astrotrac on that, which will then be more than capable of working with my TV 85. I can picture what a 600mm lens will capture, but at f7, is it going to be any use for imaging? I'm not particularly looking to image faint fuzzies, but M31, M42, the Pleiades....they'd be nice.

Any advice?

[ollypenrice]

With the Televue reducer flattener, which it does need, it is an outstanding scope. My regular guest, the very experienced Frans Kroon, sometimes borrows one from his club and calls it 'the perfect telescope.' www.franskroon.nl His site discusses it briefly.

You can frame imaginary pictures on the sky using the free ccd calculator (it Googles) once you've put in chip size and focal length.

Olly

[osbourne one-nil]

Thanks - that sounds promising! I hope it's as outstanding compared to other scopes with DSLRs and not just CCD cameras? It's going to be a while before we get into that.

The focal reducer is certainly affordable compared to getting a specific imaging scope, and would make the telescope so versatile, although I love it already!

That Frans is rather good!

[ollypenrice]

It will be great with a DSLR. It is fast enough and has a large enough flat field. Frans used DSLRs until last year so many, if not all, his TV85 images were done that way.

The key thing with a scope for DSLR imaging, assuming a flat field, is speed. You are limited by thermal noise to quite short subs (5 to 8 mins?) so a fast F ratio comes to the rescue.

Olly

[MonicaA]

Hello, new member and first post. :-)

I'm wondering how the Tele Vue 85 would perform with the Canon EOS 60Da, with its enhanced H-alpha sensitivity. I'm thinking of chromatic aberration in the IR range of the spectrum. I'm primarily interested in DSO imaging, and the 60Da is a given factor, as I have aleady bought one :-) Will the TV focal reducer for the 85 change the CA?

From the Tele Vue web site I see the designed back focus is 56 mm (+/- 2 mm, if I remember correctly). This seems fine for the 60Da, but will it totally preclude using a Starlight Xpress motorized filter wheel, which adds 29 mm? (I'm thinking of a future possible automation path, using a sky glow filter for light frames and a blocked filter position for dark frames.)

I'm open to other suggestions for a first telescope for the 60Da, but I prefer something of comparable quality, and not significantly more expensive. Maybe a triplet would be better? I plan to use this on an EQ-6 or AZ-EQ6 mount (or worst case HEQ5 Pro, if I decide against the mount weight, after all...)

Monica A

[Zakalwe]

Why would you use a filter wheel with a DSLR?

[MonicaA]

To have the possibility to take dark frames controlled by the PC. Probably won't go that route anyway, after some more pondering it makes more sense to use the night time for light frames, and build a library of dark frames for various parameters separately.

Any thoughts on the IR question? Might not be that important for DSO's anyway?

[Zakalwe]

Just put the lens cap one the scope for dark frames

[ollypenrice]

Just put the lens cap one the scope for dark frames

Surely nobody uses a filterwheel to take darks. You'd be far better advised to take darks of comparable temperature in a thoroughly dark and cosmic ray resistant box like a fridge! (Take the bulbs out!) Your library idea is a much better bet. Flats can be done any time if you have a leak proof liightpath (manual filter wheels are not light proof, for example) and don't move the camera in the OTA. What you don't want to be doing is wasting imaging time on anything but the lights.

TV85 versus triplet? Hard to say. I'd rather have a TV doublet than many peoples' triplet, certainly at the price you'd pay for a TV85 second hand. They are extremely undervalued. Mechanically they won't give you misery with slipping Crayfords and sagging focusers and they can easily be re-collimated in time, should that be necessary. The focuser is single speed but a long wand or lever attached to the knob removes that problem, or you could add a motorized focuser or even... a crazy contraption like mine (which is the best fine focus I've ever used, I reckon.)

Olly

[Zakalwe]

You'd be far better advised to take darks of comparable temperature in a thoroughly dark and cosmic ray resistant box like a fridge! (Take the bulbs out!)

Erm...the light goes out when the door is closed...... :grin:

[MonicaA]

Thanks for the input The fridge approach sounds good (with or without the bulbs ). I think I'll get a TV 85 with the FR/flattener then. I'd rather pay for known good quality that lasts than to chance on unknown QC and durability. The advantage of lower price may be eaten up by hazzle, waiting time and shipping costs (at least a statement to justify the price for myself.... )

I take it that the IR aspect probably is a non-issue with the TV 85 / 60Da combination? After all people have been using IR modified DSLRs for some years.

[swag72]

I have a very good quality doublet, a Pentax 75SDHF. There is no way I would ever have moved on to any triplet from there other than a Tak. A good doublet in my experience is a very, very good find indeed. The build quality is well worth every penny you spend as well.

If I had a choice to make between a generic triplet and a quality doublet, it would be the doublet.

[RT65CB-SWL]

Have you checked out TeleVue's website?

The TV85 was recommended on their website a few years ago. I have the Ranger (no longer made) and they say/said that the Ranger was not recommended. I have taken photos (afocal) of terestial objects and lunar & solar with a digital compact camera (they may not win any photo competitions, but it is fun seeing what can be done with basic equipment). I did the 1999 eclipse with a 35mm SLR and the 2004 transit of Venus too with both the digital compact and 35mm SLR. I have tried some planetry phography (Venus, Mars, Jupiter & Saturn) but they are/were not brilliant.

I am planning on buying a mono DMK-21 second hand from a member from my local astro society so that will prove interesting with my other OTA's (C6/SCT & ETX105) including the Ranger on an alt-az mount.

[ollypenrice]

Erm...the light goes out when the door is closed...... :grin:

How do you know???

Olly

[Zakalwe]

How do you know???

Olly

C'mon, don't pretend that you've never crawled inside a fridge to check???